Historical changes and related coastal processes, gulf and mainland shorelines, Matagorda Bay area, Texas

[From the U.S. Government Printing Office, www.gpo.gov]

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HISTORICAL CHANGES AND RELATED COASTAL

PROCESSES, GULF AND MAINLAND
SHORELINES, MATAGORDA BAY AREA.,

TEXAS

property Of CSC Library

J. H. McGowen and J. L. Brewton

C collecrion

DEPARTMENT OF COMMERCE NOA A
COASTAL SERVICES CENTER
2234 SOUTH HOBSON A VWE
CHARLESTON Sc 29405-2`4

BUREAU OF ECONOMIC GEOLOGY
The University of Texas at Austin
Austin, Texas 78712
W. L. Fisher, Director
(Y IN COOPERATION WITH

THE GENERAL LAND OFFICE OF TEXAS
Bob Armstrong, Commissioner COASTAL Kilk-
INFORMATION CENTER
Z 1975

FOREWORD

Change, both natural and man induced, is a significant and defining element of the
Coastal Zone. Man-induced change, by definition, can be controlled if desired. The work of
nature, however, is altered and modified with much more difficulty, if, at all, and attempts
to do so commonly lead to unintended results.

Prudent use and adequate management of. the Coastal Zone must consider natural
changes..These changes.are expressed primarily 'in changes ofnatural boundaries-changes in
position of shorelines, changes in position of lines of vegetation, and changes in boundaries
of wetlands, among others. These changes assume particular significance when an eroding
(and changing) shoreline transgresses coastal structures or when natural boundaries that are
also legal boundaries, such as those marking.line of vegetation or boundaries between fresh
and tidal wetlands, change.

The best assessment of change is over the long term. In such manner., distinctions can
-be made between temporary variations and long-term change. In this report, the technique
of historical monitoring has been specifically developed. Through mapping of specific,
significant boundaries on vintage photographs and charts, taken at varying periods over the
past 125 years, long-term direction, amount, and extent of change are determined. Through
comparable historical monitoring or mapping of land use and land use activities,
man-induced changes can be determined and, importantly, distinguished from natural
change. A more accurate evaluation of man's impact can be made.

In 1971, the Texas General Land Office and the Bureau of Economic Geology, The
University of Texas at Austin, initiated on a cooperative basis a comprehensive pilot study
of Matagorda Bay and environs. The first phase of that study was an analysis of historical
changes and the related coastal processes, herein reported. Techniques of historical
monitoring developed in this study have been utilized by the Bureau of Economic Geology
to determine long-term changes of the entire Texas Gulf Coast. The second part of the
Matagorda pilot study addressed in detail the biologic, physical, and chemical characteristics
of sediments in Matagorda Bay.

We believe that this project, historical in its orientation, gives us a better ability to
make intelligent decisions for the future.

Bob Armstrong
Commissioner, General Land Office

W. L. Fisher-
Director, Bureau of Economic Geology

CONTENTS

Abstract 1 Sand Point area 21
Introduction 2 Powderhorn Lake 21
Acknowledgments 2 Lavaca Bay North Area 21
General Setting 3 West shoreline- Lavaca Bay 22
Geologic History of the Matagorda Bay Area 6 North shoreline- Lavaca Bay 22
Pleistocene history 9 East shoreline-Lavaca Bay 22
Holocene history 9 Cox Bay 22
Modern history 10 Carancahua Bay Area 22
Prehistoric development 10 North shore-Matagorda Bay 22
Historic development 10 Carancahua Bay shoreline 23
Historical Shoreline Monitoring: General Methods Keller Bay shoreline 23
and Procedures Used by the Bureau of Economic Salt Lake and Redfish Lake 23
Geology 11 Tres Palacios Bay Area 23
Definition 11 Palacios Point-Oliver Point 23
Sources of data 11 Oliver Point-Tres Palacios Creek 23
Procedure 11 Tres Palacios Creek-Turtle Point 24
Factors affecting accuracy of data 11 Turtle Point-Turtle Creek 24
Original data 12 Turtle Creek-Sartwelle Lakes 24
Topographic surveys 12 Oyster Lake 24
Aerial photographs 12 Summary 24
Interpretation of photographs 12 Marsh Distribution, 1856-1957 25
Cartographic procedure 13 General wetland trends 25
Topographic charts 13 Marsh changes re sulting from natural processes 25
Aerial photographs 13 Marsh changes resulting from man's activities 26
Measurements. and calculated rates 13 Coastal Processes and Short-Term Shoreline Changes 26
Justification of methods and limitations 14 Coastal processes 26
Vintage Cartographic /Photographic Materials Used in Short-term shoreline changes, 1957-1972 26
the Matagorda Bay Area Study 15 Open Gulf shorelines 27
Presentation of Data in Map Form 15 Bay shorelines 33
Effect of Astronomical Tide on Mapping Waterlines Cliffed shorelines 33
on Vintage Aerial Photographs 15 Shorelines characterized by shell
Gulf and Mainland Shoreline Changes, 1856-1957 16 beaches and berms 35
Brown Cedar Cut Area 16 River-influenced shorelines 40
Gulf shoreline 16 Shoreline segments dominated by
Bay shoreline of Matagorda Peninsula 16 salt marsh 42
Mainland shoreline 18 Shorelines dominated by spoil
Colorado River Area 18 outwash 43
Gulf shoreline 19 Stability of Shorelines 44
Bay shoreline of Matagorda Peninsula 19 Sediment availability 44
Mainland shoreline 19 Subsidence 45
Deltaic shoreline 19 Shoreline orientation 45
Shell Island Reef Area 19 Direction of sediment transport 45
Gulf shoreline 19 Distinction Between Natural and Man-Induced Shore-
Bay shoreline of Matagorda Peninsula 19 line Changes 46
Mainland shoreline 19 Natural shoreline changes 46
Oyster Lake 20 Man-induced shoreline changes 46
Pass Cavallo Area 20 Conclusions 53
Gulf shoreline 20 Shoreline Change 53
Bay shoreline-Matagorda Peninsula and Marsh Area Change 54
Matagorda Island 20 Future Studies 55
Mainland shoreline 20 References 56
Marsh islands 21 Appendix A: Changes in Shoreline for the Period
Lavaca Bay South Area 21 1856-1957 58
West Matagorda Bay 21 Appendix B: Changes in Marsh Area for the Period
West Lavaca Bay 21 1856-1957 59
Cox Bay 21 Appendix C: Profiles of Gulf and Mainland Beaches 61

ILLUSTRATIONS

Figures--

1. Locality map of the Matagorda Bay area 4 16. Locations of trenches dug into shell beaches,
2. Geologic map, Matagorda Bay area 5 berms, and spits 37
3. Slope of the lower 20 miles of coastal plain 6 17. Distribution of shell beaches, berms, and spits
4. Temperature and rainfall distribution for Bay in the Magnolia Beach-Indianola area, 38
City, Port Lavaca, Palacios, and Port O'Connor 7 18. Wave refraction at Arena Cove, California 40
5. Location of weather stations in the Matagorda 19. Map of the growth of the Colorado delta during
Bay area 7 the period 1908-1941 41
6. Percentage frequency of surface wind direction
20. Area increase of the Colorado delta, in acres,
(annual) 7
7. Sea-level changes related.to glacial and inter- for the period 100.8-1953 42
glacial stages 8 21. Net annual longshore drift 45
8. Slopes of Gulf and mainland beaches 17 22. Locations of jetties in the Port O'Connor area,
9. Index of shoreline change and marsh distribu- and changes in the shoreline and nearshore sand
tion maps 18 distribution during the interval 1934-1956 48
10. Shoreline types of the Matagorda Bay area 28 23. Distribution of bay-margin sand north of the
11. Short-term erosional and accretionary shore- jetties at Port O'Connor 49
lines of the Matagorda Bay area 29 24. Accretion and erosion associated with Mata-
12. Effects of Hurricane Carla, 1961., on a segment gorda .Ship Channel jetties 49
of Matagorda Peninsula beginning about 1.5 25. Shoreface profiles in the area of the north jetty,
miles west of theColorado River 31 Matagorda Ship Channel 50
13., Relationship between direction of wave 26. Shoreface profiles in the area of the south jetty,
approach and longshore drift 32 Matagorda Ship Channel 51
14., Postulated sequence of events leading. to the 27. Dredging of Dog Island Reef for oyster shell 52
development of an 'erosional unconformity on
Matagorda Island 34
15. Cuspate shell beaches, oyster reefs,. and, spoil Plate-
outwash along the north shore of Matagorda
Bay, w est of the Colorado delta 36 1. Gulf and mainland beach profiles. In pocket

MAPS

1. Matagorda Bay System: Gulf and mainland shorelines 2. Matagorda Bay System: marsh distribution

Brown Cedar Cut Area In pocket Brown Cedar Cut Area In pocket
Colorado River Area In pocket Colorado. River Area In pocket
Shell Island Reef Area In pocket Shell Island Reef Area In pocket
Pass cavallo Area In pocket Pass Cavallo Area In pocket
Lavaca Bay South Area In pocket Lavaca Bay South Area In pocket
Lavaca Bay North Area In pocket Lavaca Bay North Area In pocket
C Iarancabua Bay Area In pocket Carancahua Bay Area In pocket
Tres Palacios Bay Area In pocket Tres Palacios Bay Area In pocket

TABLES

Tables--

1. Pleistocene glacial and interglaciaI episodes 6 3. Comparison of changes along bay-shore shell
2. Comparison of erosional, rates of.cliffed shore- beaches; filed field measurements (1957-1972)
lines determined from field measurements and historical monitoring (1856-1957) 39
(1957-1.972) and from historical monitoring 4. Erosional and accretionary rates (1957-1972)
(1856-1957) 33 and physiographic units associated with marshes 43

HISTORICAL CHANGES AND RELATED COASTAL PROCESSES,
GULF AND MAINLAND SHORELINES,
MATAGORDA BAY AREA, TEXAS

J. H. McGowen and J. L. Brewton'

ABSTRACT

Most of the Gulf and bay shorelines of the Shoreline stability (accretion, equilibrium, or
Aatagorda Bay area are in an erosional state. erosion) is a function of the interplay among
Historical shoreline monitoring, during the interval several geological processes such as wind, waves,.
1856-1957, and field measurements, made in tides, the kinds and volumes of sediment con-
1971-1972, document direction and rate of shore- tributed to bays and the Gulf of Mexico, storm
line change for a 116-year interval. Average frequency, and compactional subsidence, and
erosional rates of 16 and 22 feet per year occur at slump which is restricted to the cliffed shoreline
Brown Cedar Cut and along the northeastern part segments. Erosion dominates the coastal scene
of Matagorda Island, respectively. Erosional rates
along bay shorelines range from less than 1, foot primarily because of a deficit of sand supplied to
per year to 15 feet per year. the area. In general, the shores of large bays are
eroded more rapidly than those of small bays; the
All shoreline segments are not erosional. Parts fetch of large bays is great and waves tend to be
of central and western Matagorda Peninsula were large. Also, bay shores that face into the prevailing
either in equilibrium or in an accretionary phase wind erode rapidly. Shoreline erosion is relatively
during 1856-1957, as was Matagorda Island near slow along high cliffed shorelines that lie in the lee
the western limit of the area of investigation. Field of prevailing southeast winds. Northers are gen-
measurements made in 1971-1972 indicated that erally accompanied by high-velocity, short-
most of the Matagorda Peninsula shoreline was duration winds. These winds generate rather large
erosional. Several bay shoreline segments exhibited waves, which erode north-facing shorelines.
net land gain for the 116-year interval. Such areas
as bayhead deltas, spits which developed down- Exceptionally high tides and large waves are
current from erosional headlands, and bay shores
that are adjacent to spoil-disposal sites are cur- produced by hurricanes. Storm surge (storm tides)
rently accreting. and large waves severely erode coastal barriers and
peninsulas. Hurricane Carla J1961) eroded the
Marshes generally decreased in area during the shoreline of Matagorda Peninsula as much as 800
period 1856-1957. Known causes of marsh decline feet in a few hours. Low-relief barrier islands and
are shoreline erosion which occurs under normal peninsulas, such as Matagorda Peninsula, are easily
sea and storm conditions, inundation by sediments breached by storm surge, and large volumes of
related to storm washovers, and burial of wetlands sediment eroded from the shoreface and beach are
beneath dredge spoil. A few marshes have increased
in size; the most notable expansion is the marsh transported into the bays. This volume of sediment
associated with the Colorado delta. Most of the is effectively removed from the sediment transport
Colorado delta, which is some 7,000 acres in area, system operating in the nearshore zone of the Gulf
was constructed between 1929 and 1936. of Mexico and is stored in the Matagorda Bay
system. Erosion of the shoreline of the Modern
Continental Oil Company, Ponca City, Oklahoma. Matagorda Peninsula by storms is irreparable.

INTRODUCTION

The Texas Gulf Coast consists of erosional expand with larger oil imports and a shift from
deltaic headlands and of peninsulas and barrier fossil fuel to nuclear-powered generating plants.
islands, which separate bays and lagoons from the
Gulf of Mexico. There are about 367 miles of Gulf The objectives of this study were: (1) to
shoreline and about 1,425 miles of lagoon, bay, document the direction and magnitude of shoreline
and estuary shoreline in Texas. Climate of the change; (2) to present some possible causes of
Texas Coastal Zone is mild with average annual change; and (3) to make this information available
temperature ranging from 69'F in the east to 74'F to those who might be interested in shoreline
in the south. A mild climate, coupled with access stability as it will affect man-made structures.
to relatively wide sand beaches, fishing, and other Man's activities in the Coastal Zone have grown
water-related sports, makes the Texas7 Coast a without an adequate knowledge of shoreline
prime tourist attraction. Natural resources within stability. About 60 percent of the Texas Gulf
the coastal plain and the relative ease of trans- shoreline is now erosional, with rates up to 80 feet
porting raw materials and finished products via per year locally. Principal causes of erosion are
intracoastal waterways have attracted industry to natural, but certain of man's activities may have
the region. increased erosional rates. Natural causes of shore-
line retreat, such as sand deficiency and relative
In recent years the Texas Coastal Zone has rise in sea level, pose problems that cannot be
experienced a rather dramatic change. Approxi- readily solved. It is imperative, therefore, that rates
mately one-fourth of the State's population now and directions of shoreline change be measured and
resides in the region, and population is steadily that man's Coastal Zone activities proceed in
increasing. Consequently, shoreline property is in concert with this natural change.
exceeding demand for construction of permanent
homes, second homes, condominiums, hotels and The Matagorda Bay area was chosen for study
motels, and for recreational use. Similarly,, indus- because it is one of the segments of the Texas
trial development is also expanding significantly. Coast that has been least affected by man's
activities. A two-phase study of the Matagorda Bay
Added to an increasing population and indus- area was undertaken to determine shoreline
trial expansion within the region are a decrease in stability of Gulf and bay shorelines and changes in
supply of domestic oil and gas and a greater marsh area for the period 1856 through 1972, and
reliance on foreign oil imports to fulfill energy to map the distribution of sediment types, total
requirements. Deepwater ports or offshore mono- organic carbon, trace elements, and molluscs. This
buoy systems will be required to handle super- report presents the results of the first phase of the
tankers. Increased use of nuclear power for Matagorda Bay investigation by the Bureau of
meeting energy needs will require a number of sites Economic Geology and the General Land Office,
in the Coastal Zone. Construction in the region will undertaken between October 1971 and May 1972.

ACKNOWLEDGMENTS

Many individuals and organizations con- L. E. Garner, Bureau of Economic Geology,
tributed to this project. The project was funded in B. H. Wilkinson, C. V. Proctor, Jr., and A. W.
part by the General Land Office of Texas. Funds Erxleben, former employees of the Bureau of
to help defray some of the costs of printing the Economic Geology, and Scooter Cheatham and
shoreline maps were contributed by the Sea Grant Lee McKibben, General Land Office, assisted with
Program, Texas A&M University, through a grant the field work. R. W. Nordquist participated in'the
from the National Oceanic and Atmospheric field work and mapping shorelines on sequential
Administration, U. S. Department of Commerce. aerial photographs. Virgil Townley, Harry Smith,,
Boats were provided by E. Gus Fruh, Department
of Civil Engineering, The University of Texas at W. F. Green, and Bruce Kay granted access to their
Austin, and by the Marine Science Institute of The properties and/or provided information concerning
University of Texas at Port Aransas. history of local areas.

Special thanks are extended to the following of molluscan shell and wood collected on Mata-
people for enlightening discussions concerningthe gorda Peninsula and from Cedar Lake and Cow
problems. associated with shoreline stability: B. H. Trap Lakes.
Wilkinson, University of Michigan; L. F. Brown,
Jr., W. L. Fisher, L. E. Garner, and R. A. Morton, The manuscript was reviewed by W. L. Fisher,
Bureau of Economic Geology; K. 0. Emery, Woods L. F. Brown, Jr., E. G. Wermund, and J. R. Byrne,
Hole Oceanographic Institution; J. R. Byrne and Bureau of Economic Geology, and W. D. (Red)
Walter Leeper, Department of Geological Sciences, Oliver, Peggy Harwood,'and Rose Ann Rowlet,
The University of Texas at Austin; Peggy Harwood, General Land Office. Cartographic preparation was
the General Land Office; and Wayne Ahr and Barry by D.F. Scranton under the supervision of J. W.
Sullivan, Texas A&M University. Macon, who also supervised drafting of the illustra-
tions. Elizabeth T. Moore edited and typed the
Sam Valastro, Research Scientist, Radio- manuscripts; final editing was by Kelley Kennedy.
carbon Laboratory, The University of Texas at The paper was composed by Fannie Mae
Austin, provided, radiocarbon age determinations Sellingsloh.

GENERAL SETTING

The Matagorda Bay study area consists of relatively. flat. Maximum elevation is about 50 feet
deltaic headlands, peninsulas and barrier islands, in the northwest comer of the, area. The. slope of
large bays and estuaries, and gently seaward-sloping the upland surface is shown on figure 3. Areas of
uplands that form the mainland shoreline. lowest slope are near, Caney 'Creek and Port
Matagorda Bay system and environs .are situated in O'Connor. Small drainage systems are, in part,
parts of Matagorda, Calhoun, Victoria, and Jackson affected by the degree and direction of the slope of
Counties (fig. 1). Size of the area is approximately the land surface.
2,000 square miles, consisting of: (1) 1,470 square
miles of uplands; (2) 455 square miles of bays and Several rivers and creeks discharge water and
estuaries; and (3) 75 square miles of.peninsulas, sediment into the Matagorda Bay system. The
barrier islands, and tidal deltas. larger streams, such as the Colorado and.Lavaca
Rivers and Garcitas Creek have constructed bay-
Gulf beaches in the area are about 65 miles head deltas along Ithe bay margins. The largest of
long. They range in composition and texture from these deltas, the Colorado, has prograded. Com-
terrigenous fine. sand to shell and rock fragment pletely across Matagorda Bay.
gravel. The bay shoreline, consisting of wetlands,
deltas, sand and shell beaches, and, almost vertical Climate of the Matagorda Bay area is humid
Cliffs, is approximately 235 miles.long. subtropical (U. S. Dep .artment of - Commerce,
Three general physiographic elements charac- 1958-1969). Rainfall and te'niperature.data (fig. 4)
terize the study area., These are: (1) Matagorda are almost identical for, four weather stations in the
Peninsula and Matagorda Island; (2) the Pleistocene vicinity of Matagorda Bay (fig. 5). Rainfall distri-
uplands; and (3) rivers and small streams that bution graphs show two peaks, one, in June and the
dissect the uplands. Matagorda Peninsula ranges in other in September, which coincide with thunder-
width from 0.75 to 1.0 mile, and has an average storm and hurricane occurrences, respectively.
elevation of about 7 feet. Dunes are rare on the
Wind data from the Victoria Weather Station
peninsula, but. some isolated dunes attain heights
of - 25 feet. Only the eastern 7.5 miles of Matagorda records indicate that surface winds are chiefly
Island occurs in the study area. The islan Id ranges in onshore (fig. 6). Prevailing winds for the period
width from 1.25. to 1.5 miles. Foremisland dunes up 1951-1960 were from the south-southeast, whereas
strongest winds during the same period were from
to 30 feet high (Wilkinson, 1973) are well devel- the northwest.
oped on Matagorda Island.

Pleistocene uplands, which are underlain by Hurricanes and tropical storms are naturally
fluvial-deltaic and strandplain deposits (fig. 2), are occurring phenomena of the Atlantic, Caribbe an,

NAVIO

GARCI rAS
DEL TA A VA CA
DELrA

SWAN
LAKE

PORT BAY CITY
LAVACA POINT
COMFORT

Cox Q)
Y CARANCAHUA Q@
BA Y
L
-4
SAY PA LAC I
SAND POINT WELL
POINT
COLORAD
PO OysrER DEL7A LAKE AUSTIN
LAKE PALACIOS LAKE
PORT POINT MATAGORDA
OCONNOR MA TA GORDA BA Y
E-SPIRITU SANro BAY EAsr AfATAGORDA
D MATAGORDA BAY SARGENT
:__::@GRE_'EENS BAYOU EACH
7MA @TA G 0 @RD A I @S BROWN CEOA R
CUT
GULF OF MEXICO

0 4 1.2
MILES

Figure 1. Locality map of the Matagorda Bay area.

and Gulf Coast areas. Hurricanes are storms of each tidal day). Tidal range is low. The mean
tropical origin with cyclonic circulation of 74 mph diurnal range at Freeport Harbor is 1.7 feet and 1.4
or higher (Dunn and Miller, 1960). During the feet at Pass Cavallo (U. S. Department of Com-
period 1900-1963, the Texas Coast was struck by merce, 1973). Tidal currents are an important
42 tropical cyclones, a frequency of one storm sand-transporting mechanism in tidal pass areas;
every 1.5 years (Hayes, 1965, 1967). Hurricanes elsewhere waves and longshore currents are the
occur most commonly during the months August principal sediment-transporting mechanisms.
and September. The effects of hurricanes on the
Coastal Zone are: (1) shoreline erosion; (2) breach- The Texas Coast is a wave-dominated coast
ing of barrier islands and peninsulas; (3) salt-water
flooding by storm surge; (4) damage to man-made (Hayes, 1965). Since prevailing wind in the
structures by flooding and wind; and (5) flooding Matagorda Bay area is from the southeast quad-
resulting from aftermath rains. rant, most waves approach the shoreline from that
direction, strike the shoreline at an angle, and set
Tides in the northern Gulf of Mexico are up longshore currents that move sediment to the
chiefly diurnal (one high and one low water level southwest.

..........

'8.

LAVA
..........

. . . . . . . . . . . . . . . LPh
........ ...

BAY
,,,,,,(;oRDA
EXPLANATION

oo.
PLEISTOCENE DEPOSITS
BAY AND MY MARGIN

F 1@ -b.b.-V -d, 1,td! it 11
s-,b .@d bl,- I

-d, -1-1bly-f-bl- I- ti@ f'--,

R, -d: --bd by -11h 11
Llk-

J Wi.d-tidli tilt; Ild
0
St-dipilm 11d, dtk, id------phy
ov..,
MODERN-HOLOCENE DEPOSITS
BARRIER ISLAND PENINSULA
11.VIAL-DEITAI.
B-h:
0 2 4 6 P.I. .. W..-d. 1,
--I.m by bb- Ild Ild -1-1,

SCALE IN MILES Sh.11 -i, bl

-d lid.1 ft..; I. with

MW- -11- by W- b., - -d! -,bl@k ,tbdi S@It -,tt. .d..d @ddy

MI 11 d1IIl 111"dll I'd - ,It 1"d 1,111,11
_d

Mbd@ pl,!,, d,,i ... ld! by I- -11h; IW,

GEOLOGIC MAP, MATAGORDA BAY AREA

Figure 2. Geologic map, Matagorda Bay area.

GEOLOGIC HISTORY OF THE MATAGORDA BAY AREA

Pleistocene, Holocene, and Modem deposits tinental glaciers. Glaciation began to affect the
constitute the uplands, bay margins, and Gulf North American continent approximately three
shoreline features (fig. 2). Water bodies, including million years B.P. (Cooke, 1973, table 3, p. 215).
bays, estuaries, and their associated fluvial systems, The sequence of Pleistocene glacial events is shown
are Holocene and Modem features. Uplands are in table I and figure 7.
underlain by deltaic sediments which were
deposited during the Sangamon Interglacial Stage Table 1. Pleistocene glacial and interglacial episodes
and a strandplain sand that accumulated during a (after Kummel, 1961).
Wisconsin Interstadial Stage. Shorelines of the
Matagorda Bay area began developing their present
configuration at sea-level stillstand, about 3,000 to Glacial Interglacial
2,500 years B.P. (before present). The term still- (Low Sea-Level Stand) (High Sea-Level Stand)
stand implies a halt in the rise of sea level. The
dates given for stillstand are those reported by Holocene
Curray (1960), Nelson and Bray (1970), and Wisconsinan
Frazier (1974). Illinoian Sangamon
Depositional and erosional features of the Kansan Yarmouth
Texas Gulf Coast were created, indirectly, by Aftonian
alternate growth and reduction in size of con- Nebraskan

4e 11
@7,

3.6' per mile Della
Lavaca
06/to 0 5 10 rnde@
0 Scale
4.4' per mile

Lavaca 8ay
2. 8' per mile

Pori
L-c a A

2.0' per mile
1.7' per mile
Pdoci.@* .1.8'per mile

Colorado
Dell,
...........
Molag-do Bay Lake
A'SI'n

Pori
O'Connor

EaV N010�701dl Bly
E5.0"d, SaNo 80y"=ZZ,

Gulf of mexlco

Figure 3. Slope of the lower 20 miles of coastal plain. Slope gradient and direction are shown for six areas along the
Sangamon delta plain and Wisconsin strandplain. Slope directions converge. along the axis of Lavaca Bay.

100-

90-

80- 5-
0- 70-
4-
@i 60-
E
50
&
40-
2
30

Jan. Feb. Mar. Apr. May 'June July Aug.' Sept. Oct. Nov. I Dec. 4pr. 1 May 1 June 1 July Aug.' Sept. Oct. Nov. Dec.
say City 19 year average Bay City - 19 year average
Port Lavaca 22 year average Port Lavaca - - - - - - - - - 22 year average
Polocios ............................ 2 1 year average Palocios .......................... 28 year average
Port dConnor - - - - - - - 22yearaverage Port dConnor - - - - - - - 2 2 year average

Figure 4. Temperature and rainfall distribution. for Bay City, Port Lavaca, Palacios, and Port O'Connor. (A) Mean
monthly temperature. (B) Mean monthly precipitation. Data from U. S. Dept. Commerce, Climatological summary, Bay City,
1943-1961, andfrom Env. Sci. Serv. Adm., U. S. Dept. Commerce, Palacios 1941-1968, Port Lavaca 1947-1968, and Port
O'Connor, 1948-1969.

Ba*YCity

Port P locios Wind co nditions calm
vaca 6% of the time

N
Port O'Connor 0 %GO N
Of
0-% 20%

Figure 5. Location of weather stations in the Matagorda Figure 6. Percentage frequency of surface wind direction
Bay area. (annual). Data from Victoria weather station (U. 9. Depart-
ment of Commerce, Local Climatological Data, Victoria,
Texas, 1958-1969).

THOUSANDS OF YEARS BEFORE PRESENT
0 200 400 600 Boo 1000 1200

wP
_j P
SEE FIG. 5B DEPOSITION DEPOSITION DEPOSITION VALLEY CUTTING

@WA LEVEL

OUITH AFTONIAN
SANGAM IN E LACIAL INTERGLACIAL
INTER- E STAGE
GLACIAL
STAGE KANSAN NEBRASKAN
WISCONSIN ILUNOIAN GLACIATION GLACIATION
GLACIATION GLACIATION After Fisk (1944) and Berna

THOUSANDS OF YEARS BEFORE PRESENT
0 50 too 156

VALLEY
SEE FIG. 5C DEMSITIONI CUTTING DEPOSITION VALLEY CUTTING DEPOSITION

t LATE EARLY
WISCONSIN WISCONSIN
GLACIATION GLACIATION SEA LEVEL'
PEORIAN SANGAMON
MODERN- INTERGLACIAL INTERGLACIAL
HOLOCENE STAGE STAGE

After Fisk (1944) and Bernard & LeBlanc (19652

YEARS BEFORE PRESENT (B.P)
TODAY 5,000 10,000 15,000 20,000 ......

LATE
w
w 50 MODERN HOLOCENE PLEISTOCENE
(WISCONSIN
_j GLACIATION)
w
U, too-

w
Z W N EXTENDS TO
< w 0
uJ 150 0 0 APPROXIMATELY
0 0 1.2 MILUON
CL
T YEARS B.P.
Z
w
U)
w
cr
Q_ 200

Proposed Sea Levels:
_ Frazier-in press
250 - Curray-1960
w Nelson 8 Bray-1970

Figure 7. Sea-level changes related to glacial and inter-
glacial stages.. (A) Generalized Pleistocene sea-level variations
and associated erosional and depositional episodes. (B)
1@@ @@N <A LEVEL
@TE@ IN NGAMO IN
@A
PEORIAN rERG A AL
or N IN RGLACIAL LC
LZ@E STAGE STAGE

and '..' a
er @1,60941)@d@.. @0961)

Generalized sea-level changes during Late Wisconsin glaciation.
(C) Proposed sea-level changes during the last 20,000 years
(after Fisher and others, 1973).

Pleistocene History characterized by several advances and retreats of
the continental ice sheet and fluctuations in sea
Alternate deposition and erosion occurred., level. During one of the advances and associated
along the coastal plain in. response to waxing and lowering of sea level, a soil was developed on the
waning of continental glaciers (fig. 7). With the Sangamon delta plain. The following Irise in sea
growth of glaciers, there was a lowering of sea level flooded part of the area covered by this soil.
level, and streams were entrenched across the During subsequent stillstand, sand delivered to the
coastal plain and continental shelf. Melting glaciers Gulf of Mexico by several local streams con-
returned water to the sea creating.a rise in sea level structed a. strandplain in the area of Port O'Connor
and renewed sedimentation in the area of the (McGowen and others, 1972; Wilkinson, 1974;
present outer coastal plain (outer coastal plain is Wilkinson and others, in press). Where observed in
the general area extending inland from the Gulf of outcrop and by subsurface methods, the strand-
-Mexico a distance of 50 miles). The sediments plain sand rests on the soil horizon.
,exposed within the outer coastal plain were
deposited during one Pleistocene interglacial stage During the late Wisconsin, sea level was
and one interstadial stage, probably during the lowered approximately 390 feet (Curray, 1960) to
Sangamon Interglacial Stage and a Wisconsin Inter- 450 feet (LeBlanc and Hodgson, 1959). According
stadial Stage. to Curray, shoreline position at that time was at or
near the edge of the continental shelf. Streams
During the Sangamon high stand of sea level, entrenched their courses across the coastal plain
most of the outer coastal plain, extending from the and continental shelf in response to a changing
bay shore inland, was constructed by sediment base level.
delivered to the area by the ancestral Brazos- Holocene History
Colorado and 'San Antonio-Guadalupe Rivers. The
coastal plain was built seaward into the Gulf of Sea level began to rise approximately 18,000
Mexico by prograding. deltas. The Pleistocene delta years B.P. (fig. 7). This rise marks the beginning of
lyingto the east.of the Modem Lavaca River.was
the Holocene Epoch. Several temporary stillstands
constructed by the ancestral . Brazos-Colorado during Holocene sea-level rise produced barrier
Rivers, and the delta to the west -is the Pleistocene
islands and lagoons on the continental shelf similar
San Antonio-Guadalupe delta. The. slope of the
to Modem barriers and lagoons of the Texas Coast
coastal plain surface (fig. 3) reflects the overlap of (Frazier, 1974). The sea reworked these deposits as
these two major deltaic systems.
it resumed. its landward migration with rising sea
Sangamon deposits consist of fluvial and level. River valleys were filled with sediment during
this Holocene transgression across the continental
distributary gravel and sand, interdistributary and shelf.
overbank mud, and bay-estuarine mud and shell. In
some outcrops along the shores of the Matagorda Erosion associated with Wisconsin low stand
Bay system, red to brown bay mud containing of sea level produced numerous valleys in the
oyster reefs is overlain by thin progradational Matagorda 'Bay area. The largest of these valleys
deltaic sequences. Deltaic muds and sands are was scoured by the Lavaca-Navidad fluvial system.
chiefly red or brown. Commonly, both mud and Smaller systems, such as Tres Palacio's and
sand have been extensively calichified. Distributary Carancahua Creeks, were probably tributary to the
channel filt is coarse-grained silt and very fine- Lavaca-Navidad Rivers. Data from subbottom
grained sand. Channel-fill fluvial sand and gravel profiling and coring indicate that the depth .of the
were recognized in only one bay-shore outcrop. It Lavaca-Navidad valley ranged from about 100 feet
consists of a lower granule to small pebble gravel near the head of the Modem Lavaca Bay to at least
unit (containing abundant vertebrate remains) 125 feet in the vicinity of Port O'Connor. Water
which grades upward into fine-grained sand. Sedi-@ from the Gulf of Mexico first invaded the Lavaca-
mentary structures are well preserved in the fluvial Navidad estuary 11,000 to 10,500 years B.P. These
deposit, but structures have been obliterated in dates were derived from Frazier's (1974) sea-level
adjacent Pleistocene units.by caliche replacement. data.

The Wisconsin Glacial Stage which began A relict shorelne at 45 to 60 feet below sea
about 130,000 years B.P. (Cooke, 1973) was level was reported by Frazier (1974) to have

developed between 10,000 and 7,500 years B.P. Rivers was put into the longshore drift system; at
During this time interval, water from the Gulf of this time Matagorda Peninsula began to develop.
Me xico probably exerted an influence on the The Brazos and Colorado bayhead deltas were
Lavaca-Navidad estuary as far north as the present characterized by distributary sands and inter-
head of Lavaca Bay. distributary muds, but deltas constructed by these
same rivers in the open Gulf of Mexico were similar
Transgression was resumed at about 7,500 to the present Brazos delta; they were high-
years B.P. Sea level reached its present position destructional, wave-dominated deltas (Scott and
between 3,000 and 2,500 years B.P. (Curray, 1960; Fisher, 1969, p. 11-29).
Nelson and Bray, 1970; Frazier, 1974). During the
latter phases of Holocene transgression, estuaries While the Brazos and Colorado Rivers were
were being filled by a sequence of fluvial and filling their estuary, the north and west shores of
estuarine sediments, and parts of the Pleistocene Matagorda Bay were probably open to the Gulf of
strandplain sand were being reworked and trans- Mexico. At this time,. mudflats and marshes were
ported landward to form nuclei for Matagorda .developing in the area of Lake Austin and shell
Island (Wilkinson, 1973). beaches were being constructed along most of the
remaining shoreline. West of Pass Cavallo, incipient
Modem History islands had coalesced to form Matagorda Island.

Modem history of the Matagorda Bay area
dates from stillstand, 3,000 to 2,500 years B.P., to Subsequent to the filling of the Brazos-
the present. Development of the Modem shoreline Colorado estuary, sand was transported toward the
may be divided into prehistoric and historic cate- southwest by persistent longshore drift. Matagorda
gories. Prehistoric development is based chiefly on Peninsula was constructed by spit accretion across
interpretation of field data, and documentation, of the . bay and estuarine muds. Growth of the
peninsula eventually separated Matagorda Bay
historic development is from both published from the Gulf of Mexico. Since Matagorda Penin-
records and field observations. sula has been erosional throughout much of its
history, it does not display its original accretionary
Prehistoric Development grain.
Following stillstand, rivers. began to.fill their
estuaries by progradation of bayhead deltas. Sedi- Approximately 1,000 years ago, the lower
ment derived from several major fluvial systems part of the Colorado River, known today as Caney
directly influenced the I development of shoreline Creek, was captured in the area between Wharton
features in the Matagorda Bay area. Among these and Columbus by a headward-eroding stream.
are the Brazos, Colorado, and Lavaca-Navidad After its capture, the Colorado began discharging
Rivers, and Garcitas Creek. into Matagorda Bay in the vicinity of the small
town of Matagorda.
At stillstand, the Brazos and Colorado Rivers
weredischarging into a common estuary which had Historic Development
an estimated average depth of 25 feet, a width of
30 miles, and a length (measured from the Gulf Configuration of Gulf and mainland shore-
shoreline to bayhead) of 22 miles. With the use of lines has not changed significantly since the first
sediment volume data for theBrazos and Colorado reliable coastal charts were produced in
Rivers published mi the Nineteenth Report. of 185.6-1859. Some local changes have resulted from
Texas Board of Water Engineers (1950, p. 161), it man's activities. Accretion of the north shore of
is apparent that the Brazos. and Colorado Rivers Matagorda Bay (between Lake Austin and Oyster
could have filled their estuary in 1,200 years. While Lake) and the north shore of Espiritu Santo Bay
these rivers were filling their estuary, and before resulted from dredging the Intracoastal Canal.
Matagorda Peninsula was constructed, much of the Construction of Matagorda Ship Channel jetties has
suspension load delivered to the Gulf of Mexico by produced changes along the Gulf shoreline. Sand
these streams was transported into Matagorda and that is transported to the southwest by longshore
Lavaca Bays. Upon reaching the Gulf of Mexico, currents is trapped along the north jetty, thereby
bed-load material from the Brazos and Colorado accreting the shoreline. Since most of the sand is

trapped by the north jetty, the Gulf shoreline lying the town of Matagorda. A large volume of sedi-
between the south jetty and Pass Cavallo conse- ment had accumulated in the river because of the
quently is eroding. greatly reduced flow. Upon release of the logjam,
sediment was rapidly transported to the bay
Perhaps the most obvious change in the creating a delta that prograded completely across
Matagorda Bay area was the growth of the Matagorda Bay. In 1936, a channel was dredged
Colorado delta, during 1929-1935, from a small through Matagorda Peninsula, and the Colorado
45-acre delta to a complex delta of almost 5,000 River began to discharge into the Gulf of Mexico.
acres. Rapid deltation resulted from removal of a Tiger Island Channel was dredged from the river to
log jam from the river in 1929 (Wadsworth, 1941, west Matagorda Bay in the early 1950's. Since then
1966). The log jam extended inland 46 miles from a small delta lobe has developed in Matagorda Bay.

HISTORICAL SHORELINE MONITORING

GENERAL METHODS AND PROCEDURES USED BY THE
BUREAU OF ECONOMIC GEOLOGY

Definition (1:24,000 or 1 inch 2,000 feet) are used for this
purpose. Topographic charts and aerial photo-
Historical Shoreline Monitoring is the docu- graphs are either enlarged or reduced to the precise
mentation of direction and magnitude of shoreline scale of the topographic maps. Shorelines shown
change through specific time periods using accurate on topographic charts and sediment.-water interface
vintage charts, maps, and aerial photographs. mapped directly on sequential aerial photographs
are transferred from the topographic charts and
Sources of Data aerial photographs onto the common base map
mechanically with a reducing pantograph or opti-
Basic data used to determine changes in cally with a Saltzman projector. Lines transferred
shoreline position are near-vertical aerial photo- to the common base map are compared directly
graphs and mosaics .and topographic charts. and measurements are made to quantify any
Accurate topographic charts dating from 1850, changes in position with time.
available through the Department of Commerce,
National Oceanic and Atmospheric Administration Factors Affecting Accuracy of Data
(NOAA), were mapped by the U. S. Coast Survey
using plane table procedures. Reproductions of Documentation of long-term changes from
originals are used to establish shoreline position available records, referred to in this report as
(rnean high water) prior to the early 1930's. Aerial historical monitoring, involves repetitive sequential
photography supplemented and later replaced mapping of shoreline position using coastal charts
regional topographic surveys in the early 1930's; (topographic surveys) and aerial photographs. This
therefore, subsequent shoreline positions are is in contrast to short-term monitoring which
mapped on individual stereographic photographs employs beach profile measurements and/or the
and aerial photographic mosaics representing a mapping of shoreline position on recent aerial
diversity of scales and vintages. These photographs photographs only. There are advantages and disad-
show shoreline position based on the sediment- vantages inherent in both techniques.
water interface at the time the photographs were
taken. Long-term historical monitoring reveals trends
which provide the basis for projection of future
Procedure changes, but the incorporation of coastal charts
dating from the 1850's introduces some uncer-
The key to comparison of various data needed tainty as to the precision of the data. In contrast,
to monitor shoreline variations is agreement in short-term monitoring can be extremely precise.
scale and adjustment of the data to'the projection However, the inability to recognize and differ-
of the selected map base; U. S. Geological Survey entiate long-term trends from short-term changes is
7.5-minute quadrangle topographic maps a decided disadvantage. Short-term monitoring also

requires a . network of stationary, permanent tant variations in mapping precision. The sediment-
markers which are periodically reoccupied because water interface can be mapped with greater preci-
they serve as a common point from which future sion on larger scale photographs, whereas the same
beach profiles are made. Such a network of boundary cqn be delineated with less precision on
permanent markers and measurements has not smaller scale photographs. Stated another way, the.
been established along the Texas Coast and even if line delineating the sediment-water interface repre-
a network was established, it would take consider- sents less horizontal distance on larger scale photo-
able time (20 to 30 years) before sufficient data graphs than a line of equal width delineating the
were available for determination of long-term same boundary on smaller scale photographs.
trends. Aerial photographs of a scale less than that of the
topographic base map used for compilation create
Because the purpose of shoreline monitoring an added problem of imprecision because the
is to document past changes in shoreline position mapped line increases in width when a photograph
and to provide basis for the projection of future is enlarged optically to match the scale of the base
changes, the method of long-term historical moni- map. In contrast, the mapped line decreases in
toring is preferred. width when a photograph is reduced optically to
match the scale of the base map. Furthermore,
Original Data shorelines mechanically adjusted by pantograph
methods to match the scale of the base map do not
Topographic surveys.-Some inherent error change in width. Fortunately, photographs with a
probably exists in the original topographic surveys scale equal to or larger than the topographic map
conducted by the U. S. Coast Survey [U. S. Coast base can generally be utilized.
and Geodetic Survey, now called National Ocean
Survey]. Shalowitz (1964, p. 81) states ". . . the Optical aberration causes the margins of
degree of accuracy of the early surveys depends on photographs to be somewhat distorted and shore-
many factors, among which are the purpose of the lines mapped on photographic margins may be a
survey, the scale and date of the survey, the source of error in determining shoreline position.
standards for survey work then in use, the relative However, only the central portion of the photo-
importance of the area surveyed, and the ability graphs are used for mapping purposes, and
and care which-the individual surveyor brought to distances between fixed points are adjusted to the
his , task." Although it is neither possible nor 7.5-minute topographic base.
practical.to comment on all of these factors, much
less attempt to quantify the error they represent, Meteorological conditions prior to and at the
in general the accuracy of a particular survey is time of photography also have a bearing on the
related to its date; recent surveys are more accurate accuracy of the documented shoreline changes. For
than older surveys. Error can also be introduced by example, deviations from normal astronomical
physical changes in material on which the original tides caused by barometric pressure, wind velocity
data appear. Distortions, such as scale changes and direction, and attendant wave activity may
from expansion and contraction of the base introduce errors, the significance of which depends
material, caused by reproduction and changes in on the, magnitude of the measured change. Most
atmospheric conditions, can be corrected by photographic flights -are executed during calm
cartographic techniques. Location of mean high weather conditions, thus eliminating most of the
water is also subject to error. Shalowitz, (1964, effect of abnormal meteorological conditions.
p. 175) states ". . . location of the high-water line
on the early surveys is within a maximum error of Interpretation of Photographs
10 meters and may possibly be much more
accurate than this." Another factor that may contribute to error
in determining rates of shoreline change is the
Aerial photographs.-Error introduced by use ability of the scientist to interpret correctly what
of aerial photographs is related to variation in scale he sees on the photographs. The most qualified
and resolution, and to optical aberrations. aerial photograph mappers . are those who have
made the most observations on the ground. Some
Use, of aerial photographs of various scales older aerial photographs may. be of poor quality,
introduces variations in resolution with concomi- especially along the shorelines. On a few photo-

graphs, both the beach and swash zone are bright Aerial photographs.-Accuracy of aerial pho-
white (albedo effect) and cannot be precisely tograph mosaics is similar to topographic charts in
differentiated; the shoreline is projected through that quality is related to vintage; more recent
these areas, and therefore, some error may be mosaics are more accurate. Photograph negative
introduced. In general, these difficulties are quality, optical resolution, and techniques of com-
resolved through an understanding of coastal piling controlled mosaics have improved with time;
processes and a thorough knowledge of factors that thus, more adjustments are necessary when work-
may affect the appearance of shorelines on ing with older photographs.
photographs.
Cartographic procedures may introduce minor
Use of mean high-water line on topographic errors associated with, the transfer of shoreline
charts and the sediment-water interface on aerial position from aerial photographs and topographic
photographs to define the same boundary is charts to the base map. Cartographic procedures do
inconsistent because normally the sediment-water not increase the accuracy of mapping; however,
interface falls somewhere between high and low they tend to correct the photogrammetric errors
tide. Horizontal displacement of the shoreline inherent in the original materials such as distor-
mapped using the sediment-water interface is tions and optical aberrations.
almost always seaward of the mean high-water line.
This displacement is dependent on the tide cycle, Measurements and Calculated Rates
slope of the beach, and wind direction when the
photograph was taken. The combination of factors Actual measurements of linear distances on
on the Gulf shoreline which yield the greatest maps can be made to one-hundredth of an inch
horizontal displacement of the sediment-water which corresponds to 20 feet on maps with a scale
interface from mean high water are low tide of 1 inch = 2,000 feet (1:24,000). This is more
conditions, low beach profile, and strong northerly precise than the significance of the data warrants.
winds. Field measurements indicate that along the However, problems do arise when rates of change
Texas Gulf Coast, maximum horizontal displace- are calculated because: (1) time intervals between
ment of a photographed shoreline from mean photographic coverage are not equal; (2) erosion or
high-water level is approximately 125 feet under accretion is assumed constant over the entire time
2
these same conditions. Because the displacement of period; and (3) multiple rates (--'@-Fn, where n repre-
the photographed shoreline is almost always sents the number of mapped shorelines) can be
seaward of mean high water, shoreline changes obtained at any given point using various combina-
determined from comparison of mean high-water tions of lines.
line and sediment-water interface will slightly
underestimate rates of erosion or slightly over- The beach area is dynamic and changes of
estimate rates of accretion. varying magnitude occur continuously. Each
photograph represents a sample in the continuum
Cartographic Procedure of shoreline changes and it follows that measure-
ments of shoreline changes taken over short time
Topographic charts.-The topographic charts intervals would more closely approximate the
are replete with a 1-minute-interval grid; transfer of continuum of changes because the procedure
the shoreline position from topographic charts to would approach continuous monitoring. Thus, the
the base map is accomplished by construction of a problems listed above are interrelated, and solu-
1-minute-interval grid on the 7.5-minute topo- tions require the averaging of rates of change for
graphic base map and projection of the chart onto discrete intervals. Numerical ranges and graphic
the base map. Routine adjustments are made across displays are used to present the calculated rates of
the map with the aid of the 1-minute-interval shoreline change.
latitude and longitude cells. This is necessary
because: (1) chart scale is larger than base map Where possible, dates when individual photo-
scale; (2) distortions. (expansion and contraction) graphs actually were taken are used to determine
in the medium (paper or cloth) of the original the time interval needed to calculate rates, rather
survey and reproduced chart, previously discussed, than the general date printed on the mosaic.
require adjustment; and (3) paucity of culture Particular attention is also paid to the month, as
along the shore provides limited horizontal control. well 'as year of photography; this eliminates an

apparent age difference of one year between cally all based on a geodetic network which
photographs taken in December and January of the minimized the possibility of large errors being
following year. introduced. They therefore represent the best
evidence available of the condition of our
Justification of Method and Limitations coastline a hundred or more years ago, and the
courts have repeatedly recognized their com-
The methods used in long-term historical petency in this respect . . .
monitoring carry a degree of imprecision, and
trends and rates of shoreline changes determined Because of the importance of documenting
from these techniques have limitations. Rates of changes over a long time interval, topographic
change are to some degree subordinate in accuracy charts and aerial photographs have been used to
to trends or direction of change; however, there is study beach erosion in other areas. For example,
no doubt about the significance of the trends of Morgan and Larimore (1957), Harris and Jones
shoreline change documented over more than 100 (1964), El-Ashry and Wanless (1968), Bryant and
years. An important factor in evaluating shoreline McCann (1973), and Stapor (1973) have success-
changes is the total length of time represented by fully used techniques similar to those employed
observational data. Observations over a short herein. Previous'articles describing determinations
period of time may produce erroneous conclusions of beach changes from aerial photographs were
about the long-term change in.coastal morphology. reviewed by Stafford (1971) and Stafford and
For example, it is well established that landward others (1973).
re treat of the shoreline during a storm is accom-
panied by sediment removal; the sediment is Simply stated, the method of using topo-
eroded, transported, and temporarily stored off- graphic charts and aerial photographs, though not
shore. Shortly after storm passage, the normal absolutely precise, represents the best method
beach processes again become operative and some available for investigating long-term trends in
of the sediment is returned to the beach. If the shoreline changes.
shoreline is monitored during this recovery period,
data would indicate beach accretion; however, if
the beach does not accrete to its prestorm position, Limitations of the method require that
then net effect of the storm is beach erosion. emphasis be placed first on trend of shoreline
Therefore, long-term trends are superior to short- changes with rates of change being secondary.
term observations. Establishment of long-term Although rates of change from map measurements
trends based on changes in shoreline position can be calculated to a precision well beyond the
necessitates the use of older and less precise limits of accuracy of the procedure, they are most
topographic surveys. The applicability of topo- important as relative values-, that is, do the data
graphic surveys for these purposes is discussed by indicate that erosion is occurring at a few feet per
Shalowitz (1964, p. 79) who stated: year or at significantly higher rates. Because
sequential shoreline positions are seldom exactly
"There is probably little doubt but that parallel, in some instances it is best to provide a
the earliest records of changes in our coastline range of values such as 10 to 15 feet per year. As
that are on a large enough scale and in long as users realize and understand the limitations
sufficient detail to justify their use for quanti- of the method of historical monitoring, results of
tative study are those made by the Coast
Survey. These surveys were executed by com- sequential. shoreline mapping are significant and
petent and careful engineers and were practi- useful in coastal zone planning and development.

VINTAGE MATERIALS USED IN THE MATAGORDA BAY AREA STUDY

Two types of data were collected to docu- approximately 1:24,000. Other data used were
ment direction and rates of shoreline change: (1) U. S. Geological Survey topographic maps
historical data from charts, maps, and aerial photo- (1946-1947) and two vintages of Tobin aerial
graphs; and (2) field data. Historical monitoring photomosaics (1934-1937 and 1956-1957), also at
was accomplished for the years 1856-1859, a scale of 1:24,000. A set of U. S. Department of
1934-1937, 1946-1947, 1952-1953, and Agriculture stereophoto graphic pairs (1952-1953),
1956-1957, specifically to document long-term at a scale of 1:10,000, completes the list of vintage,
changes. Field observations and measurements materials used for compiling the Matagorda Bay
were made to document changes that occurred shoreline change map. All charts, maps, and
between 1957 and 1971-72.
photos, with exception of the stereophotos, were
Baseline data for this study are U. S. Coast at approximately the same scale. The base on
Survey charts for the years 1856-59. These charts which the vintage data were compiled was at a
were reproduced photographically to a scale of scale of 1:24,000.

PRESENTATION OF DATA IN MAP FORM

Each of the five vintages of Gulf and main- have an overall erosional trend, it may display
land shorelines is color coded and presented on a short-term accretionary trends; these trends will be
series of accurate base maps. Measurements of depicted by the graphs.
distances between the oldest and youngest shore- Long-term erosional and accretionary shore-
lines are adequate to show long-term shoreline lines are highlighted on the maps by color, red for
trends. Intermediate shorelines may not all show erosion and green for accretion. Width of the color
the same trend (erosion or accretion). To aid in bands are indicative of the amount of erosion or
reading the shoreline map, graphs were prepared at accretion experienced by a particular shoreline
selected intervals along the shoreline. The segment. The 100-year shoreline change can be
1856-1859 shoreline serves as the 'base line for determined by measuring distances between the
these graphs. Although a shoreline segment may color-coded, vintage shorelines.

EFFECT OF ASTRONOMICAL TIDE ON POSITION OF WATERLINES
ON VINTAGE AERIAL PHOTOGRAPHS

In general, differences in position of mean degrees and shell beaches 3.9 degrees. Slopes of
high-water line, as shown on topographic charts, Gulf beaches are roughly bimodal with 44 percent
and waterline, as mapped on aerial photographs, occurring in the 2- to 3-degree range and 47
were discussed in the section on Interpretation of percent in the 4- to 5-degree range. There are three
Photographs. Slopes of Gulf and mainland beaches types of mainland beaches: (1) terrigenous sand;
were measured in the Matagorda Bay area for the (2) shell gravel; and (3) caliche and shell gravel
purpose of showing the effect of tides on historical veneer over Pleistocene mud. Slopes of mainland
monitoring. beaches range from less than 1 degree to 11 degrees
(fig. 8B). Sand beaches average 5.25 degrees, shell
Slopes of Gulf and mainland beaches were beaches 6.3 degrees, and caliche and shell gravel
beaches have average slopes of 5.4 degrees. Main-
measured and the horizontal distance between land beaches are steeper than Gulf beaches; 37
flood and ebb tide was determined. Gulf beaches percent of the mainland beaches occur in the 3- to
consist of sand and shell with slopes of 1.5 to 6.0 5-degree range and 50 percent range from 6 to 7.5
degrees (fig. 8A). Sand beaches average 2.75 degrees.

Assuming a 2-foot mean tidal range along the slopes, and associated horizontal distances between
Gulf shoreline, the horizontal distance between ebb and flood tide positions are 3- to 5-degree
flood and ebb tide along the 2- to 3-degree beaches beaches and 6- to 7.5-degree beaches. Horizontal
would be 60 and 40 feet, respectively, and the distance between ebb and flood tide is 8 and 7
horizontal distance along, the 4- to 5-degree feet, respectively. These differences would not be
beaches would be 30 and 23 feet, respectively. detected on aerial photographs.
Accuracy of measurement at the 1:24,000 scale
used in the historical monitoring program is on the Parts of the mainland shoreline are almost
order of �40 feet. The difference between ebb and vertical cliffs. Here the horizontal difference be-
flood tide would have an effect on waterline tween flood and ebb tide is insignificant. Low-lying
position, indicated on aerial photos, where beaches areas, such as marshes, are inundated by astro-
slope 2 degrees or less, but the difference probably nomical and wind tides. Extent of inundation may
would not be detected on beaches with higher not be discernible on conventional black-and-white
slopes. aerial photographs because marsh plants tend to
obscure the tidal waters. Regardless of whether the
Range of astronomical tide in Matagorda Bay tidal cycle is ebb or flood, however, the bay margin
is on the order of 0.5 to 0.7 foot. A 1-foot tidal of the salt marsh is commonly defined by Spartina
range was used for determination of the horizontal alterniflora. Height of S. alterniflora is greater than
distances between ebb and flood tide along main- the tidal range and is not inundated by the flood
land beaches. Two dominant beach classes, their tide.

GULF AND MAINLAND SHORELINE CHANGES, 1856-1957

The purpose of this section on historical Gulf Shoreline
monitoring is to document the direction and rate
of long-term shoreline change and to present some The Gulf shoreline has been chiefly erosional
of the probable causes of change. Elaboration on since 1856. Minor accretion occurred between
the interaction of coastal processes and shoreline 1946 and 1956, and between 1952 and 195,6 (see
stability is deferred, however, until short-term fluctuation graphs for stations 1 through 6).
shoreline changes are considered. Maximum erosion (1856-1956) of 1,580 feet was
recorded at station 3. Least amount of erosion was
Long-term trends of Gulf and mainland shore- 880 feet at station 6. Yearly erosional average was
lines are presented on eight maps, each at a scale of about 11 feet, with total land loss for this 100-year
1:24,000 (in pocket). Four of the maps display period being about 1,575 acres. This is the most
only the mainland shoreline. These are-Lavaca rapidly eroding shoreline segment in the study
Bay South, Lavaca Bay North, Carancahua Bay, *area. Several factors contribute to rapid shoreline
and Tres Palacios Bay areas (fig. 9). Both Gulf and retreat. These are: (1) Matagorda Peninsula is a
mainland shorelines occur on the remaining maps-- thin sand body; (2) there is a sand deficit; (3)
Brown Cedar Cut, Colorado River, Shell Island waves approach the shoreline at a high angle; (4)
Reef, and Pass Cavallo areas. In the following eustatic rise of sea level has increased over the past
discussion, shoreline trends are presented for each 50 years (K. 0. Emery, personal communication);
map area. (5) the low peninsula is frequently washed over
during storms; and (6) there is compactional
Brown Cedar Cut Area subsidence of underlying deltaic muds.

Salient features of this area are: (1) Mata- Bay Shoreline of Matagorda Peninsula
gorda Peninsula, which is tied at its.east end to a
deltaic headland; (2) an inactive tidal delta (near With the exception of the two tidal pass areas,
station 13); (3) an active tidal delta (Brown Cedar the bay shoreline has been chiefly erosional. Land
Cut); and (4) east Matagorda Bay. area accreted to this shoreline amounts to about

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Figure 8. Slopes of Gulf and mainland beaches. Measurements are representative of slopes lying between the lower berm
and the toe of the forebeach.

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Figure 9. Index of shoreline change and marsh distribution maps.

424 acres, whereas land loss was about 298 acres. trend since 1934. Accretion ranged from a maxi-
Most of the accretion, 337 acres, occurred in the mum of 400 feet at station 14 to a minimum of 80
tidal pass areas. West of Brown Cedar Cut, approxi- feet at station 21. Maximum erosion of 760 feet
mately 210 acres of land were lost through erosion. was measured at.station 19, and least erosion was
Maximum erosion of 600 feet was recorded at recorded at station 22. In 1856, Dressing Point was
station 8, and least erosion, 240 feet, occurred in tied to the mainland. Net land loss of about 501
the area of station 12. acres occurred along the mainland shoreline.

Erosion of Matagorda Peninsula bay shore Colorado River Area
results from waves generated by north winds.
Gulf and bay shorelines of Matagorda Penin-
Mainland Shoreline sula have been chiefly erosional since 1856. Main-
land shoreline was mostly erosional until a logjam
Mainland shoreline, the area between stations was removed from the Colorado River and the
14 and 22, was primarily erosional during the Intracoastal Canal was dredged. Following the
interval 1856-1957. Shoreline fluctuation graphs removal of the log jam in 1929, a delta rapidly
indicate short-term accretionary periods since prograded across Matagorda Bay. To the east of the
1934. Accretion resulted from spoil outwash. delta, the mainland shoreline accreted by spoil
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Stations 14, 18, and 21 all display an accretionary outwash.

Gulf Shoreline (12.5 square miles).. Except for the Tiger Island
Gulf shoreline erosion decreases westward. Channel area, the delta is depositionally inactive.
This shoreline segment experienced short-term Shell Island Reef Area
accretion (sections 2-6). Station 1 has shown
overall net accretion for the 100-year period. Both Matagorda Bay and Matagorda Peninsula
Maximum shoreline retreat of 300 feet was widen somewhat to the west. Long oyster reefs
recorded at station 4, and the minimum of 80 feet extendat right angles from the mainland shoreline
was recorded, at station 2. Net land loss of 245 into west Matagorda Bay. Matagorda. Peninsula is
acres was recorded for this shoreline segment. divided -into two sections on this map; these are
treated as a single unit in the following discussion.
The westward decrease in erosion and slight
accretion between Spring Bayou and the mouth of Gulf Shoreline
the Colorado River result in. part from low,
continuous, fore-island dunes that prevent sand During the period 1856-1957, the shoreline
transport across the peninsula into Matagorda Bay. east of . Greens Bayou was mostly erosional,'where-
as to the west of Greens Bayou,, the shoreline was
Bay Shoreline of Matagorda Peninsula accretionary. Maximum accretion of 260 feet was
The bay shoreline was erosional during the recorded at, station 4, and minimum accretion of
historical monitoring period. Three stations (9, 10, 40 feet occurred at station 3. Maximum erosion of
500 feet occurred atstation 11, and the minimum
and 11) show insignificant short-term accretion.
of 150 feet was recorded at station 13. Net land
Accretion. near TigerIsland and Greek Island is gain.of 121, acres Io.ccu.rred to the west of. Greens
related to Colorado River deltation. Maximum T
erosion of about 560 feet was recorded at station 7 Bayou, and land loss of about 652 acres was
and a minimum of 120 feet at station 11. calculated for the shoreline between stations 3 and
11.
Minor accretion was shown as small beaches Bay Shoreline of Matagorda Peninsula
in embayed areas and as spits downcurrent from
erosional segments. Net land loss was about 532
Erosion exceeded accretion along the. bay
acres.
shoreline during the 1 00-year period. Land loss was
Mainland Shoreline approximately 1,807 acres.
With the exception of the area of station,16, The bay shoreline between stations 8 and 21
the mainland shoreline is accretionary. The is highly irregular. The serrated bay shoreline
shoreline segment bounded by stations 13 and 16 results from. the scouring of storm channels. Storm
is regarded as, mainland shoreline;.the I .arge accre- breaching is common east of Greens Bayou, but is
tionary area. to the west is the Colorado delta. All uncommon west of Greens Bayou because of the
of the mainland shoreli InIe, except the station 16 continuous fore-island dunes.
area, is accretionary. Bay-shore accretion. occurs during hurricanes.
Most of. the accretion occurred after 1934. Sediment . is transported across the peninsula
Accretion values range from 1,050 feet at station through storm channels; it accumulates in the bay
15 to 1,680 feet at station 13. Spoil outwash is the as small islands (station 21). Extremeerosion takes
cause of accretion. place during storms,, at which time the peninsula
may -be segmented. At station 20, about 2,520 feet
Net land gain was about 675 acres. Part of the of the peninsula was eroded by storms. Minimum
sediment which accreted this shoreline was derived erosion of 120 feet (1856-1957) was recorded at
from the Co lorado. River. station 6.

Deltaic Shoreline Mainland Shoreline

In 1957, the subaerial. Colorado delta, which The mainland shoreline has been accreting
was chiefly marsh, covered an area of 8,000 acres since 1934. West of station 27, the shoreline,

consisting chiefly of shell beaches and berms, was Matagorda Island was erosional between sta-
erosional. Shell beaches and berms also form the tions 2 and 6, and accretionary west of station 2.
shoreline east of station 27. Prior to removal of the Maximum erosion of 2,200 feet was recorded at
Colorado River log jam and dredging of the station 3, and maximum accretion of 620 feet at
Intracoastal Canal, shell beaches also existed west station 1. Net land loss between stations 2 and 6
of station 27. Now the shoreline west of station 27 was about 816 acres, and net land gain in the area
is composed of spoil outwash. of station 1 was about 47 acres.

Accretion began after 1934 (fluctuation Bay Shoreline-Matagorda Peninsula and Matagorda Island
graphs 22, 23, 24, and 25); rates of accretion
increase from west to east. Land gain from spoil The bay shoreline of Matagorda Peninsula
outwash was about 1,079 acres. Land loss of about begins at Decros Point and extends to the east map
80 acres occurred in the area of stations 26 and 28. limit, and the bay shoreline for Matagorda Island
A net. gain of about 1,000 acres was experienced lies between the south bank of Saluria Bayou and
by the mainland shoreline during the interval the west map limit.
1886-1957.
Approximately 2,200 feet of accretion was There are a few local accretion areas along the
recorded at station 22, and minimum accretion of bay shore of Matagorda Peninsula, but principal
direction of change has been erosional. Net land
150 feet occurred at station 25. Maximum erosion loss was about 501 acres.
of 330 feet occurred in the area of station 26; the
minimum was 160 feet at station 28. Areas of erosion alternate with accretion
Oyster Lake along the bay shore of Matagorda Island with
erosion occurring in the area of stations 12 and 13,
Only the southern accretionary shore Of and accretion occurring between Lighthouse Cove
Oyster Lake is shown on this map. Here, spoil an .d Saluria Bayou. The tidal delta was the
outwash has created about 97 acres of new land. pnncipal site of sedimentation. Net land gain was
about 123 acres.
Pass Cavallo Area Mainland Shoreline
Pass Cavallo, a major tidal pass, separates the There are two mainland shoreline segments on
erosional Matagorda Peninsula from Matagorda this map. One is sit .uated along the west shore of
Island. This is the only major pass on the Texas Matagorda Bay between Saluria Bayou and the
Coast that has not been physically altered by man. northwest map limit. The other is along the north
A ship channel was dredged through Matagorda shore of Espiritu Santo Bay between Port
Peninsula in 1965, and since then Pass Cavallo has
begun to shoal. The vintage shorelines displayed on O'Connor and the west map boundary.
the Pass Cavallo area map all predate dredging of . The west shoreline of Matagorda Bay was
the ship channel. dominated by erosion. Maximum erosion of 1,200
Gulf Shoreline feet and a minimum of 370 feet were measured at
stations 7 and 10, respectively. Net land loss was
The Gulf shoreline is defined by the areas about 770 acres. Rapid erosion was documented
lying between' station 23 and Decros Point, and between Port O'Connor and the north map bound-
Saluria Bayou and station 1. Erosion exceeded ary. Here, relatively nonresistant Pleistocene sand
accretion during the period of historical moni- is subjected to wave erosion. Erosion of the
toring, and local changes in shoreline stability were shoreline south of Port O'Connor has also been
effected by the Matagorda Ship Channel jetties.. rapid. Here, almost pure sand beaches are attacked
by unimpeded waves which approach the area from
The Gulf shoreline of Matagorda Peninsula at the northeast and southeast.
station 23 was in equilibrium from 1856 to 1957.
Shoreline retreat of approximately 1,460 feet was The north shoreline of Espiritu Santo Bay is
recorded at station 25. Net land loss was calculated erosional along Dewberry and Blackberry Islands;
to be about 289 acres. net land loss was 113 acres. Shoreline accretion

ranging from 140 to 190 feet was measured The overall direction of change has been
adjacent to the Intracoastal Canal; net land gain erosional. Indian Point (station 12) exhibited the
was approximately 633 acres. maximum accretion of 360 feet, whereas 200 feet
of shoreline recession was measured at station 17.
Marsh Islands Net land loss for this area was about 49 acres.

Farwell, Grass, and Bayucos Islands are emer- Cox Bay
gent parts of Pass Cavallo tidal delta. The islands
which are bounded by Big Bayou and Barroom Bay The bay shore between stations 18 and 22 is
are also part of the tidal delta. Most of the islands chiefly low-relief vertical cliffs with a few marsh
are vegetated with salt-tolerant plants. Land loss in areas such as the head of Huisache Cove. In this
the areas of Bayucos, Grass, and Farwell Islands area, accretion (58 acres) and erosion (59 acres)
amounted to about 188 acres. Net land gain in the were virtually equal. Between 1856 and 1957, the
Big Bayou--Barroom Bay area was about 81 acres. trend in shoreline change was to erode promon-
tories and deposit sediment in the small reentrants.
Lavaca Bay South Area
This shoreline segment lies in the lee of the
The amount of physical energy (waves, tidal prevailing southeast wind. North wind, however,
and longshore currents) expended along the bay generates waves that break. on the south shore of
shoreline varies from place to place depending Cox Bay.
upon (1) shoreline orientation, (2) width of the
bay, and (3) the extent of bay segmentation Sand Point Area
resulting from spoil islands adjacent to dredged
channels. Wave energy appears to be more intense Erosion dominates this shoreline segment.
along the north and west shores of Matagorda Bay Maximum land loss (116 acres) occurred between
and the west shore of Lavaca Bay. Smaller waves Sand Point and the northeast map limit. Accretion
attack the shores of small, enclosed water bodies was measured in marsh areas along the south shore
such as Powderhorn Lake, Chocolate Bay, and of Keller Bay.
Keller Bay.
A maximum of 500 and a minimum of 20
For convenience of discussion, the shoreline feet of shoreline retreat were recorded at stations
has been divided into five segments: (1) west 25 and 23, respectively. Net land loss was approxi-
Matagorda Bay; (2) west Lavaca Bay; (3) Cox Bay; mately 130 acres.
(4) Sand Point area; and (5) Powderhorn Lake.
Powderhorn Lake
West Matagorda Bay
Powderhorn Lake is a water body with its
This part of the bay shoreline lies between longest dimension oriented transverse to the pre-
station 1 and Indian Point; it is predominantly vailing southeast wind. Because of its orientation,
erosional. The shoreline at stations 2 and 10 small waves from the southeast do not significantly
accreted between 1856 and 1934. An equilibrium erode the north shore; sedimentation exceeds
shoreline existed in the area of station 10. Approx- erosion. The opposite is true for the south shore.
imately 580 feet of shoreline retreat occurred at High-velocity, short-duration north winds generate
station 1. Net land loss was approximately 162 waves that erode the south shoreline. Net.land loss
acres. was approximately 49 acres.

West Lavaca Bay Lavaca Bay North Area

From Indian Point to the north map limit, the The north bay shore receives sed iment from
bay shore is diversified. It is made up of shell Garcitas Creek and Lavaca River. At Port Lavaca
beaches and berms, almost vertical bluffs, shell and in the Mitchell Point area, some shoreline
spits, and marshes. This shoreline segment has not changes have resulted, from man's activities. Many
experienced the dramatic changes that characterize of the man-made changes are directly related to
some of the previously described areas. dredging activities. For this discussion, the shore-

line was divided into four sections based on Calculations made for the shoreline segment south
shoreline orientation: (1) west, shoreline of Lavaca of State Highway 35 indicate that at least 95 acres.
Bay from the map boundary northward to Placedo of bay bottom were covered with spoil.
Creek; (2) north shoreline of Lavaca Bay from
Placedo Creek eastward to station 8; (3) east Cox Bay
shoreline of Lavaca Bay from station 8 to Mitchell
Point; and (4) Cox Bay shoreline from Mitchell Historical shoreline data for the period
Point to the southeast map boundary. 1856-1957 document a net land gain of about 39
acres. Field observations made in the winter of
West Shoreline, Lavaca Bay
1971 and spring of 1972, however, revealed that
Erosion exceeded sedimentation, and net land the shoreline was in an erosional state. Net gain
loss was approximately 162 acres. Noble Point and and loss of land for the period 1856-1972 were
station 13 experienced the most erosion, 1,200 and approximately equal.
600 feet, respectively. Noble Point was a large
marsh area; loss of wetland area was about 128 Carancahua Bay Area
acres.
Within this map area there are: (1) large water
The principal areas of accretion were stations bodies characterized by large waves generated by
14 and 16. Spoil dredged from boat basins, in the prevailing southeast winds; (2) enclosed bays that
vicinity of station 16, created about 24 acres of are elongate transverse to the prevailing wind; and
new land. At least two factors contributed to (3) small, shallow, enclosed water bodies charac-
sedimentation near station 14. These are: (1) a terized by small waves.
concave shoreline; and (2) sediment discharged
into the bay through drainage ditches. For convenience of discussion, the shoreline
North Shoreline, Lavaca Bay was divided into four segments. Grouping of
shoreline segments was made on the basis of
There was a net land gain of about 34 acres relative wave intensity, shoreline orientation, and
along the north shore. Shoreline accretion is degree of enclosure. The segments are (1) the north
attributed to sediment delivered to the bay by shore of Matagorda Bay, including part of Turtle
Placedo Creek, Garcitas Creek, and the Lavaca Bay, (2) Carancahua Bay, (3) Keller Bay, and (4)
River. Erosion is restricted to cliffed shorelines in Salt Lake and Redfish Lake.
the stations 10-12 area. The amount of shoreline
retreat was 720 and 580 feet at stations 12 and 10, North Shore, Matagorda Bay
respectively.
Land loss in these two areas amounts . to The shoreline was erosional during
1856-1957, except for two small accretionary
approximately 138 acres. Approximately 137 acres areag--a shell spit at Well Point, and a salt marsh
of new marsh in Garcitas Cove resulted from between station. 2 and Carancahua Pass.
sedimentation at the mouth of Garcitas Creek.
East Shoreline, Lavaca Bay The banks of Carancahua Pass were highly
erosional. At station 3, shoreline retreat was about
The Lavaca River strongly influences shore- 1,540 feet and about 1,480 feet at station 15.
line stability north of State Highway 35. South of Beaches and berms in the Carancahua Pass area are
the highway, man dominates shoreline activities. composed of 80 to 90 percent shell, and erosion is
There has been a net gain of about 103 acres along attributed to a decrease in shell production within
the east shore. Sediment for shoreline accretion the bay.
was derived from the Lavaca River and from
material dredged from the bay bottom for a This part of the bay shore is fronted by a
turning basin. wide bay and is, therefore, subjected to the forces
of breaking waves generated by southeast winds.
Deltation at the mouth of the Lavaca River Land loss resulting largely from wave activity was
created approximately 96 acres of new land. about 342 acres.

Caraneahua Bay Shoreline derived from Pleistocene uplands and from
Carancahua Bay.
The east shore is. in the lee of the southeast
wind, but is open to waves approaching from the Tres Palacios Bay Area
north. The opposite is true for the west shoreline.
Most of the Tres Palacios Bay shore is
Erosion has dominated the east shore since relatively protected from waves approaching from
1856. Maximum erosion of 280 feet was recorded the southeast. The orientation of spits, which
at station 6; at station 11, directly across the bay, occur between Palacios Point and Oliver Point,
420 feet of shoreline retreat.was measured during indicates that southeast waves generate longshore
the same penod. Small areas of spit and marsh currents which transport sediment northward.
accretion occur between sections 5 and 6. Net land Depositional 'grain preserved as beach ridges along
loss amounts to about 83 acres. the same point suggests that net longshore drift
was to the south in 1856.
The west shore has undergone almost equal
amounts of accretion and erosion; there was a net The shoreline, was divided into the following
land loss of about 29 acres., Accretion occurs segments based upon shoreline orientation and
downdrift. from erosional cliffs (see, section 8) and degree of enclosure of water bodies: (1) Palacios
along concave shoreline segments (between sec- Point to Oliver Point; (2) Oliver Point to the
tions 9 and 11 and 11 and 12). Shoreline.con- mouth of Tres Palacios Creek; (3) mouth of Tres
-figuration is continually - changing in the area Palacios Creek to Turtle Point; (4) Turtle Point to
between sections 12 and 14. Here, shell spits the mouth of Turtle Creek; (5) mouth of Turtle
accrete across entrances to Salt Lake and Redfish Creek to Sartwelle Lakes; and (6) Oyster Lake.
Lake; spits are breached during storms.
Palacios Point-Oliver Point
Keller Bay Shoreline
Between Palacios Point and Oliver Point, the
Both the east and west shores of Keller Bay, shoreline trend was erosional. All. shoreline changes
which are modified by processes identical, to those were natural exc ept for the area just north of the
operating in Carancahua Bay, are chiefly erosional. dredged channel. Sediment accumulated as spits
downcurrent from erosional headlands. Successive
Net land loss along. the east shore was about changes in.size, shape, and orientation of spits. are
47 acres. Up to 260 feet of shoreline retreat was illustrated at Palacios Point and at.stations 2 and 3.,
recorded at station 20, which is near the bayhead. In the winter of 1972, the spit at Palacios Point
Bluffs that front the bay are up to 10 feet high and was attached to the headland at both. its upcurrent
slumping is probably the dominant cause of shore- and downcurrent ends.
line retreat. Sedimentation occurred along two
concave shoreline segments at stations 17 and 19. Approximately 1,040 feet of erosion was
recorded at station 2. Material eroded from station
Approximately 61 acres of land were eroded 2 was moved downcurrent and was deposited at
from the west shore of Keller Bay. In the area of station 3, accreting the shoreline about 520 feet.
station 22, where maximum erosion of 370 feet Although the shoreline between Palacios Bayou
was recorded, there is evidence that.erosion was and Oliver Point is concave, it was also eroded. The
prevalent prior to 1856. shoreline was erosional because waves approaching
from the north struck the area at a high angle. Net
Salt Lake and Redfish Lake land loss was about 199 acres.

Salt and Redfish Lakes were initially parts.of Oliver Point-Tres Palacios Creek
Carancahua Bay. They were cut off from the main
body of water by the accretion of spits. The This segment of the bay.shore lies in the lee
shoreline of Salt Lake accreted approximately 8 of the prevailing southeast wind and, -therefore, it
acres since 1856, but there was a loss of about 28 is not significantly affected by waves generated by
acres along the Redfish Lake. shore. The long-term the southeast winds. However, waves approaching
trend has been for the lakes to fill with sediment from the north do erode the shoreline.

Several accretionary pockets are present along the dominant process along this part of the bay
this predominantly erosional bay shore, e.g., the shore. Deposition was recorded in local reentrants
southeast shore of Coon Island Bay and a small and to the west of Buttermilk Slough where shell
area lying between stations 11 and 12. Accre- beaches and berms are common.
tionary areas are protected by oyster reefs (Coon
Island) and shell spits. Deposition and erosion were approximately
the same between 1856 and 1957. There was a net
Approximately 350 to 440 feet of shoreline land loss of approximately 23 acres.
retreat were recorded at stations 12 and 13,
respectively. Each of these areas is a promontory, Oyster Lake
upon which wave energy is focused. Net land loss
was ap Iproximately 73 acres. Oyster Lake is a small, shallow, tidally influ-
enced water body that is connected to Matagorda
Tres Palacios Creek-Turtle Point Bay through Palacios Bayou and the Intracoastal
Canal.
Shoreline accretion prevailed between the Most of the south shore is accretionary.
mouth of Tres Palacios Creek and Grassy Point, Erosion dominates the other shoreline segments.
whereas erosion dominates the shoreline between Erosion amounting to 150 feet was recorded at
Grassy Point and Turtle Point. Sediment is station 7. Spoil outwash has accreted the south.
supplied to the north shore by river flooding and shore 1,040 to 1,480 feet at stations 5 and 4,
by wind tides produced by the southeast wind. respectively. There was a net land gain of about
Erosion in the Grassy Point-Turtle Point areas 220 acres.
results from a paucity of sand-size sediment
supplied to the area and from relatively large waves Summary
from the southeast. Net land gain for the shoreline
segment between the mouth of Tres Palacios Creek Most of the Gulf and mainland shorelines of
and Grassy Point was about 78 acres.
the Matagorda Bay area were in an erosional phase
from 1856 through 1957. The erosional shoreline
Only one significant accretionary area occurs trend was established prior to any major activities
between Grassy Point and Turtle Point. This is a of man which could have caused a change in
spoil area created by dredging boat harbors at shoreline stability.
Palacios; 'accretion amounts to about 9 acres. Net
land loss for the Grassy Point-Turtle Point shore- Man's activities tend to accelerate Gulf shore
line was about 91 acres. erosion by depleting the sand supply. Sedimenta-
tion has been localized by jetties that trap sand on
Turtle Point-Turtle Creek their upcurrent sides. Erosion is initiated or accel-
erated on- the downcurrent sides of jetties. The
Waves that approach from the north affect principal effect of man's activities in the bay area
this shoreline segment more than waves produced was shoreline accretion.
by southeast wind. This is demonstrated by the
fact that erosion exceeds accretion, and that the Natural accretion of the Gulf shoreline was to
rate of erosion increases as Turtle Bay widens the southwest of Greens Bayou and Pass Cavallo.
westward. Shoreline retreat of 320 feet was Accretion to bay shores occurred at the heads of
recorded at station 20. Net land loss was about 45 bays as bayhead deltas, at the terminus of tidal
acres. channels as flood deltas, and on the back side of
Matagorda Peninsula as washover deposits.
Turtle Creek-Sartwelle Lakes
Appendix A summarizes the shoreline changes
Depositional and erosional shoreline segments (1856-1957) of the Matagorda Bay area in terms of
alternate in this area. Waves from the southeast are acres of land acereted or eroded.

MARSH DISTRIBUTION, 1856-1957

Marshes were mapped on the same, charts, tion, and relative sea-level change.. River diversion
maps, andaerial photographs that were utilized for and impoundment, construction of dams across
shoreline mapping. Five vintages of marsh distribu- tidal creeks, dredging of channels, and creation of
tion could not,be displayed on a single set of maps, spoil mounds are some of man's activities that
and, therefore, long-term changes in marsh area produce change in marsh area.
were determined by comparing the oldest coastal
charts and the youngest aerial photographs. Marsh Change Resulting From Natural Processes
The 19.56-1957 marsh distribution was A decrease in marsh area was recorded along
mapped on Tobin photomosaics (scale .1: 24,000). the bay shore of Matagorda Peninsula. The
Field work (winter 1971 through spring .1972)
verified the photo interpretation and also docu- 1856-1859 marsh was widely distributed, whereas
the 1956-1957 marsh was more restricted in area.
mented the fact that certain marsh areas had been
filled or dammed by man in the interim period of Between 1856 and 1957,. themarsh area decreased
through erosion and deposition. The Gulf shore line
1957-1972. Wetlands were mapped by the U. S.
Coast Survey in 1856-1859. The distinction was was eroded and marsh . deposits were locally
not made exposed,in the swash zone.. Burial of marsh by
. however, between salt marsh and fresh-
water marsh. The 1856-1859 marsh boundaries washover deposits and erosion along the.bay shore
further reduces marsh area.
were determined by comparing the 1934-1937,
1952-.1953,. and 1956-1957 photomapping with
the U. S. Coast Survey charts. Erosion and deposition have decreased the
marsh area of the Pass Cavallo flood delta. Marsh
A set of eight maps at a scale of 1:48,000 has been destroyed by erosion as Pass Cavallo
shows the distribution of marshes during migrates westward, and burial of marsh by wash-
1856-1859 and 1956-1957. Marshes are color over deposits has also reduced marsh area.
coded. The 1856-1859 marsh is represented by
diagonal red lines and the 1956-1957 marsh is Much of the bay, shore is composed of marsh.
shown in solid green.. An overlap of colors depicts Marsh is commonly eroded by waves approaching
the persistence.of the marsh for at least 100 years. from the. south or north. C Ionstruction of beaches
Extinct marshes are shown in red only, and a single
and beims at the bay margin is coincident with
green color indicates areas of marsh expansion. marsh erosion.

Marsh maps have the. same designation as
shoreline change maps--Brown Cedar Cut, Small, enclosed water bodies, such as Powder-
Colorado River, Shell Island Reef, Pass Cavallo, horn Lake, Chocolate Bay, Salt Lake, and Redfish
Lake, are less affected by wave erosion than are the
Lavaca Bay South, Lavaca Bay North, Carancahua
Bay, and Palacios Bay areas (fig. 9). larger water bodies. Marshes associated with the
small enclosed water bodies, however, also
General Wetland Trends decreased in size. Sediment was washed into the
marsh from adjacent slopes.
Many of the marshes in. the Matagorda Bay
area decreased in size from 1856 to 1957. Some of Marsh area increased. where streams discharge
the 18564859 maps, however, did not extend far directly into the bays, for example: (1) at the head
enough up some of the bays and their associated of Lavaca Bay (Placedo Creek, Garcitas Creek, and
creeks and rivers for a .vAlid.comparison to be made Lavaca River).; (2) at the head of Carancahua Bay
between the oldest and youngest marshes. (Carancahua Creek); and (3) at the head of Tres
Palacios Bay (Tres Palacios Creek). Marshes asso-
Significant marsh changes occurred on ciated with the Holocene Brazos-Colorado delta
Matagorda Peninsula, in the Lake Austin area, and (Lake Austin area). increased in size . during
on the Colorado delta. Changes in marsh area result 1856-1957. A probable cause.of marsh expansion
from both natural. processes and man's activities. was compactional subsidence of formerly surficial
Natural causes are shoreline erosion, sedimenta- deltaic deposits.

Three areas on the bayside of Matagorda large marsh areas. This kind of activity converts
Peninsula exhibited an increase in marsh area. marshes into fresh-water lakes. Examples of
These are marsh islands associated with active and marshes that were dammed are: (1) Blind Bayou
inactive tidal channels. Two of these tidal channels area (Lavaca Bay South area); (2) Huisache Cove
occur on the Brown Cedar Cut area map.. The (Lavaca Bay North area); (3) Piper Lakes and a
western channel, Brown Cedar Cut, is active; the
ot .her channel, no .w closed, lies at the east end of marsh along the 'north shore of Carancahua Bay
east Matagorda Bay. A third tidal pass area, Greens near the Jackson-Calhoun County line (Carancahua
Bayouj is also closed (Shell Island, Reef area map). Bay area); and (4) Buttermilk Slough. (Palacios Bay
area).
Marsh Change Resulting From Man's Activities
A large marsh has developed on the Colorado
Man's activities may either destroy or create delta. Within the next few years, the course of the
conditions that promote marsh growth. Some Colorado River will be diverted, and it will
marshes were buried by spoil along the bay margin; discharge into Matagorda Bay between Culver Cut
see the mainland shoreline on the Brown Cedar
Cut, Colorado. River, Shell Island Reef, and Pass and Middle Channel (Colorado River area map). A
Cavallo, maps. Spoil outwash has created conditions new delta will be constructed in this area and there
favorable for marsh growth in the vicinity of should be an increase in salt marsh in that area.
McNabb Lake (Colorado River area) and Fresh-
water Lake (Shell Island Reef area). Expansion and/or decrease in marsh area is
summarized in appendix B. Net loss or gain for the
Several dams have been constructed across period 1856-1859 through 1956-1957 is expressed
tidal creeks and between the bays and relatively as. acres.

COASTAL PROCESSES AND SHORT-TERM SHORELINE CHANGES

Observations of coastal processes operating on Gulf and mainland shorelines may be dras-
Gulf . shorelines of Matagorda Peninsula and tically altered during the approach, landfall, and
Matagorda Island began in the winter of 1970 and inland passage of hurricanes (Hayes, 1967; Scott
continued through the fall of 1973. Similar obser- and others,. 1969; Shepard,, 19,73). Storm-surge
vations were made along the mainland shoreline flood and attendant , breaking waves erode Gulf
during the winter of 1971 and spring of 1972. shorelines a few tens to a few hundreds of feet.
Short-term shoreline.changes were measured in the Washovers along barriers and peninsulas are com-
field, and an attempt was made to correlate these mon, and salt-water flooding may be extensive
changes with coastal processes. along mainland shorelines.
Coastal Processes Rivers and small streams normally flood in
Processes that constructed and that are the spring and early fall. Flooding corresponds
presently. modifying shorelines. in the M .atagorda. with spring thunderstorm activity and the
Bay area are astronomical and wind tides,. long- hurricane season. Rivers may flood as a result of
shore currents, normal. wind and waves, hurricanes, regional rainfall, but the smaller streams may be
river flooding, and slump along cliffed shorelines. activated only by. local thunderstorms. Effects of
river flooding are: (1) overbanking into floodbasins
In the Gulf Coast region, astronomical tides and onto delta plains; (2) progradation of bayhead
are low, ranging from a maximum of about 2 feet and oceanic deltas; and (3) flushing of bays and
along the Gulf shoreline to about 0.5 foot in the estuaries.
bays.. Wind regime greatly influences coastal
processes by raising or lowering water level along Short-Term Shoreline Changes, 1957-1972
both Gulf and mainland shorelines, and by gen-
erating waves and. longshore currents, (Price, 1954; The direction and rate of shoreline change for
Hayes, 1965; Watson, 1968; Watson and Behrens, the period 1856-1957 were determined by using
1970). vintage charts, maps, and aerial photographs. A

field study was conducted from October 1971 Beaches composed of shell and rock frag-
through May 1972,, for the purpose of docu- ments characterize the rapidly eroding shorelines.
menting shoreline changes which occurred after Shell and rock fragments are derived primarily
1957. Profiles were measured with alidade and from Pleistocene and Holocene deposits which are
stadia rod along Gulf and mainland shorelines. being eroded from the shoreface and inner conti-
nental shelf. These materials are direct evidence
Beach profiles were - measured in 1971-72 that the volume of terrigenous sand is low. The
from the waterline to a known geographic point, relative abundance of shells of shelf and bay
which distance could be compared to the distance species indicate that relict paralic deposits are the
between the 1957 waterline and the same geo- chief sources of coarse sediment that compose
graphic point. At each profiling station, physio- beaches and ramps of Matagorda Peninsula.
graphic setting and sediment composition were
described. Normally, the profiles were extended Beaches between Caney Creek and Pass
into the bays and Gulf of Mexico to water depths Cavallo have a high shell and rock fragment
from 1 to 3 feet. Sediment characteristics, fauna content (fig. 10). Shell material consists of both
and/or flora were determined for. the shallow, bay. and Gulf species, with bay species being more
nearshore parts of. the profiles. Also, marsh and abundant. East of the mouth of the Colorado
upland flora were described wherever a profile River, Crassost.rea virginica and Rangia,sp. are the
crossed these communities. Observations of coastal most common species.. Radiocarbon ages for
processes were made at. each profiling station; Crassostrea virginica shell collected from this beach
observations were. also made at selected intervals segment range from 860 to 37,000 years B.P.,
between profile stations. indicating that offshore Pleistocene and Holocene
Within the Matagorda Bay area, there are two deposits are sources of oyster shell. West of the
broad classes of shorelines: (1) open Gulf shoreline mouth of the Colorado River, three bay species
which extends from the vicinity. of Caney Creek on (Crassostrea virginica_ Y-ereenaria campechiensis
the northeast to about 4.5. miles west of Pass texana, and Rangia cuneata) are more common
Cavallo (fig. 1); and (2) mainland shoreline. A wide than Gulf species. Locally, one, of these bay species
range of variation in sediment types and physio- may be more abundant than the, other two, but
graphic features was encountered within the throughout this segment of, beach, Rangia cuneata
Matagorda Bay region. These variations reflect past is the dominant species. Rock fragments are
common between Caney Creek and Pass Cavallo;
geologic history , of the area, coastal processes
currently - operating on the shoreline, sediment they occur most frequently between Caney Creek
availability, and, to a certain degree, man's and.the Colorado River. Rock fragments range in
activities. size from granule-size gravel. to boulders up to
2-foot-maximum diameter. Small rock fragments
Characteristics of Gulf and mainland shore- are compact, and, large fragments are - platy to
lines, based upon field observations, are presented bladed (clast morphology, after Sneed and Folk,
in the following sections on "Open Gulf 1958). Pleistocene distributary sand, beach rock,
Shorelines" and "Bay Shorelines." reef flank sediments, and carbonate lacustrine
deposits are the sources of rock fragments.
Open Gulf Shorelines
Profiles of the Gulf beaches (figs. 10, 11, and
In general, Matagorda Peninsula beaches are pl. I) show that shell beaches are narrower and
characterized by a mixture of terrigenous sand, steeper than sand beaches. Evidence.that most shell
shell, and rock fragments. Matagorda Island, on the beaches are erosional is the common occurrence of
other hand, has beaches composed of terrigenous marsh deposits in the swash.zone. An exception to
sand (Wilkinson, 1973). Composition of beach the erosional nature of shell beaches is shown at
sediment is a good indication of sand availability profiles 10, 11, and 12 (fig. 11). Terrigenous sand
and stability of a particular shoreline segment content of these shell beaches, is higher than those
(McGowen and Garner, 1972; McGowen and to the east. This segment of Matagorda Peninsula is
others, 1972). Most of @Matagorda Peninsula has slightly convex seaward; it lies along the axis of a
been in an erosional condition for at least 118 buried Pleistocene fluvial sand body (fig. 2). The
years, whereas Matagorda Island has just recently shoreline convexity, short-term, accretionary trend,
shifted from an equilibrium to an erosional phase. and increase in terrigenous sand are probably

Cliffed shoreline with Pleistocene deposits exposed
in the swash zone

Shell and rock fragment beaches

Terrigenous sand beaches

Marsh-dominated strondline

Shoreline altered by man's activities

Port
Lovaco

9@1 @O

Keller
WOOS

710 P,10,io

MArAGORDA BAK
ke
MotogordG Austin
Port O'Connor

ESPIRIrU SAWO
84), rAsr mAr46oRDA aA),
rj

S111f of AASWO

0
12

Figure 10. Shoreline types of the Matagorda Bay area.

related to the Pleistocene sand source, which 9 to 25 feet, occur along segments of Matagorda
underlies the shoreface in the vicinity of profiles Peninsula that have broad sandy beaches.
10, 11, and 12.
Some of the principal questions about Gulf
The distribution of sand and shell beaches is shorelines in the area involve the location of sand
shown on figure 10, erosional and accretionary and shell beaches and the factors which cause
rates are shown. on figure 11, and beach profiles are shoreline erosion. Sand and shell beaches are the
shown on plate I. Profiles of sand beaches on plate products of sediment availability and coastal
I are represented by numbers 4, 5, 13, 14, 15, 16, processes that are operating in the area. Sediment
28, 30, 31, and 32 (appendix C, profiles. 23 and sources are the Brazos and Colorado Rivers, the
30). The occurrences of fore-island dunes and Holocene Brazos-Colorado delta, and the inner
beaches consisting of shell and rock fragments are continental shelf and shoreface. The principal
mutually exclusive. Coppice mounds and low, coastal processes responsible for sediment trans-
discontinuous sand dunes are associated with shell port and deposition are wind, waves and attendant
beaches. Fore-island dunes, ranging in height from longshore drift, and hurricane storm surge.

O'er Gulf and mainland beach profiles run in
compared with shoreline position

12
Location of profiles

Average yearly erosional rate (25
14
Average yearly accretion rate (25
52
51 +2 53 Swan Lake 0 Stable shoreline segment
-1 Caan C 10
PORT 54 /V
LAVACA

-2
10
50 +-2
'q8 -7 55 59
47 -20 N-6 Q
46 Keller -5 66
45 190Y - 6@ .2
44 -5 57 58 64* -1.4 67
56 1 652 63 6 4 8
@
3 A A- V 4 0 -2 PALACIOS
51 1,+ 1 -0. 0.5 -4 _6 -3
42 _4 69 68 Miles

Powderhorn
0*8 Adatagoroa BGy -3t, Oyster
71 ake
41 -32 73
PORT 42 MATAGORDA
O'CONNOR 4 4-10 74 -2
IN .75 77
Greens 80YOU -13 + 076
ESPIrdu Santo Bay -2 -3 +12 -44
-2 35 30 0 -2 2726 25 24 23 4_1 el 20 19 1 East lwalworda Bay
3 3 34 1 6 39 4 6 A 6 A 0 A A
C:Z> A3 - -12 -0+4 -11 -10 -12 -4-8 -2 - 10 11 411 161 13
u
-16 +4 _, T 10
.2 0 9 8,
+7
Go// of Ivexice 15 +3 +; -9 +8

Figure 11. Short-term erosional and accretionary shorelines of the Matagorda Bay area. At each profile, which was run in I
measured from the waterline to a known geographic point. These distances were then compared with the 1956-57 distances from the w,

Modem rivers are not providing a significant energy events, the entire peninsula migrates bay-
volume of sand to. the longshore drift system. The ward. Major hurricanes scour storm channels
Brazos River discharges its sediment load about 17 through the peninsula and build lobate sand bodies
miles east of the study area. The Brazos transports that project into the bay. At this.time, coarse shell
a large volume of sand and mud to the Gulf, but and rock fragments accumulate in.the interchannel
beaches only five miles west of the river mouth Are areas as shell ramps. Storms that raise the water
retreating at rates of 25 to 40 feet per year. The level less than 5 feet activate a few storm channels,
Brazos carries a large suspension load, approxi- and sand -is transported through these channels
mately 993 x 106 cubic feet per year. The toward the bay area, building small washover fans.
morphology of the present Brazos delta (con- Most of the present beach of Matagorda Peninsula
structed since 1929, Nienaber, 1963) and the west of the mouth of the Colorado River postdates
direction of wave approach and wave, refraction Hurricane Carla.
limit the volume of sand that will be contributed
to the beaches of Matagorda Peninsula. Under normal wind and tide conditions,
sediment is moved onshore and alongshore to the
Promontories, such asthe Brazos delta, cause southwest. Erosion is not as severe under normal
waves , to refract. Sometimes wave refraction sea conditions as during storms, but steep, short-
creates currents that flow counter to the dominant period waves are especially erosive in the area of
direction of southwest longshore drift. Hayes and the Holocene deltaic headland. There is selective
others (1970), in their discussion of offset inlets, sorting of terrigenous sand, shell, and rock frag-
explain the mechanism of countercurrent genera- ments. Turbulence of. breaking waves tends to keep
tion by wave refraction. The Brazos delta, by the fine- to very fine-grained terrigenous sand in
causing wave refraction, creates a local drift system suspension, making it readily available. for long-
that is directed eastward along its western periph- shore transport. The large, heavy shell and rock
ery. This countercurrent system is possibly one of fragments travel at a slower rate than sand, thereby
the mechanisms that retards westward sand trans- forming a lag in the upcurrent areas; terrigenous
port along Matagorda. Peninsula. sand tends to be concentrated.in the downdrift
direction (figs. 11 and 13).
The Colorado River contributes about 250 x
106 cubic feet of suspension load to Matagorda Two other factors are involved in sand and
Bay and the Gulf of Mexico annually. Annual sand shell distribution. First, .. the Pleistocene and
contribution is estimated to be about 20 x 10' Holocene sedimentary sources have a high mud/
cubic feet. Sediment and water discharge is divided sand ratio. Secondly,.tidal passes are areas in which
between west Matagorda Bay and the Gulf of sand is concentrated. In the areas where
Mexico; the sediment volume delivered to each has Pleistocene and Holocene deposits are bemig
not been determined. Sand delivered to the Gulf eroded, the longshore current system is sand
by. the Colorado River, however, causes accretion. deficient; erosion occurs because the longshore
of the shoreline for a distance of about 1 mile west current has the capacity to. transport a greater
of the river mouth. Beyond. that point, the sediment load. Since the shoreline is eroded
shoreline is mostly erosional. throughout most of its length from Caney Creek to
Pass Cavallo and because drift is to the west, sand
In 1957, Matagorda Peninsula had a well- load tends to increase in the direction of longshore
developed beach, shell ramp, and wind,tidal flat drift. This, in part, explains local development of
(fig. 12). Hurricane Carla (1961) breached, the sand beaches. The volume of sand within the
peninsula in many places (fig. 12). Field measure- longshore drift . system also increases, in areas
ments made in the spring of 1971 (fig. 12, and pl. underlain by.Pleistocene fluvial sand.
I, profiles 19 and 23). indicate that during
Hurricane Carla the shoreline. was eroded 450 to Sand is stored in the bays within flood deltas.
600 feet. Shepard (1973) reported that as much as Sand also accumulates on the barrier islands
800 feet of shoreline erosion occurred west of the immediately downdrift from tidal inlets; this
Colorado River. In addition to eroding the beach downdrift accumulation of sand produces a Gulf-
area, hurricanes and tropical storms transport ward offset of barrier islands adjacent , to tidal
sediment onto the back.side of barrier islands and channels. Downdrift offset is another indication of
into the adjacent bay. Through these brief but high a sand deficient system (Seelig . and - Sorensen,

31
MA U GORDA 48AY

A GUL F OF MEXICO

0 2

MILES
Wind-fidal flat and marsh Shell romp and vegetated flat Beach Bare sand
B

A SHELL RAMP
BEACH
6 VEGETATED FLAT
--,Erosional JD-TIDAL FLAT
4 Escarpment
BAY MARGIN
2
OL@
U.

co I/
4 0 1,000 2,0 0
I,--
FEET &E.G
C

Figure 12. Effects of Hurricane Carla, 1961, on a segment of Matagorda Peninsula beginning about 1. 5 miles west of the
Colorado River. (A) Matagorda Peninsula as it appeared in 1957. (B) Matagorda Peninsula shortly after the passage of
Hurricane Carla. This shoreline segment was eroded.as much as 800 feet. (C) Profile across Matagorda Peninsula (May 1971);
parts of the shoreline had accreted 500 feet.

- -- - ----------
17)

'eN @w
oo

MATAGORDA BAY AREA
0

0 100 200 300

Figure 13. Relationship between direction of wave approach and longshore drift.
Waves generated by prevailing southeast wind strike the shoreline at an angle. The
northeast segment of the wave begins to feel bottom before the southwest segment,
thereby generating currents that move alongshore toward the southwest.

1973). Tidal channels along Matagorda Peninsula shorelines; (2) shorelines characterized by shell
have opened and closed several times. For example, beaches and berms; (3) river-influenced shorelines;
Greens Bayou was openedby hurricanes in 1943, (4) shoreline segments dominated by salt marsh;
1961, and 1967, and closed shortly after passage of and (5) shorelines dominated by spoil outwash.
the storms. Pass Cavallo was closed at least once.
With the closing of a tidal pass, the shoreline is Cliffed shore lines.-The. distribution of cliffed
straightened when sediment -is eroded from the shorelines is shown on figure 10. Cliffed shoreline
downcurrent island (fig.. 14). Sediment eroded profiles are shown on figure 11. A comparison of
from Matagorda Island moved to the southwest by these profiles (pl. 1, profiles 37, 41, 46, 47, 51, 55,
longshore currents where it accumulated causing 59, 62, 63, 65, 66, 71; appendix C, profiles 37, 41,
accretion of a shoreline segment without any 47, 51, 55, 59, and 62) with figure 11 indicates
significant increase in the overall sand budget. that cliffed shorelines are erosional. Cliffed shore-
lines increase in height, toward the heads of Lavaca,.
Bay Shorelines Keller, Carancahua, and Tre.s.Palacios Bays (fig..1).
Commonly, as cliff heights increase, erosional rates
There are a variety of overlapping bay shore- decrease.
line types (fig. 10) in Matagorda Bay. Most of the
bay shoreline is eroding; rates of erosion (for the Field measurements and historical monitoring
interval 1957-1972) range from 1 foot. to. 25 feet both indicate that cliffed shorelines., have been
per year. Equilibrium and accretionary shorelines erosional for at least the past 116 years (from 1856
are rare; accretionary -rates range from 0.5 foot to to 1972). A comparison of erosional rates for the
3.0 feet per year. interval 1856-1957 with field measurements
(1957-1972) indicates that there has been an
Five types of shorelines characterize the increase, in erosional rates for most of these
Matagorda Bay system. Shorelines are classified on shoreline segments. (table 2).
the basis of elevation and gradient, composition
and caliber of materials constituting beaches, dom- The height of cliffed shorelines, in Matagorda
inance of vegetation,and degree of alteration by Bay generally 'increases northward; erosional rates
man's activities. The shoreline types are: (1) cliffed consequently decrease northward. Lowest cliffs are

Table 2. Comparison of erosional rates of cliffed shorelines
determined from field measurements (1957-1972) and from historical
monitoring. (1856-1967).

Field Measurements (1957-1972) Historical Monitoring (1856-1957)
Station Erosional Rate Station Erosional Rate
,(yearly av.) (yearly av.)

41 -32 feet 41 - 7 feet
46 -20 46 -1.4
47 - 7 47 -0.6
51 -1.5 51 .5.2 t
57 -0.4 57 -2.5 *
58 .9.0 58 -3.8 *
62 -4.0 62 -1.7 *
63 -6.0 63 -1.5 *

tThis segment is near the head of Lavaca Bay. A decrease inerosional rate
m ay result from increased sediment delivery. through Lavaca-Navidad
Rivers and Garcitas Creek as a result of increase in area of.cultivation.

*This area is adjacent to that part of Matagorda Bay that is being dredged
for oyster shell. Increased erosional rates may result from destruction of
marine grassflats and decrease.in numberof Orassostrea virginica clumps
(personal communication, Mr. H. C. Smith, Dec. 20, 1971).

&CLOSING OF TIDAL PASS
B.TIDAL PASS CLOSED, DELTA
BECOMES A WASHOVER,
BEACH EROSION
C. TIDAL PASS OPENED, TIDAL DELTA
ERODED, BEACH ACCRETION

................ ...
... . . ....... .

B.
. . . . . . . . . . . . .

Figure 14. Postulated sequence of events leading to, the development of an erosional unconformity on Matagorda
Island. Sequence C is the pres;Ient confieuration of Matagorda Peninsula and Matagorda Island in the vicinity of Pass Cavallo.

about 4 to 5 feet, and the highest cliffs stand. about area. to the swash zone where they form beaches
20 feet above bay level. The most rapid erosional along small reentrants, or a gravel veneer over the
rates occur along shoreline segments,that face into Pleistocene.
the southeast wind (pl. 1, profiles 41, 46, 55, 59,
62, and 63; appendix C, profiles 41, 55.1 59, and Shorelines at Gallinipper Point (appendix C,
62). Each of these shoreline segments, except the profile 47), north of Port Lavaca (appendix C,
area of profile 41, is eroded into Pleistocene deltaic profile 51) and along the south shore of smaller
deposits (distributary sand, interdistribu.tary mud). bays (pl. I, profiles 65, 66, 71) erode less rapidly
The cliffed shoreline at profile 41 is eroded into a than other cliffed . shorelines because they are
lower, muddy, deltaic sand, and an upper, clean, either in the lee of southeast winds or they are near
incoherent strandplain sand. Since this shoreline the heads , of bays where rivers discharge their
segment is situated near Pass Cavallo, it is affected sediment load. Profile 47 (appendix C) istypical of
by both tidal currents and waves. Th e bay margin Pleistocene deposits exposed along Alamo Beach-
in, the vicinity of profile 41 is, characterized by a Gallinipper Point. This shoreline segment erodes
broad sandflat with marine grass and oyster more rapidly in the winter when winds are from
clumps. the north., Cliffs are 17 to 20, feet high in the area
of profile 51. Pleistocene distributary sands and
The shoreline in the area of profiles 55 and 59 interdistributary muds are exposed in cliff faces.
faces into the southeast wind. Cliffs have. been Shoreline recession. results from wave erosion and
eroded into Pleistocene muds that accumulated slumping (profile 51, appendix C).. In the imme-
along the distal. end of abandoned Sangamon diate area of.. this. shoreline segment, the bay is,
deltas. Erosional rates here are less. than those. floored by Pleistocene mud.
along, the western shoreline of Matagorda and
Lavaca Bays because fetch is relatively short across Parts of the Matagorda Peninsula bay shore-
Keller and Carancahua Bays and Pleistocene muds line are characterized by low cliffs. Erosional
offer more resistance to wave erosion than sandy escarpments have been cut into barrier-flat sands
deposits. The bay bottom immediately offshore and marsh deposits consisting of muddy sand.
from profiles 55 and 59consists of Pleistocene Escarpments are about 1 to 4 feet, high, and,
mud...Beaches are virtually nonexistent in these erosional rates are 2 to 3 feet per year. Erosion is
areas, but there is generally a veneer of caliche and greatest during the winter when winds are from- the
shell gravel over the eroded Pleistocene surface. north.
Most of, the shell is derived from Crassostrea
virginica. During exceptionally high wave condi- Cliffed shorelines have developed primarily
tions, gravel composed of caliche and oyster shell is from lateral cutting of Pleistocene, de posits by
deposited upon these erosional escarpments wind@-generated wa ves. They are developed, to a
forming a thin berm. lesser. degree, on th e b.ayside of barriers and
peninsulas. Shorelines 'of bays having a.large fetch
Erosional rates along the north shore of erode rapidly, particularly in areas where cliff
Matagorda Bay (profiles 62 and 63) are inter- height is low and. where cliffs consist of incoherent
mediate between... those of the west shore of sand. Along most of these shorelines, the only
Matagorda and Lavaca Bays and the northeast coarse sediment (sand size or greater) available to
shore of Keller and Carancahua Bays. Cliffs are cut the, wave and longshore - drift. system is derived
into Pleistocene muds, and, like the shorelines of from the cliffs and from molluscs living in the
Keller and Carancahua Bays, beaches are rare; there adjacent, shallow, bay-margin areas. Where oyster
are shell beaches to the east and west of, this area clumps are abundant, they provide coarse material
(Carancahua Pass and Well.Point). To the south of that may be deposited at the base. of cliffs, thereby
the Carancahua Pass-Well Point area, there is a retarding wave erosion.
relatively broad shoal up. to 0.25 mile wide
developed on eroded Pleistocene sediments. A sand Shorelines characterized by shell beaches and
veneer overlies the Pleistocene for a distance of berms.-Parts of Matagorda Bay and Lavaca Bay
about 200 feet -from the cliff; this 200-foot zone is shorelines are characterized by shell beaches (fig.
characterized by bare Pleistocene mud with some 10). Prominent shell beaches occur along the north
caliche clasts, burrows, and oyster clumps. Oysters and west shores of Matagorda Bay and the west
and caliche fragments are transported from this and south shores of Lavaca Bay. Prior to excava-

tion of the Intracoastal Canal, shell beaches were Composition of shell beaches is variable. At
continuous between the West Branch of the Dog Island, whole and fragmented oyster shell
Colorado River and Palacios Point. This shoreline makes up most of the deposit. In the Shell Lakes
segment was, cuspate, and oyster reefs were. off- area, the ridges consist almost entirely of frag-
shore from each salient. Shell Island Reef and Mad mented shell ranging in size from coarse sand to
Island, Reef appear to be bayward extensions of pebbles. Oyster shell is the most abundant con-
these salients (fig. 15). stituent; however, shelf species are common. One,
of the highest shell ridges lies just to the east of
Numerous shell beach segments were profiled Carancahua. Pass. The ridge is fronted by.
(fig. 11, profiles 42, 43, 44, 45, 48, 49, 50, 56, 57, 1,250-1,3,00 feet of salt marsh, which is underlain
58, 68, 70, .72, and 73; pl. I). Generally, salt by shell, suggesting that the shell ridge has
marshes lie between shell beaches and Pleistocene remained virtually unchanged since it was
uplands. A few shell deposits are spits that are tied deposited. The ridge at Carancahua Pass consists of
to Pleistocene headlands; others have accumulated shell debris, with oyster shell being. the most
upon gently sloping, eroded, Pleistocene deposits. abundant type. Caliche fragments constitute about
Heights of shell beaches and berms range from 1.0 5 percent of the deposit. Bay species, other than
foot to 9.5 feet and widths from 80.to.900 feet. oysters, and Gulf species were present; among Gulf
Thicknesses of shell beaches and berms were species identified was the "surf clam," Donax.
determined at Dog Island, Shell Lakes, Carancahua
Pass, Indianola, and Magnolia Beach (figs. 10 and In the Magnolia Beach-Indianola area (fig.
16); thicknesses range from 1 foot to 8 feet. Only 17), there are two prominent beach ridges and
two trenches completely penetrated the shell several older shell berms that were deposited upon
deposits; both were in the Indianola-Magnolia escarpments cut into. the Pleistocene. Thickness of
Beach area. these deposits is 3 to 8 feet. There is a wide range

M-r.Sh .......... ...

Crob Lo

.. ..... ...
Loke
A H--
Mcd
MA
- r---n

S e I i'd

SPOIL OUTWASH

1956 SHORELINE

MATAGORDA B A Y

0 1/4 1/2 1 mile
SCALE

Figtire 15. Cuspate shell beaches, oyster reefs, and spoil outwash along the north shore of Matagorda Bay, west of the
Colorado delta. The shell beach marks the position of the 1856 shoreline. Mad Island and Shell Island reefs appear to have
been attached to cusps of the shell.beach. The 1956 shoreline lies some 1,100 to 2,500 feet bayward of the shell beach. The
shoreline accreted from spoil outwash.

Bay
Port
Lavaca

CCARANCC HU Palocios
MAGNO EACH- PASS
L'A 8'
INDIANOLA AREA
U
0 0
malagorda Bay
SHELL DOG
LAKES ISLAN Matagorda
A

Port
O'Connor. Eas A-fotGg0rdC
say

GUL F_ OF MEXICO

0 4 12

MILES

Figure 16. Locations of trenches dug into shell beaches, berms, and spits.

in texture and composition of materials composing characterized by steep foresets that dip in the
the shell. ridges. Some deposits, e.g. in the area of direction of channel migration, horizontal bedding,
the 3-foot trench, consist mostly of oyster shell and a few poorly developed soil zones. Oyster shell
with a few rock -fragments. Others, such as in the is the most abundant component; however, there
area of. the 4-foot trench south of Blind Bayou, are many shelf- and inlet-related. species in this
consist almost entirely of shell debris from coarse. deposit (Parker, 1960.; Andrews, 1971), such as
sand to pebble size. Shell was derived from both Macrocallista nimbosa, Eonitia ponderosa,
bay and Gulf species. D-achycardium muricatum, Polinices duplicatas,
Busycon contrarium, and R spiratum plagosus.
A large shell ridge extending from Blind This shell deposit accumulated as a northward
Bayou to Old Town Lake (fig. 17) is up to 8 feet migrating spit across a tributary of Matagorda Bay.
thick; it is separated. from the Pleistocene uplands A second . spit is represented by the present
and Modem beach by salt marsh. This ridge has beach-berm system (fig. 17). With. the exception of
been mined for road metal. Faces of some of the shell removed for road material, the older spit has
shell pits display graded bedding, channel fill remained virtuallyintact since it was deposited.

'T>

EXPLANATION

Modern shell beach
and berm

Holocene beaches
and spits
Salt marsh

Ponds
=45 Beach profile
Trench locality
8.0 Total thickness in
feet
5.7+ Thickness from sur-
face to water tabl
in feet

77 7.
0+
\3Q
..................

-PLEIS
TOCENE..
0 112 1
UPLAND
MILE

Figure 17. Distribution of shell beaches, berms, and spits in the Magnolia Beach-Indianola area. See figure 16 for
location.

A comparison of field measurements and beaches in the areas of profiles 57 and 73 are
historical monitoring (table 3) indicates that most backed by Pleistocene and older Holocene
of the shell beaches were erosional during the deposits; during storms these deposits build
period 1856-1972. Three segments, defined.by upward. In the area of, profile 58, the low shell
profiles 42-45, profile 48, and profiles 56-58, beach is backed by wind-tidal flat, marsh, and a
experienced an overall increase in rate of erosion water body. During. storms, shell from the beach
for the period 195,7-1972. Each of these shoreline washes into the marsh and lake. Erosional rates in
segments is fronted by relatively wide bays on these three areas are controlled by height of the
which large waves are generated by southeast or beach-berm system and the physiography of the
north winds. Remaining shell beach areas display a adjacent area.
decrease in rate of erosion. With exception of
profile 73, each of these beaches is associated with Shell beaches that form parts of southern bay
somewhat. le ss expansive water bodies. Shell shorelines are depicted by profiles 48, 56, and 68
beaches have an average yearly erosional rate that (pl. I and appendix C). These shoreline segments
is significantly less than cliffed shorelines (shell erode at rates of 2 to 3 feet per year. Waves
beaches, 2.6 feet; cliffed shorelines, 10 feet). generated. by northers, . are the . chief erosional
agents. The beach and berm in the vicinity of
Based on direction of prevailing wind alone, profile 48 is 9 to 10 feet high. It consists chiefly of
the north and west shorelines should erode. the oyster shell; live Crass.ostrea virginica were found
most rapidly. Profiles 42-45 are along. the west offshore in water about 3 feet deep. North of
shoreline. Two of these areas (42 and 44) are profile 48, fetch is greater than in the other areas.
erosional; one is, accretionary (43) and one is, in It is unlikely, however, that winter storms ever
equilibrium (45). Shoreline configuration in this generate waves sufficiently large to construct, a
area probably localizes erosion or accretion. Profile 10-foot-high berm.. Large waves, associated with.
45 is typical of the Modem shell beach, berm, and hurricane storm-surge ebb, probably constructed
marshes of the Magnolia Beach area (appendix C). this beach and berm couple. Parts of this berm
have. been removed. for road material. The shell
Most of the shell beaches along the north berm in the area of profile 56 is about 2.5 feet
shore of Matagorda Bay are erosional. At profiles high;.it consists of subequal amounts of caliche
57 (appendix C) and 73, erosional rates are 0.4 and gravel and oyster shell. Live oyster clumps were
1.0 foot per year, whereas at profile 58, the found along the shallow bay margin in water I to 2
erosional rate is 9 feet per year. Shell berms and feet deep. A relatively wide salt marsh lies behind

Table 3. Comparison of changes along bay-shore shell beaches, field
measurements (1957-1972) and historical monitoring (1856-1957).

Field Measurements (1957-1972) Historical Monitoring (1856-1957)
Station Erosion or Accretion Rate Station Erosion or Accretion Rate
(yearly av.) (yearly av.)

42 - 4.0 feet 42 4.5 feet
43 + 1.0 43 0.0
44 5.0 44 2.8
45 0.0 45 +3.6
48 2.0 48
49 + 6.0 49
50 +10.0 50
56 - 3.0 56 +0.8
57 0.4 57 - 2.5
58 9.0 58 3.5
68 3.0 68 0.4
70 1.2 70 6.4
72 3.0 72 -10.3
73 1.0 73 - 1.5

the berm, and marsh deposits were exposed in the River-influenced shorelines.-Where a river
swash zone. Winter storms are responsible for discharges into a bay, its velocity decreases and its
development of this berm. sediment load is . deposited forming a bayhead
delta. The Trinity, Colorado, and Guadalupe bay-
Profiles 70 and 72 (fig. 11 and appendix C) head deltas have been studied extensively
are typical of shell beaches that occur along east (Wadsworth, 1941, 1966; McEwen, 1963; Kanes,
bay shores. Erosional ratesat profiles. 70 and 72 @1965, 1970; Bounia and.Bryant, 1969; Donaldson
are 1.2 and 3 feet per year, respectively. The shell and others, 1970; Manka and Steinmetz, 1971). To
beach in the area of profile 70 is backed by a wide date, there are no data on the Lavaca and Garcitas
salt marsh. Oyster reefs which lie offshore from bayhead deltas which are building into Lavaca Bay
profiles 70 and 72 are effective in damping waves (fig., 1). The Garcitas delta has not prograded
and reducing erosional rates. significantly beyond the head. of Lavaca Bay,
whereas. the Lavaca delta has prograded about 2.7
Shell beaches erode less rapidly than other miles into the bay. For the most part, deltas are
bay shoreline types because shell material remains accretionary features. However, because of the
in the swash zone, whereas, very fine-grained sand lateral shifts in sites of sediment input, one deltaic
and silt are kept in suspension in the swash and segment may be accretionary, whereas another
breaker zones, and subsequently come to rest in segment may be undergoing erosion. The two
water 1 to 2 feet deep. The most rapidly eroding deltas at. the head of Lavaca Bay are experiencing
shell beaches are those which face into, the south- growth primarily in the immediate vicinity of the
east wind and those that.are backed by marsh or river mouths. The Colorado River builtits delta
shallow water bodies. Configuration of shorelines across Matagorda Bay, a distance of about 4 miles,
determines, in part, whether a particular segment between 1929 and 1935 (Wadsworth, 1966).
will be erosional or , accretionary. Waves are Figures 19 and 20 show t he growth.of the delta; by
normally focused on promontories (fig. 18), 1941, the delta had almost completed its growth.
thereby accelerating erosion. Orthogonals diverge
along concave shorelines, waves decrease in.height, In. the area of. Tiger Island Channel, the
physical energy decre ases, and. sediment Colorado delta is prograding about 28 feet per
accumulates. year; elsewhere it is in a destructive phase. A delta

0 10,000

Fe et

DIVERGENCE OF ORTHOGONALS
PRODUCES LOW WAVES IN
THIS AREA (EMBAYMENT)

-A,

CONVERGENCE OF ORTHOGONALS
PRODUCESHIGH WAVES IN
THIS AREA (PROMONTORY)

Figure 18. Wave refraction at Arena Cove, California (after Wiegel, 1964; reprinted by permission of Prentice-Hall, Inc.,
Englewood Cliffs, New Jersey).

Garcitas Creek delta is protected by
Pleistocene headlands from waves approaching
N
from the southeast. Within the estuary, the deltaic
shoreline is relative ly stable. Some sediment has
accreted to marsh islands at the mouth of Garcitas
Miles
Creek. Sedimentation rates, thickness of deltaic
deposits, the ratio of mud to sand, and rates of
compactional subsidence are not known.
,.,RACO'
0A
GOP The Lavaca delta is undergoing erosion along
ATA
K@
DAY most of its perimeter. The area between the Lavaca
River and Venado Lakes was once the site of
deltation. This shoreline segment, which is now
straight, was cut back by wave and current activity.
6 AY Since 1934, parts of this shoreline have eroded
p0A
MATAGO from 1 to 5 feet per year, whereas other parts show
no change. While the abandoned delta was under-
W
going erosion, the active delta prograded about 2.7
. . .. . miles into Lavaca Bay.. Shoreline accretion is now
Delta 0@d Shorelire 1908 (45 acres) - -limited to the immediate area of the moutli of the
U__ D
elta 1930 (1780 acres) N=
Delta 1936 (4890 acres)=
Lavaca River. Accretion rates near the river mouth
Delta 1941 (7100 acres) C=
were about 4 feet per year for the interval from
(After Wadsworth, 1966)
1957-1972. The western margin of the delta was in
equilibrium over the.same time period. Thickness
Figure 19. Map of the growth of the Colorado delta of the delta, sand and mud ratio, frequency of
during the period 1908-1941 (after Wadsworth, 1966; overbank flooding, and compactional subsidence
reprmted by permission of the author). are not known for Lavaca delta.

The largest marsh areas in the Matagorda Bay
is destroyed by erosion and compactional subsi- system are associated with, deltas. These include
dence (Scruton, 1960). Compactional subsidence is the active Colorado, Lavaca, and Garcitas deltas
rapid where prodelta mud is thick, where sedimen- and the inactive Holocene Brazos-Colorado delta.
tation rates were rapid prior to abandonment, and Delta plains of the active deltas are covered with
where deposits are young. The Colorado delta salt marsh, brackish marsh, and fresh-water marsh.
prograded rapidly as a consequence of a large The Holocene Brazos-Colorado delta (most of this
sediment volume and a shallow receiving basin; delta lies to the east of the Matagorda Bay area) is
maximum depth of the basin was about 6 feet characterized by marshes that are broken by tidal
(Kanes, 1970). Total thickness of the delta is .8 to channels, lakes, and ponds. Expansion of lakes and
10 feet, and maximum thickness of the prodelta is ponds indicates that the area is subsiding (Kolband
4 to 5. feet (Manka and Steinmetz, 1971). The thin Van Lopik, 1966).
prodelta mud precludes excessive compactional
subsidence. Bay muds that are 10 to 14 feet thick In order for marshes to propagate, there must
and estuarine deposits up to 80 feet thick underlie be a rather constant relationship between the delta
the delta. plain and sea level. If there is excessive vertical,
accretion, marsh vegetation is replaced by gras ses,
The Colorado delta is eroding 6 to 8 feet per shrubs, and trees. If, on the other hand, the marsh
year along its eastern margin, but the western delta surface subsides rapidly, the plants drown, and
margin has remained relatively stable during the waves and currents erode the area.
interval 1957-1972. Subsequent to diverting river
discharge into the Gulf of Mexico, oysters had At the present, marsh surface-water level
begun to colonize the offshore . area of east relationships of Garcitas, Lavaca, and Colorado
Matagorda Bay. Shell berms and beaches now deltas are stable. Apparently subsidence and sedi-
accentuate parts of the deltaic shoreline of east mentation rates are balanced. Delta-plain and
Matagorda Bay. marsh deposits are derived from rivers and bays.

1953
7098.22 ACRES 7200-00 AC. EST
7000

6000

5000
4889.88 ACRES

4000
Cr
1933
3470 ACRES

3000

2000
1930
1000 1780.46 ACRES

1908
45.8 ACRES
0
19b8 10 15 20 25 30 35 40 45 50 55
Y E A R S

Figure 20. Area increase of the Colorado delta, in, acres, for the period 1908-1953 (after
Wadsworth, 1966; reprinted by permission of the author).

When the streams overflow their banks, fine along the back sides of shell berms that tie
sediment is added to the delta plain. Southeast Pleistocene headlands together (the south shore of
winds create wind tides that inundate parts of the Keller Bay), at bay margins of spoil outwash, and
bayhead deltas. These wind-driven waters transport along the mainland shoreline between the Colorado
fine sediment onto the delta plain. River and Oyster Lake.

The normal succession of marsh types from Most of the marsh areas are undergoing
bayward inland is salt marsh, brackish marsh, and erosion. Table 4 shows the erosion or accretion
fresh-water marsh. Some marsh areas on the Lavaca rate for the marsh areas measured in the field and
delta do not exhibit the normal floral succession their associated physiographic features.
(figs. 10 and 11; appendix C, profile 54).
Several profiles were measured across marshes
Shoreline segments dominated by salt that face into the southeast wind. Cliffs occur
marsh.-The most extensive marshes are associated landward of three of the marsh areas (fig. 10; pl. 1,
with deltas (figs. 1 and 2). Salt marsh is also profiles 52, 61@ 64; appendix C, profiles 52 and
associated with barrier islands and peninsulas; salt 61). Two of the marshes are accretionary; the
marsh of this type is situated between Brown other is erosional. The remaining two profiles are
Cedar Cut and the Colorado delta. Other marshes representative of marshes associated with spoil
occuron the flood-tidal delta at Pass Cavallo, along outwash (fig. 10; pl. I, profiles 74 and 75;
CRES,_-@

minor reentrants and mouths of lesser streams, appendix C, profile 75); one of these marshes is

Table 4. Erosional, and accretionary rates Marshes that are not associated with deltas
(1957-1972) and physiographic units associated with receive sediment from the adjacent bay and from
marshes. the erosion of headlands. Under normal wind and
wave conditions, the marsh at profile 56 is supplied
Nature of Associated sediment from Keller Bay. Storm washovers from
Station Accretion Erosion* Physiographic Unit Matagorda Bay transport sand and shell into the
marsh. The equilibrium marsh at profile 67 is
52 +2 Cliffed shoreline affected by waves generated by both north and
56 -3 Shell berm south winds. Measurements made along profile 67
60 -2 Cliffed shoreline
61 +0.5 Cliffed shoreline during a norther indicated that water level was 1.5
64 -2 Cliffed shoreline feet below normal bay level. Spartina alterniflora
67 0 0 Cliffed shoreline marsh was completely emergent and the high salt
68 -3 Shell berm marsh extended 3 feet above bay level. South or
70 -1.2 Shell berm we Ist winds inundate the marsh and a poorly
74 -13 Spoil outwash
75 +3 Spoil outwash defined, wind-tidal flat occurs at about 1.5 feet
above mean high water.
*Erosion and accretion rates are yearly averages for the
time interval 1957-1972. Two profiles were measured across marshes
that form parts of the eastern shorelines of
erosional, the other - accretionary. Accretionary Carancahua and Matagorda Bays (figs. 10 and 11;
marshes, which are backed by cliffs, lie downdrift appendix C, profiles 60, 70). Both marshes were
from eroding headlands. Other marshes (e.g., being eroded, but the rate was slightly less at
profile 64) associated with cliffed shorelines occur profile 70 where oyster reefs lie offshore. Each
in concave shoreline areas. marsh is rather broad, and under normal condi-
tions, they are inundated by about 0.5 foot of
In east Matagorda Bay, marshes that front the water. Water levels were measured along profiles 60
southeast wind are developed upon spoil outwash. and 70 during a norther. Water level at profile 60
The marsh at profile 75 is accretionary and receives was 0.75 foot below normal and 1.5 feet below
its sediment from the erosional shoreline to the normal at profile 70. A wind-tidal flat is developed
east and from reworked spoil adjacent to the 1.5 feet above normal bay level at profile 60, and
Intracoastal Canal. The spoil area. that supplies at about the same elevation to the north of profile
sediment to the, marsh at profile 75 has about 5 70. Low and high marsh are well developed in the
feet of relief and is 600 to 950 feet from the bay area of profile 60; Distichlis spicata dominates the
margin. The erosional marsh at profile 74 received high marsh. There is no low marsh at profile 70;
sediment from a spoil area having about 7 feet of the high marsh is characterized by three floral
relief; the spoil was 1,200 to 1,450 feet from the zones (appendix C, profile 70).
bay mar gin. Marshes that face the southeast wind
are flooded by wind tides which have a range of a Shorelines dominated by spoil outwash.-Bay
few inches to approximately 2 feet. shorelines adjacent to the Intracoastal Waterway
have been significantly altered. The areas that have
Marshes mostly affected by waves that been affected the most lie between Caney Creek
approach from the north were also profiled (figs. and the town of Matagorda, between the Colorado
10 and 11; pl. I, profiles 56, 679 68; appendix C, delta and Oys .ter Lake, and to the west of P O'rt
profile 56). Two ofthe marshes (profiles 56 and O'Connor (figs. 1 and 10). Approximately 40 miles
68) areeroding at rates of about 3 feet per year. of canal were dredged through marshes and shell
The other marsh area (profile 67) has been in beaches which bordered Matagorda and Espiritu
equilibrium since 1957. Santo Bays. The canal was initially 12 feet deep
and 200 feet wide.
The marsh at profile 56 is about 1,000 feet
wide and is a rather uniform 0.75 foot above bay With the completion of the Intracoastal
level; it increases to 1.5 feet above bay level where Waterway, approximately 500,544,000 cubic feet
salt-marsh vegetation gives way to Spartina of spoil were placed adjacent to the bay margin.
spartinae (sacahuista) and Tamarix gallica (salt Depth measurements made in the Intracoastal
cedar). Waterway in the spring of 1973 indicated that the

canal had been deepened about 6 feet, thereby periods, when the volume of spoil is low and when
increasing the spoil volume to approximately dense vegetation inhibits transport of sediment by
750,800,000 cubic feet. sheetwash.

Results of dredging and spoil disposal adja- Profile s 75 and 76 (figs. 10 and 11; appendix
cent to bay margins are destruction of certain C, profiles 75, 76) are characteristic of bay
physiographic and environmental units, and filling shorelines that have been affected by dredging and
of the bays through shoreline accretion (fig. 15). spoil outwash. The shoreline at profile 76 was
Accretion from spoil outwash is not a continuous experiencing erosion when. this study was termi-
process. Accretion occurs during dredging opera- nated. The shoreline was. eroding because the
tions and during heavy rains when sheetwash volume of spoil was relatively low. At the same
transports sediment from spoil mounds into the time, the shoreline at profile 75 was accreting. The
bay. Bay shorelines that have accreted as a result of volume of spoil was great, and the distal part of the
dredging operations also go. through periods of spoil outwash was densely vegetated by marsh
erosion. Erosion occurs during extended dry Plants.

STABILITY OF SHORELINES

The term stability refers to the accretionary, uplands, and from molluscs that live along the bay
equilibrium, or erosional condition of a particular margin. . Sources of sediment for Matagorda Penin-
shoreline segment. The specific, condition may sula and Matagorda Island are the Brazos and
result from long-term, annual, or short-term Colorado Rivers, as well as erosion of Holocene
processes (Wiegel, 1964). Short-term stabil 'ity, as Brazos-Colorado delta, and erosion of Pleistocene
used in this report, refers to shoreline changes that deposits exposed on the inner continental shelf.
occur during a time interval of months or a few
years. Long-term stability reflects shoreline Most of the shorelines associated with the
changes over a period of a few decades on up to a Matagorda Bay system Iare eroding, a fact which
century or more. 'Shorelines that have a long-term
erosional history may accrete under certain short- indicates that the sediment volume. supplied to
term wave conditions, and similarly shorelines with Gulf and b .ay shorelines is insufficient to balance
a long-term accretionary trend may be erosional the amount of sediment removed by waves and
under certain short-term wave conditions (Wiegel, longshore drift. The nature of beaches in the
1964). Matagorda Bay area is a good indicator of the
Over the short term, the beach may be condition of shoreline stability. There are very few
pure sand beaches along the mainland shoreline.
erosional, accretionary, or in equilibrium, de- Sediments of mainland beaches are a mixture of
pending upon wave conditions at the time the
observations were made. Steep waves that develop sand, shell, and rock fragments; shell and rock
fragments are the most common constituents.
in deep'water affect erosion or accretion at a
moment in time. Steep waves tend to erode the Beaches composed of shell and rock fragments
beach, whereas flat waves have the opposite effect. indicate that virtually no sand is supplied currently
to these beaches by fluvial systems. Accretionary
Long-term shoreline trends are controlled by mainland shorelines generally coincide with river
sediment availability, subsidence,, shoreline orienta- mouths.
tion, existence - of promontories, and direction of
sediment transport. In general, the long-term trend There are two contrasting areas along the Gulf
of Matagorda Peninsula has been erosional, and the shoreline: (1) the area from Pass Cavallo westward
.trend of Matagorda Island has been accretionary or (Matagorda Island); and (2) the area from Pass
in equilibrium. Mainland shorelines have been Cavallo eastward to the boundary of the study area
chiefly erosional during the same time interval. (Matagorda Peninsula). Composition of materials
making up these two areas is different. Matagorda
Sediment Availability Island is composed chiefly of terrigenous sand,
whereas shell and rock fragments compose a
Sediment supplied to the mainland shore is significant part of Matagorda Peninsula. The rate of
derived from rivers, from erosion of Pleistocene delivery of terrigenous sand to Matagorda Penin-

sula is now, and probably was in the past, slower 97* 916. 96. 30-
than the delivery rate to Matagorda, Island. Among
the more obvious lines of evidence are the follow- 111
ing: (1) Matagorda Peninsula has a long erosional
history, whereas Matagorda Island is just entering Golvesto.n
an erosional phase; (2) accretionary grain through-
out Matagorda Island indica tes rapid sand accu- 29'
mulation, whereas Matagorda Peninsula only
exhibits accretion near Decros Point; (3) the two
areas have cont rasting widths, with a broad, barrier
sand body indicating an excess of terrigenous sand
<5
and a narrow island suggesting the opposite; and _,@1_28o
-Corpus N 11\
(4) there is a contrasting sediment composition in Christil
the two areas. GULF OF @aAR SEPT
4P
Subsidence k41-

ZONE OF
In some areas of the Texas Coastal Zone, CONVERGENCE
subsidence is a major cause of shoreline retreat.
There has been some subsidence in the northeast 0
part of the study area as a consequence of
dewatering of Holocene Brazos-Colorado deltaic 26*
deposits. Transgression in the area from Caney 0 2 50.iles
Creek to about halfway to the mouth of the SCALE
Colorado River probably resulted from a low sand
supply and compactional subsidence. Subsidence Figure 21. Net annual longshore drift (after Watson,
changes the relative position of sea level and would 1968).
by itself be sufficient to cause shoreline retreat.

Shoreline Orientation In the Matagorda Bay area, waves approaching
from the east, east-southeast, and southeast
Interaction between the direction of wave generate southwestward-flowing longshore cur-
approach and shoreline orientation determines the rents. Rate of longshore drift is greatest when wave
direction of longshore drift (fig. 21). Prevailing approach is from the east, because the angle
wind and, hence, the direction of wave approach, is between wave approach and the shoreline is at a
from the southeast. Near. the shore, the shallow maximum. Waves approaching from the south and
bottom begins to exert a drag (friction) on waves; south-southeast create longshore drift and sedi-
it is in this shallow water zone that shoreline ment transport to the northeast.
orientation begins to exert its influence on break-
Direction of Sediment Transport
ing waves and nearshore currents. The Texas Gulf
shoreline is concave to the southeast, According to The dominant sediment. transport direction,
Watson (1968): as indicated by natural tracers, is both onshore
(from the inner shelf and shoreface) arid along-
"An onshore wind blowing onto a concave shore. to the southwest. Natural tracers are frag-
shoreline will produce wave fronts normal to
the wind direction. These wave fronts move ments of lacustrine limestone, calcite-cemented
shoreward and . are incompletely refracted. As sandstone blocks, plates of beach rock, caliche
they break, the waves generate a longshore nodules, and vertebrate 'and invertebrate fossils.
current due to their oblique approach to the During hurricanes, these material's are eroded from
shoreline. This current is strongest at, the Pleistocene.and Holocene deposits exposed on the
greatest distance from the central point where
the waves approach the shore at the greatest shoreface and inner continental shelf and trans-
angle. The current decreases in magnitude ported to barriers and peninsulas.
toward the center where. it diminishes to zero
because the waves Iapproach parallel to the Morphological features of barrier islands and
shoreline at the point where the wind direction peninsulas indicate that dominant sediment trans-
is normal to the shoreline and no current. is
generated." port is southwestward. Ridges and swales at Decros

Point indicate that Matagorda Peninsula grew to west where part of it accumulates on beaches.
the southwest by spit accretion. Coarse shell and rock fragments remain behind to
be transported onshore by a storm surge.
Rock fragments and shell, which are derived
from bay species, occur on beaches throughout the Coarse materials are probably more abundant
extent of Matagorda Peninsula. The greatest con- in relict sediment in the nearshore zone adjacent to
centration of shell and rock fragments is on the eastern Matagorda Peninsula than to the west.
eastern two-thirds of the peninsula. The day-to-day Onshore winds contribute to the concentration of
process of breaking waves and longshore currents coarse materials by selectively removing sand-size
selectively transports terrigenous sand to the south- particles fromback-beach areas.

DISTINCTION BETWEEN NATURAL. AND
MAN-INDUCED SHORELINE CHANGES

Natural changes in the Coastal Zone are those cent to shallow bay margins which are inhabited by
changes that would occur even in the absence of clumps of Crassostrea virginica.
man. Such natural changes result from, the inter-
play among the various coastal processes an .d the Marsh areas, particularly those associated with
interaction of these processes with sediments that theJarger fluvial systems, are least likely to erode.
are available for. construction and maintenance of Erosion is retarded by the ability of plant roots to
shoreline features. Man's role Iin producing shore- trap and bind sediment particles. Some marshes are
line changes generally'has been to bring about a undergoing erosion; these are situated on aban-
state of natural disequilibrium by interrupting the doned delta lobes and along bay margins of barriers
progression of natural coastal processes and by and peninsulas.
decreasing the sediment budget. Bay shorelines also erode because of -a sedi-
ment deficit. Maximum sediment input is localized
Natural Shoreline Changes at the mouths of Garcitas Creek, Lavaca and
Colorado Rivers. The Matagorda Bay system con-
Historical monitoring and field studies have, stitutes a large water body which is conducive to
shown that most of the shoreline changes in the generation of rather large waves. Waves strike much
Matagorda Bay area are the result of natural of the bay shoreline at an angle and are refracted,
processes. During the past 116 years, the dominant thereby setting up longshore drift. The north
trend of Gulf and bay shorelines has been shorel ine of Matagorda Bay is at the present a
erosional. relatively straight segment, which results from
wave erosion and longshore drift (the area between
The Gulf shoreline of the Matagorda Bay area Well Point and Sand Point is an example). Waves
is -erosional because there is a.deficit of sand-size erode the Pleistocene headlands, and longshore
sediment. Based upon calculations of Volumes I of currents transport granular materials to the west
sand eroded from the inner shelf and shoreface, where they accumulate in spits (Turtle Point, Sand
sand volume delivered to the Gulf by the Brazos Point, and Rhodes Point).
and Colorado Rivers, and the volume of sand
trapped by the north jetty of. the. Matagorda Ship The natural trend is for the bay shorelines to
Channel, it is concluded that materials derived retreat, thereby increasing the bay area. Matagorda
froin shelf and shoreface erosion constitute over 60 Peninsula is also retreating; the peninsula will
percent of the current sand budget for this ultimately migrate across Matagorda Bay and
shoreline segment. attach itself to the bay-shore segment that lies
between Chinquapin and Palacios Point.
Bay shorelines are mostly erosional. Shoreline
segments that face into the predominant southeast Man-Induced Shoreline Changes
wind or face the north and that are adjacent to
water bodies with significant fetch tend to erode A few areas of shoreline change are definitely
rapidly. Erosion is retarded along shorelines adia- related to man's activities in the Coastal Zone.

Some of man's activities that affect shorelines, or shoreline erosion and accretion adjacent to Mata-
that have the potential of affecting shorelines are: gorda Ship Channel jetties was determined from
(1) construction of dams across major fluvial data (1964 through 1971) provided by the
systems; (2) river diversion; (3) land use in major Galveston Office, U. S. Army Corps of Engineers.
drainage basins; (4) construction of bulkheads, Twelve profiles (figs. 25 and 26) were constructed
groins, and jetties; (5) mining beach materials; (6) from the Corps data. The volume of sand that
shell dredging; (7) dredging canals; and (8) dune accumulated adjacent to the north jetty over the
alteration. A few of these are ongoing activities in seven-year period was about 45,000,000 cubic feet
the Matagorda Bay area, and others have been (about 6,400,000 cubic feet per year). The shore-
conducted in the past. line adjacent to the south jetty has undergone
erosion during the 1964-1971 interval (fig. 24);
Consequences of three of man's activities have approximately 51,500,000 cubic feet of sand was
been documented in the Matagorda Bay area. The removed from the beach and shoreface during the
rapid shoreline accretion which followed dredging period 1964-1971 (fig. 26).
of the Intracoastal Waterway, and progradation of
the Colorado delta across Matagorda Bay are Activities of man' in the drainage basins of
examples of shoreline changes produced by man's major streams that discharge into bays and the
activities. jetties cause changes in shoreline sta- Gulf of Mexico also affect shorelines. In most areas
bility by trapping sand transported by, longshore of the Texas Coastal Zone, the magnitude of
currents. There are two jetty systems in the shoreline changes resulting from altering, major
Matagorda Bay area that are currentl y being used, fluvial systems has not been determined. Increase
one at Port O'Connor and the other on the Gulf in agricultural activity in drainage basins increases
side of Matagorda Peninsula. sediment yield, and dams constructed across major
Data on the jetties at Port O'Connor have streams impound water and sediment. Suspension
been derived from 1934 and 1956 photomosaics, load volumes have been calculated for the major
1969 NASA photographs, and data supplied by the streams, but at this time the volume of bed load
Galveston Office, U. S. Army Corps of Engineers. carried. by these streams is not known. Other
There are two sets of jetties at Port O'Connor. The unknowh factors are lag time between, man's
oldest jetties (fig. 22, area 1), shown on 1934 activities (increased sedimentation or reservoir con-
photographs, have been abandoned; a second set of struction) in the drainage basin and shoreline
jetties (fig. 22, area 2) was constructed in 1939 and changes related to these activities. Examples of
improved in 1965. Pleistocene strandplain sand in stream alteration are: (1) the diversion of the
the Port O'Connor area yields sediment that is Brazos River in the Freeport area; and (2) removal
transported southward by tidal and longshore of the log jam on the Colorado River. Diversion of
currents. Jetties at Port O'Connor prevent south- the Brazos River has caused: (1) destruction of the
ward movement of this sand. These jetties are n ot old Brazos delta (Seelig and Sorensen, 1973); (2)
the cause of downdrift. erosion because the trend construction of a new delta at the river mouth
during 1856-1957 also had been erosional. The (Odem, 1953; Nienaber, 1963; Seelig and
jetties trap sand that would normally move south- Sorensen, 1973); and (3) development of a
ward to an area already experiencing erosion. By trapping mechanism (local reversal of longshore
1969, sand had accreted to a point where it began currents) for most of the sand transported to the
to bypass the jetties and move into the Intracoastal Gulf of Mexico by the Brazos River. Rivers have
Waterway (fig. 23). Sand dredged from the, canal is not been intensively studied with respect to the
placed south of the jetties where it is picked up by role they play in shoreline'stability.
the current system and transported toward the
Gulf of Mexico. Bulkheads have been constructed to retard
erosion of. cliffed shorelines in several areas along
Matagorda Ship Channel (fig. 24) was com- the mainland shore. Such areas are: (1) Port
pleted in 1965. Initially,.the north jetty extended O'Connor along the west shoreline of Matagorda
approximately 5,000 feet beyond the shoreline Bay; (2) Olivia along the north shore of Keller Bay;
into about 4 fathoms of water. Since 1964, the (3) near the mouth of Keller Creek; and (4) along
shoreline has accreted from 100 to 950 feet updrift the east shore of Carancahua Bay west of the
from the north jetty (fig. 24). The nature of mouth of Fivemile Branch.

48 EXPLANATION
FED Pleistocene subaerial sand

Modern subaerial flood delta

Modern subaqueous sand

Mainland shoreline 1934

......... Mainland shoreline 1956

Erosion 1934-1956

Accretion 1934-1956

Seaward limit of subaqueous sand 1934

Seaward limit of subaqueous sand 1956

L. Erosion 1934-1956
Accretion 1934-1956 PORT O'CONNOR

I Pre-1934 jetty system
2 1939 Jetty system

2
0011,

956

934

MILES

IV
N/

1956
qY,

1934

Figure 22. Locations of jetties in the Port O'Connor area, and changes in the shoreline and nearshore sand distribution during the interva.
1934-1956.

Bay-Margin
Port Sand
Ocorn 0,1 Ord'
MOO 00Y

Sand Bar
43poil Island,

Matagorda Peninsula
s s rs
Vegetate P P1 Flat

Breaker 0 16 32
Matagorda Island I I
Thousands of Feet

GUL F OF MEXI CO

Figure 23. Distribution of bay-margin sand north of the jetties at Port O'Connor. The map was derived from 1969
NASA photography. Between 1956 and 1969, the bay-margin sand had accreted at least as far as the bayward extent of the
north jetty.

MA TA GORDA B A K

Z
@J MATAGORDA PENINSULA

Co
1 1) 12 13 1 15 16 IT Is 19 20
'o -o _o

0 5
0
Miles
NORTH JETTY

GULF OF MEXICO

Figure 24. Accretion and erosion associated with Matagorda Ship Channel jetties. Accretion occurs adjacent to the
upcurrent (north) jetty; erosion occurs downcurrent from the south jetty.

SE NWO

---------- Accretion
1964 1971 5

13 2

--------------
---------- ccretion 4
------ ------------

1971 1964

Erosion

3
---- ---------- Accretion 4

W
1967 -T6-6-9-:r 52
Erosion

17 2

Accretion 4
--------------- 1971
- - - - - - F9 @72
5
1964
E,osion

3
------- Accretion
1967-1 - - - - - ----- - - --- ----- ------------------------
5

I
20 1971 2

3
1964 ccretion
4
----------------
I 9@; 5

10 9 8 7 6 5 4 3 2 1 0
THOUSANDS OF FEET

Figure 25. Shoreface profiles in the area of the north jetty, Matagorda Ship Channel. The upper set of profiles at each
station includes profiles for 1960, 1964, 1967, 1969, and 1972. Accretion or erosion was determined for each profile by
comparing the oldest and youngest data. See figure 24 for profile locations.

SE 0

1964
----- ----------- 191,
3

---------- 4
-0-

- - - ----------- 1971

4
-----------

I
4
----------- 197
------------------------ ----- -
4

1964 197,

-------------
6 969 4
---------- I--
@960@

711
2
1964

969
--------------- -- ---------------------- 4
---------------------------- - -------- I---------------------- 5
960J

10 1971

----------
1967@ ------------ W(I@ -------------- ---- -- --- A-tw
--------------------
........ ..
-----------
9691

11 14 11 12 11 1. 1 1 4 0
THOUSANDS OF FEET

Figure 26. Shoreface profiles in the area of the south jetty, Matagorda Ship Channel. The upper set of profiles at each
station includes profiles for 1960, 1964, 1967, 1969, and 1971. Accretion or erosion was determined for each profile by
comparing the oldest and youngest data. See figure -24 for profile locations.

1 "'K

p p
7x- ME a E& 6ORK-
, km

3 v-1
"N

IF
. ..... .F 0 IN,
""4
@@v
'01 NIZIM

Scale
0, 0.5 1.0

Mile

Figure 27. Dredging of Dog Island Reef for oyster shell (USDA photos, 1953). Dog Island Reef is situated west of the
Colorado delta.

Shell dredging has in the past and is presently along the older Holocene deposits. A few shell pits
being conducted in the Matagorda Bay system. were operated on the marsh side of Modern shell
Dredging in the early 1950's destroyed Dog Island beaches. Holocene beach ridges, which are up to 10
Reef (fig. 27). Dredging operations,,are proceeding feet high, provide protection for man-made struc-
near the north shore of Matagorda Bay in the tures during storms. Mining of shell from these
general area between Carancahua Pass and Sand ridges created gaps that can be easily breached
Point. The effects of shell dredging,on shoreline during storms. Shell removed from Modern beaches
stability are not definitely known. has not destroyed these features, but continued
In the past, Holoceneand Modem beach and mining could bring about serious erosional prob-
spit deposits were mined in the Indianola-Magnolia lems, particularly during hurricanes or tropical
Beach area. Most of the mining operations were storms.

CONCLUSIONS

Historical shoreline monitoring (for the Matagorda Island eroded at a rate of about 11 feet
period 1856-1957) and field measurements per year; combined land loss in these two areas was
(1971-1972) demonstrate that erosion is an impor- about 1,050 acres.
tant natural process along both Gulf and bay
shorelines of the Matagorda Bay area. Erosion is a During the 100-year period, the bay shore-
long-term trend. These trends were established at lines, of Matagorda Peninsula and Matagorda Island.
least 118 years ago prior to any significant modifi- (including the bay margin of the tidal delta
cation of the coastal environment by man. Ero- associated with Pass Cavallo) were in an erosional
sional shoreline trends will probably continue and state, although there were areas of local accretion.
may accelerate in the future as some of man's Average rate of erosion along Matagorda Peninsula
activities interrupt the normal movement of sand was approximately 4 feet per year; approximately
in the natural sediment dispersal system. 2,650 acres of land were lost. The bay shoreline of
Matagorda Island retreated at an average rate of
Shoreline Change about 2.0 feet per year with a land loss of about 41
acres.
For the 100-year period of historical mon-
itoring (1856-1957), approximately 8,450 acres of Most of the remaining shoreline segments of
land were lost by natural erosion of Gulf and bay the Matagorda Bay system experienced erosion
shorelines. During this same period, approximately during the 100-year period. Shorelines of Powder-
615 acres of land was gained through natural horn Lake and Carancahua Bay retreated at ave rage
accretion. Natural accretion occurred at the rates of 0..2 and 0.7 foot per year, respectively. The
mouths of Garcitas Creek and Lavaca River, along west shorelines of Matagorda and Lavaca Bays
the Gulf shore of Matagorda Island near the west eroded at about 2.6 feet per year, and the north
limit of the study area, and at ephemeral tidal shore of Matagorda Bay between Sand Point and
passes such as Brown Cedar Cut and Greens Bayou. Well Point eroded at an average rate of about 2.4
Since 1929, certain of man's activities have caused feet per year. Other shoreline segments retreated at
local accretion. Through man-induced accretion, a rates between the maximum displayed by west
total of 9,600 acres has been gained at the mouth Matagorda-Lavaca Bay and the minimum displayed
of the Colorado River, along segments of the north by Powderhom Lake and Carancahua Bay. Total
shore of Matagorda and Espiritu Santo Bays, and in land loss from these areas was approximately 2,200
the Port Lavaca-Point Comfort area. acres.

Gulf shorelines have experienced the greatest Waves are the dominant erosive mechanism
amount of erosion. Matagorda Peninsula eroded at acting on both Gulf and bay shorelines. Prevailing
a rate of about 5 feet per year during the 100-year southeast winds generate waves that erode south-
period; approximately 2,600 acres of land were and east-facing shoreline segments. Northers gen-
lost. The shoreline along Pass Cavallo retreated at a erate rather large waves which erode north- and
rate of about 33 feet per year, and parts of west-facing shoreline segments. Huge waves

generated by hurricanes erode both Gulf and bay stored within the river was available for transport
shorelines. Erosion of several hundred feet of Gulf to the bay; rapid deltation ensued. Initial rates of
beaches, dunes, and shell ramps occurs when shoreline accretion were about 1 mile per year.
hurricanes such as Carla, 1961, make landfall in the
Matagorda Bay area. Parts of the Gulf Intracoastal Waterway were
dredged across coastal lands adjacent to the north
Natural accretion in the Matagorda Bay area shores of Matagorda and Espiritu Santo Bays.
was local and relatively insignificant when com- Sediment dredged from the canal was deposited as
pared with the widespread erosion. Accretion spoil banks upon marshes along the bay margins
occurred in a protected bay at the mouth of and within the bays. Spoil, deposited directly in
Garcitas Creek where over the 100-year period the bays by the dredging process, or subsequently
there were approximately 132 acres of land gain. washed into the bays by sheetwash, has filled
Although Lavaca River is larger than Garcitas approximately 1,560 acres of the subaqueous bay
Creek, only 55 acres of new land was constructed margin. Spoil mounds and spoil outwash have
during the same time interval; the Lavaca delta has created approximately 1,560 acres of new land by
prograded into Lavaca Bay and is subject to wave shoreline accretion.
erosion.
Dredging of boat basins and turning basins in
Approximately 380 acres of land have the Port Lavaca and Point Comfort areas created
accreted to the bayside of Matagorda Peninsula in approximately 110 acres of new land.
the form of emergent segments of tidal deltas.
These areas occur at the extreme eastern part of Approximately 9,600 acres of new land ac-
east Matagorda Bay, at Brown Cedar Cut, and at creted to bay shorelines as a result of man's
Greens Bayou. Each of these areas is representative activities. Or, stated differently, 9,600 acres of bay
of ephemeral tidal passes, each created when a bottom have been filled by these activities.
hurricane scoured a channel through Matagorda
Peninsula. Brown Cedar Cut is the only one of Marsh Area Change
these passes that is presently active.. Most of the
sediment comprising these emergent tidal deltas Marshes were historically monitored during
was transported into the bay during hurrican es or the same time interval (1856-1957) as the mon-
tropical storms. itoring of shoreline changes. Comparison of the
distribution of marshes during the period from
The westernmost part of the Gulf shoreline of 1856-1957 indicates that there has been a decrease
Matagorda Island was accretionary during the in wetland areas of approximately 5,000 acres (an
100-year period of historical monitoring; approxi- average loss of 50 wetland acres per year). For the
mately 47 acres of land was added to this area. period 1856-59, there were approximately 46,000
Sand that accreted this shoreline segment was acres of wetlands, and in 1956-57 there were about
derived from the erosional segments of Matagorda 41,000 acres.
Peninsula, from along the west bank of Pass
Cavallo, and from the northeast end of Matagorda Locally there was a natural increase in wet-
Island; the sand was transported southwestward by lands during the 100-year period; for example,
longshore currents. wetlands formed at the mouths of Garcitas' Creek
and Lavaca River, in the Lake Austin area, and at
Large and small areas of shoreline accretion active and inactive tidal passes through Matagorda
have resulted from man's activities. The largest Peninsula. Most of the wetlands experienced a loss
single area, amounting to approximately 7,900 in areal extent, and generally the magnitude of the
acres of new land, is the Colorado delta which loss increased from the heads of bays toward the
prograded completely across Matagorda Bay be- barriers and peninsulas; maximum loss occurred
tween 1929 and 1936. The delta owes its existence along the bayside of Matagorda Peninsula.
to removal of a log jam which retarded water and
sediment movement into Matagorda Bay. The log Natural changes in wetland areas are brought
jam extended from the town of Matagorda up the about through shoreline erosion or accretion, and
Colorado River a distance of about 46 miles. With land-surface subsidence, and by burial beneath
removal of the log jam, sediment that had been sediment and burial loss during droughts. Man-

induced changes in wetland areas have occurred as were possibly destroyed by drought. Expansion of
a result of construction of earthen dams across marsh in the Lake Austin area has resulted
marshes and tidal creeks, burial of wetlands primarily from subsidence of the Brazos-Colorado
beneath spoil or creation of new wetland areas by delta. Subsidence in the vicinity of Lake Austin has
spoil outwash, and by changing a river regime so been on the order of 0.2 to 1.0 foot during the
that there is an increase or decrease in the volume past 3 or 4 decades (Brown and others, 1975).
of fresh water and sediment delivered to bay-
margin areas. Man has contributed directly to reduction of
marsh area. Principal activities resulting in marsh
Approximately 5,800 acres of wetlands were loss were construction of dams across marshes and
lost along the bayside of Matagorda Peninsula tidal creeks and piling of spoil in marshes adjacent
between 1856 and 1957 (a rate of about 58 acres to dredge channels and boat basins. A total of
per year). During this time, approximately 2,650 1,307 acres of marsh was dammed between 1856
acres of land were lost by erosion along the bayside and 1972. These areas are (1) the Blind Bayou area
of Matagorda Peninsula. Erosion accounts for between Indianola and Magnolia Beach (about 315
almost half of the loss of marsh area; marsh area acres); (2) Huisache Creek (approximately 200
was reduced at a rate of about 26 acres per year acres); (3) Piper Lakes (301 acres); (4) the marsh
through shoreline erosion. Burial beneath washover northwest of, Piper Lakes (114 acres); and (5)
deposits, possibly in conjunction with the drought Buttermilk Slough (379 acres). A total of 1,987
of the 1950's, accounts for about 3,150 acres of acres of marsh was destroyed when buried by
marsh reduction. dredge spoil. The three principal areas are (1) south
of Oyster Lake along the north shore of Matagorda
There was a decrease in marsh area from Bay (560 acres); (2) west of Port O'Connor along
about 4,175 acres to about 2,250 acres on the Pass the north shore of Espiritu Santo Bay (1,323
Cavallo tidal delta and along the bayside of acres); and (3) the Point Comfort area along the
Matagorda Island; this was a natural change of east shore of Lavaca Bay (104 acres).
about 1,925 acres (19 acres per year). Approxi-
mately 475 acres of marsh were destroyed by Environments favorable for marsh develop-
erosion along Pass Cavallo, and approximately ment have also been created directly and indirectly
1,450 acres were destroyed by burial beneath by man's activities. The removal of some 46 miles
sediment and perhaps by drought conditions. of log jam along the lower Colorado River (be-
tween Matagorda and Wharton) changed the river
Marshes have expanded through natural regime and transported a large volume of sediment
processes in three general areas. There was an to Matagorda Bay. Rapid deltation resulted in
increase in wetland area of about 130 acres at the creation of a delta plain covering an area of about
mouth of Garcitas Creek; this is an increase of 7,910 acres of which about 4,000 acres are
about 1.3 acres per year. At the mouth of the inhabited by marsh plants. Outwash from spoil
Lavaca River, marsh area increased by. 55 acres mounds created environments favorable for marsh
(about 0.5 acres per year); marshes expanded as plants. Two areas of spoil outwash along the north
new sediment was deposited at the river mouth. shore of Matagorda Bay are now inhabited by
Flood-tidal deltas were constructed at the extreme marsh plants. These areas are (1) east of the
east end of east Matagorda Bay, at Brown Cedar Colorado delta and south of McNabb Lake (about
Cut, and at Greens Bayou. Emergent and intertidal 316 acres); and (2) west of the Colorado delta and
segments of these deltas were the sites of marsh south of Freshwater Lake (about 267 acres).
expansion; total marsh area related to these tidal
deltas in 1957 was about 380 acres (an average Future Studies
increase of about 3.8 acres per year). The Lake
Austin area has experienced an increase in wetland The study of the Gulf and bay shorelines of
area of about 835 acres; rate of increase was about the Matagorda Bay area has documented the rates
8 acres per year. Within this area (bounded on the and directions of shoreline changes and changes in
east by Caney Creek and extending to the north- marsh area. Field observations of coastal prpcesses
west boundary of the Brown Cedar Cut Area map), were made in the winter of 1971 and spring of
there were some large patches of marsh which did 1972, and an attempt was made to explain
not appear on the 1957 photomosaies; these areas shoreline changes in terms of these processes.

This study does not answer the question- present). Bay area is increased, in part, by shoreline
Why are Texas Gulf and bay shorelines eroding9 It erosion.
does, nevertheless, suggest that a major cause is a
deficit in the volume of sand being supplied to Any future program intended to mitigate the
Gulf shorelines. It also suggests that, with the effect of shoreline erosion and loss of wetlands
exception of a few local areas such as bayhead along the Texas Gulf Coast will depend upon a
deltas, the bays have been increasing in area at least thorough understanding of the sediment budget
since stillstand (some 3,000 to 2,500 years before and sediment dispersal systems.

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marine macro-invertebrates, northern Gulf of Mexico, - 1947-1968, Climatological summary, Station Port
in Shepard, F. P., Phleger, F. B., And van Andel, Lavaca, Texas: U. S. Dept. Commerce.
T. H., eds., Recent sediments, northwest Gulf of - 1948-1969, Climatological summary, Station Port
Mexico: Tulsa, Okla., Am. Assoc. Petroleum O'Connor, Texas: U. S. Dept. Commerce.
- 1958-1969, Local climatological data, Victoria,
Geologists, p. 302-337. Texas: U. S. Dept. Commerce, Weather Bureau,
Price, W. A., 1954, Dynamic environments: reconnaissance Environmental Data Service.
mapping, geologic and geomorphic, of continental - 1973, Tide tables, east coast of North and South
shelf of Gulf of Mexico: Gulf Coast Assoc. Geol. America: U. S. Dept. Commerce, Natl. Oceanic and
Socs. Trans., v. 4, p. 75-107. Atmospheric Adm., 388 p.
Scott, A. J., and Fisher, W. L, 1969, Delta systems and Wadsworth, A. H., Jr., 1941, The lower Colorado River,
deltaic, deposition, in Delta systems in the exploration Texas: Univ. Texas (Austin), Master's thesis, 61 p.
for oil and gas: Univ. Texas, Austin, Bur. Econ. 1966, Historical deltation of the Colorado River,
Geology Syllabus for Delta Colloquium, p. 10-29. Texas, in Shirley, M. L., and Rogedale, J. A., eds.,
I Hoover, R. A., and McGowen, J. H., 1969, Effects Deltas in their geologic framework: Houston, Tex.,
of Hurricane Beulah, 1967, on coastal lagoons and Houston Geol. Soc., p. 99-195.
barriers, in CastaR-ares, A. A@, and Phleger, F. B., eds., Watson, R. 1., 1968, Origin of shell beaches, Padre Island,
Lagunas Costeras, Un Simposio: Mexico City, Univer- Texas: Univ. Texas, Austin, Master's thesis, 12 1 p.
sidad Nacional Autonoma de Mexico, UNAM- -1 and Behrens, E. W., 1970, Nearshore surface
UNESCO, Mem. Simp. Internatl. Lagunas Costeras, currents, southeastern Texas Gulf coast: Univ. Texas,
p. 221-236. Port Aransas, Marine Science Institute, Contr. in
Scruton, P. C *, 1960, Delta building and the delta sequence, Marine Sci., v. 15, p. 133-143.
in Shepard, F. P., and others, Recent sediments, Wiegel, R. L., 1964, Oceanographical Engineering:
northwest Gulf of Mexico: Tulsa, Okla., Am. Assoc. Englewood Cliffs, N. J., Prentice-Hall, Inc., 532 p.
Petroleum Geologists, p. 82-102. Wilkinson, B. H., 1973, Matagorda Island-The evolution of
Seelig, W. N., And Sorensen, R. M., 1973, Investigation of a Gulf coast barrier complex: Univ. Texas, Austin,
shoreline changes at Sargent Beach, Texas: College Ph.D. dissert., 178 p.
Station, Tex., Texas A&M Univ., Sea Grant Pub. No. -, McGowen, J. H., and Lewis, C. R., in press,
TAMU-SG-72-212,153 p. Ingleside strandplain sand of the central Texas coast:
Shalowitz, A., 1964, Shore and sea boundaries: U. S. Dept. Tulsa, Okla., Am. Assoc. Petroleum Geologists
Commerce Pub. 10-1, v. 2, 749 p. Bulletin.

APPENDIX A

CHANGES IN SHORELINE FOR THE PERIOD 1856-1957

Shoreline changes for the Matagorda Bay area are whereas only one shoreline group may occur in other map
summarized in this appendix. Shorelines have been divided areas.
into three groups: (1) Gulf shoreline, (2) the bay shorelines .Map areas are referred to by number in the following
of Matagorda Peninsula and Matagorda Island, and (3) table: (1) Brown Cedar Cut Area, (2) Colorado River Area,
mainland shoreline. Erosion or accretion associated with a (3) Shell Island Reef Area, (4) Pass Cavallo Area, (5) Lavaca
particular shoreline group is shown for each map area. All Bay South Area, (6) Lavaca Bay North Area, (7)
three shoreline groups may occur in some map areas, Carancahua Bay Area, and (8) Tres Palacios Bay Area.

Map Bay Shoreline, Barriers, and
Area Gulf Shoreline Peninsulas Mainland Shoreline
Net Loss Net Loss Net Loss
Erosion Accretion or Gain Erosion Accretion or Gain Erosion Accretion or Gain

1 -1,574.87 - -1,574.87 297.51 +424.29 + 126.78 546.37 + 44.99 - 501.38
2 256.19 + 11.02 245.17 585.56 + 53.53 - 532.03 21.11 +8,674.98 +8,653.87
3 651.98 +121.21 530.77 -1,998.99 +191.75 -1,807.24 83.55 +1,179.28 +1,095.73
4 -1,118.44 + 59.68 -1,058.76 - 328.74 +294.76 33.98 -1,056.93 + 894.37 162.56
5 649.33 + 263 7 387.63
6 498.66 + 513.0 + 14.34
7 771.29 + 197.43 579.86
8 634.62 + 363.65 270.97

For. the period 1856-1957, there For the period 1856-1957, there When the Colorado delta is
was a loss of 3,409.57 acres of land was a loss of 2,246.47 acres of land included in shoreline change, there
along the Gulf shoreline of along the bay shore of Matagorda has been 7,861.54 acres of land
Matagorda Peninsula and Matagorda Peninsula and Matagorda Island. accreted to mainland shorelines. If
Island. the Colorado delta is excluded,
then there has been a loss of 138.45
acres of land along the mainland
shoreline.

APPENDIX B

CHANGES IN MARSH AREA FOR THE PERIOD 1856-1957

Areas of marsh expansion or diminution are sum- SHELL ISLAND REEF AREA (3)
marized in this appendix. Marsheson the Brown Cedar Cut,
Colorado River, Shell Island Reef, and Pass Cavallo area Matagorda Peninsula
maps have been divided into two groups based upon (1) 1856 marsh-4,255.68 acres
whether they occur on the bayside of Matagorda Peninsula 1957 marsh-926.64 acres
and Matagorda Island, or (2) whether they are associated Mainland
with the Pass Cavallo flood-tidal delta, or (3) whether they 1856 marsh-2,738.74 acres
are situated along the mainland shoreline. Marsh associated 1957 marsh-2,100.38 acres
with the Colorado delta developed since 1930 and is -
considered as a separate category. There was a decrease of 3,329.04 acres of marsh on
Matagorda Peninsula for the period 1856-1957.
BROWN CEDAR CUT AREA (1) Causes of decreases are the same as for Map Area 1.
For the same period (1856-1957), there was a loss of
Matagorda Peninsula (Caney Creek to west limit of 638.36 acres of marsh along the mainland shoreline.
the map) Change in area resulted from slopewash and dredging
1856 marsh-2,031.74 acres of the Intracoastal Waterway.
1957 marsh-1,404.83 acres
Marsh on Matagorda Peninsula was larger in 1856 PASS CAVALLO AREA (4)
than in 1957. Loss in area was 626.91 acres for the
100-year period. Loss in marsh area resulted from Matagorda Peninsula
erosion along the bay margin, and burial of marsh by Only 1856 marsh recorded-251.68 acres
washover deposits. Marsh area along the mainland Matagorda Island
increased during 1856-1957 by about 835.12 acres. 1856 marsh-205.92 acres
This area is representative of delta plain and mudflats 1957 marsh-18.30 acres
associated with the Holocene Brazos-Colorado delta Flood Tidal Delta
prior to construction of Matagorda Peninsula. Marsh 1856 marsh-4,173.31 acres
area is expanding as the area is undergoing subsidence 1957 marsh-2,244.53 acres
and is also experiencing a rise in sea level. Mainland (includes Espiritu Santo and Matagorda Bay
shorelines)
COLORADO RIVER AREA (2) 1856 marsh-1,671.38 acres
1957 marsh-800.8 acres
Matagorda Peninsula
1856 marsh-4,752.18 acres Matagorda Peninsula had virtually no marsh for the
1957 marsh-2,795.94 acres period 1957. There was a loss of 251.68 acres of
Mainland (east boundary of the map to Colorado marsh between 1856 and 1957. Matagorda Island
delta plain) (this includes the island proper, which is the area
1856 marsh-5,193.76 acres lying to the south of a line through Fish Pond and
1957 marsh-2,951.52 acres Lighthouse Cove). There has been a decrease in marsh
Colorado Delta Plain area amounting to 187.62 acres for the 100-year
Marsh in this area developed since period. Some of the loss is attributed to filling of
1930-3,628.77 acres swales on the island (this is fresh-water marsh) and
Causes of loss of marsh area on Matagorda Peninsula part to erosion along the bay margin.
amounting to 1,956.24 acres are the same as for Map
Area 1. Flood-tidal delta (from the Fish Pond-Lighthouse
Cove area northward to the Barroom Bay area). There
Between 1856-1957, there was a loss of marsh on the was a loss of marsh for the period 1856-1957 of
mainland of about 2,242.24 acres. Two natural about 1,928.78 acres. Two . factors, both natural,
processes and one man-made structure effected reduc- appear to, be the, cause of change in marsh area. First,
tion in marsh area. Slopewash along some of the erosionalong the right @bank of Pass Cavallo removed
Pleistocene cliffs and transportation of this material considerable marsh. Secondly, deposition on the
to marshes reduced marsh area. Vertical accretion and marsh islands covered a large part of the marsh and
progradation of the Colorado delta reduced marsh raised the surface of the islands sufficiently to
area, particularly to the west of the town of prevent inundation by tides.
Matagorda. Dredging of the Intracoastal Waterway
reduced marsh area by burial of marsh with spoil. Mainland marsh shows a decrease in area amounting
Since 1930, 3,628.77 acres of new marsh developed to 870.58 acres for the 100-year period. Marsh has
on the Colorado delta plain. This marsh was in- been lost through the natural process of erosion and
directly created by man's activities (a log jam was through man's activities. Man's activities, dredging of
removed in late 1929-early 1930). the Intracoastal Waterway, resulted in burial of marsh

by spoil and by creation of a barrier (spoil mounds) There has been a decrease in marsh area for the
between marshand the bay. 100-year period amounting to approximately 253.96
acres (the loss would be even more if the 1957 marsh
LAVACA BAY SOUTH AREA (5) at the head of Tres Palacios Bay were not included in
the calculations). Most of the 1856 marsh south of
Mainland shoreline Palacios and east of Turtle Point has been removed.
1856 marsh-3,759.18 acres At Turtle Point, the marsh was destroyed by dredging
1957 marsh-4,164.16 acres shell from the Turtle Point spit. East of Turtle Point,
much of the 1856 marsh was at least 5 feet above bay
For the 100-year period, there has been an increase in water level; most of this was probably fresh-water
marsh area of approximately 404.98 acres. Most of marsh.
this gain has been in the area of Powderhorn Lake
and in the area lying between Powderhorn Bayou and Summary
Indian Point Powerhorn La ke area was not com-
pletely mapped in 1856. New marsh areas were During the mid 1800's, there was a large area of salt
created between Powderhorn Bayou and Indian Point in arsh on the bay side of Matagorda Peninsula and on the
by partial filling of shallow water bodies with flood-tidal delti. associated with Pass Cavallo. Total area of
sediment during storms and by spit accretion. the 1856 salt marsh was 15,670.51 acres. In 1957, the salt
marsh acreage was only 7,390.24. There was a decrease in
LAVACA BAY NORTH AREA (6) marsh area amounting to approximately 8,280.27 acres (a
reduction in area of about 53. percent).
Change in marsh area for the period 1856-1957 is not
accurate for this area because mapping in 1856 was Total marsh area for the mainland shoreline in
incomplete along Chocolate Bayou, Placedo Creek, 1856-1859 amounted to 30,600.85 acres. Total marsh area
Agula Creek, and the Lavaca delta. for the mainland shoreline 1956-1957 amounted to
1856 marsh-3,134.56 acres 29,927.03 acres. There was a loss in marsh area for the
1957 marsh-4,296.86 acres 100-year interval of approximately 673.82 acres (or a
reduction in area of only 2 percent).
According to the calculations of acreages,for the two
vintages of marsh, there has been a net gain of about Several areas of marsh were not mapped in 1856.
1,162 acres. Accretion of 137.73. acres occurred on Marsh areas mapped on the 1956-1957 photos but not on
the Garcitas delta; this is new marsh land. Accretion 18N-1859. charts are: (1) along Placedo Creek-648.27
of 95.51 acres occurred at the mouth of the Lavaca acres; (2) along Garcitas Creek-549.88 acres; (3) along
River; this is new marsh land. At Noble Point, 127.64 Keller Creek-97.62 acres; (4) at the head of Carancahua
acres of marsh have been lost by erosion during.the Bay-551.41 acres; and (5) at the head of Tres Palacios
period 1856-1957. Bay-453.02 acres. This is a total of 2,302.18 acres. In
order for the 1856 and 1957 comparison of marsh areas to
CARANCAHUA BAY AREA (7) be valid, the. above marsh areas must be deleted from the
1956-1957 marsh area calculations. The new value is
Complete data are not..available for this map since 27,624.85 , acres. Using this value, there was a loss of
1856 marsh mapping did not extend up Keller Creek approximately 2,976 acres of marsh for the period
and Carancahua Creek. 1856-1957. Reduction in marsh acreage results primarily
185.6 marsh-3,310.74 acres from natural causes.
1957 marsh-4,239.66 acres
Several marsh areas have been dammed in order to
There was an increase of 928.92 acres in marsh area create fresh- to brackish-water ponds. Salt marsh areas that
for the period 1856-1957. have been dammed are: (1) Blind Bayou area-312.69 acres;
(2) northwest of Carancahua Pass and northeast and across
TRES PALACIOS BAY AREA (8) the bay from Port Alto-75.5 acres; (3) Piper Lakes
area7-237.95 acres; and (4) Buttermilk Slough-379.04
With exception of the small. delta at the head of Tres acres. This is,a total of 1,005.18 acres. Total marsh area lost
Palacios Bay, the 1856 marsh mapping was complete. by natural processes and man's activities is 3,981.18 acres.
1856 marsh-3,793.50 acres Man's activities are responsible for at least 2.5 percent of the
1957 marsh-3,539..54 acres reduction in marsh area.

APPENDIX C

CHARACTERISTIC PROFILES OF GULF AND MAINLAND SHORELINE FEATURES

Details of Gulf and mainland shorelines are exhibited by the following 21 profiles. The profile numbers correspond to
those shown on figure 11. Characteristics of the five shoreline types, as shown on figure 10, are depicted by these
representative profiles.

BACKBEACH BARRIER FLAT-
Shell romp Coppice mounds

xx Salt ceda
/@@elnsline
Whole shell, shell frogments,and rock fragments Shell sand Terrigenous sand
Spring tide and shell fragments
debris line
SL

Profile 23

FORE-ISLAND DUNES

Dune field

Deflation
zone

Spring tide debris line Debris line Hurricane Corla
SL
Whole shell, shell
Terrigenous sand fragments and Terrigenous sand, some shell debris in swales
and shell fragments terrigenous sand
5

Profile 30

Barrier f lot

SL N. Terrigenous sand and shell fragments
Whole shell, S@e I I
fragments, and
5' terrigenous sand

50'

Profile 37

Shell berm (Holocene)
Wave-\ - - - - - - - - - -- -
cut Str Indploin sand
Cliff c
Bay margin Swash e (Pleistocene)

with plant roots (Pleistocene)
Mottled red and gray muddy sand

5L
9,Sflchlis
Bare sand
"er I Red and brown muddy sand
lumps WrIgNII Oyster clumps (cottered oyster clumps
(dead) both alive and dead) (Pleistocene)
5-7y Terrigenous, sand with whole and fragmented shell
50'

Profile 41
At.

------ ---- --
ej@
vvov ndp
Str. l-'n s-nYd
Sh utf (PI escen.)
cl f
ne Matti ed red and gr. muddy sand
th plant roots (Pie 'stocene)

BEACH RIDGES

Seowo I I
Shell Shell ramp Salt morsh
4013 \
Swash ramp Tropical sto-
zone debris It
Po,
Whole shell, Anae
shen fragments :Sporfinasparflnod@: Halo- i 60VO sp/colo UCO :richia
and rock frog- Oyster shell :8arrich1qAsM7hAs1 plyra,5: bu-S, I Halophytes :Sparllm
soicalo, and Uco (dornlmNly :01tem-
Terri,enous son1l, ents burrows succulents) flara
with whole and Predominantly shell frogments, some whole shel I Muddy frogmented
fragmented shell
5 shell

50'

Profile 45

Form Road 2717

Soil zone
4:p -
4 CrGSSOSf1_6o and Rcngic
,:S"- Dark gray mud, caliche nodules. (Pleistocene)
Ci CcIficnassO burrows common.
Alternating, red, very fine sand, silt, and mud.
Ripple cross-lomince and parallel laminae
S.
characterize sand and silt; mud is homogeneou
4
Some injection, features and cut-outs. Bedding
C-5 IV o/ 1 --,_,@-4 inches thick. (Pleistocene)
S'L n roc n r gmentid-
r OYster sheT/
Re ents.
(Recent)

ene)
5

Profile 47

Shell romp zone

Salt marsh Gross -
Gray mud
land
Berm- with coliche
Z swo I e nodu I es
(Pleistocene)
zone

Whole and fragmented Holop es
hytes Prairie gross
she[ I
SL Well-rounded
Shell debris

Oyster clumps
Muddy sand and
sandy mud with shell

50'

Profile 48
ras So
S'
I @and
x

(prairie grosses)

Soil zone

Mottled gray and
brown mud with
C) caliche nodules

------------

Mottled red and brown
65 mud and brown, very
fine sand (Pleistocene)

,5L
Granule caliche gravel

Red mud and
51 very f ine sand
(Pleistocene)

I
50 1

Profile 51

Prairie grosses

Dark gray soil

Reddish brown,
slightly muddy,
dc@ very fine sand
'e and coarse silt;
91C@ caliche nodules
common; no
stratif ication.
Pleistocene
C:x)II

Bay margin Salt marsh Prairie grosses and
mesquite trees

Tropical storm
debris line

--Uco burrows
SL DIsfichlis and
Sorrichic
Sporlino allernIflora Granule - bearing,
Muddy, very fine sand very f ine sand
5 and muddy sand

Profile 52

Densely vegetated wor,
brush and grosses
Shell IFresh-water marsh Salt-water marsh CP Pleistocene
cotfth berm
50YOU

\Whole ",@ Sporflno spGrlmoe and Distichlis splcolo and HGlophyles
Oyster\and ",,, Juncus (clay)
,@_Ic lumps agmentedl-I Sandy mud 5
Oyster shell Clay and I113arren
reef sand
muddy sand
50'

Profile 54
Who le

r
;@tue and
c, r s f

@O Grassland

4
,ZI
-0
c;:
Soil zone
Bay _r a-y -m t Td - -
margin'\\ (Pleistocene)
SL Veneer of calliche
frogments ond
oyster shel I
DIPIGntherc
Wrl_01711.i

5 Mud

50'

Profile 5&

Shell berm Salt marsh

U
Bay N
margin f I
SL U U AEII
MonGl7thochloe lltto@alts, SGlicornia, perennis, some blue-green algae, and UCG burrows Pure stand
Oyster flat with of D151'Clyfis
green algae Marsh mud with plants@ Sandy mud sp/Co/0
Muddy in growth position,
terrigenous overlain by shell and
sand and rock fragment gravel 5
shell debris

Solt cedar
Z matte
50,

U
,5L_
Pure stand of Mononihochloe Nforolis, Sorrio@io f,,ut-
sparl(no escens, Limonium, Sporlina, SpGrIln0e,
sportmoe and 7omorix gaffica
Sandy mud
Sandy mud
ccj
a G
Lmayr

Sol, cedar
matte

Profile 56

Storm Short prairie gross Ies Shell romp Salt marsh Fresh-water
@ierms Short prairie marsh
grosses Nononlhoclo& liftorobs with
w a S;\h@ and shrubs some Schcormo perenrils Spoltino sportinae
Bo y Coarse sand to 'W
O\Y& zone Mu
margin aliche and granule-size caliche Clay Mud with some shell gravel
_@phell gravel fragments and shell debris shel I
15-L\ /gravel
@Terrigenous sand ar@
Pleistoce e granule-pebble size
mud caliche fragments 5
nd shelf debris .1
50'

Profile 57

Public Extensively developed area at Port Alto
T.il parking
@q 0
a rea
Soil zone

0
Bay ""< (9 \ - - Pleistocene muds
margin Coarse sand to pebble-size caliche,
siderite, and shell debris - ---- -
Pleistocene m

5,0

Profile 59

Erosional escarpment

@o
@O
0.

Wind-tidal
Boy margin Sporflho flat
alternifloro marsh Solt marsh

chloe
C,11,- .,d nd 'ao 5'
b"'_' hl@-14
Fine sand MAdy,fihe sand Red, moddy fine 5and and mud Red, sandy mud
50'

Profile 60

Top of
escarpment
OS04 Oyster shel I
veneer
Prairie grosses and
Bay margin chaparral
SPCI-11@7G Debris
Salt marsh /line Oyster shel I
Gllel-nlflolv
Marsh Debris\ veneer
line spGrt1nae
I /a and short
5L Ifrulescens prairie gr @s
0 Sparfina clterniflorq, Bclis mGri-
Yster C/UMPS fimq, DIsI10711s splcatq, Uca bur- 15parfinG
Sand and mud- rows, Litfol-Ina Irrorato sparfinoe
5 dy sand Sandy mud

50'

Profile 61

f
&

Prairie grosses, shrubs, OpunlIc
Soil zone

Light brown mud with callche nodules
Swash
Bay (Pleistocene)
zone
margin Veneer of granule and pebble-size callche and
oyster she I I

Mottled red and green
mud with collche nodules
(Pleistocene)

50'
@ICJO ell"

Profile 62

Oyster
_j
Bay margin \shel Salt marsh T

5L
Oyster reef Son ive oyster clumps Wholp and Sdtls marill-, 01511chlIs SPICOIC, Ond
rogmented patches of 5,oorliao olternIflora; UCG
Shell flat, muddy shell ySter she " burrows
and rock frogmen t /arld caj@' he Mud
5'j gravel rag ment gr'_@
vel

50
Lu Drainage /Oyster
Z shell
_j
Salt marsh ditch road

Predominantly DI@71iclVls spicatc, some Sallcorn1c, peremns and BG115 MGri&nG Predominantly Bcli@ marlfl@no with some
Distlcl7M5 soicara, Uco burrows

Mud Mud and sandy mud

Profile, 70

'r. co@9
w9e
0S
'@PP9
Bay Storm berms Fresh-water Erosional surface, gentle slope
margin i@ marsh

Chaparral, short grisses, some mesquile
CD Broad, upper berm vegetated
ith Opunlid, short grosses, sparllnc sportince
shrubs and sell cedar .1uncus, short grosses
SL / and shrubs Coarse sand to granule-size shell debris Pleistocene mud
01 0,. Coarse sand to pebble-
,. - size shell and caliche Shell and rock
5 fragments, swales fragment gravel
are finer grained
than ridges

Profile 72
erms

Coarse sand
sze she 11
fragm@
are

Low salt marsh High Wind-fidol High salt marsh Wash from spoil Z
salt
f lot
marsh

5L
"3@1_11n' allmifloric, Bat's Desiccation rocks, D'shchlis, Bobs, Soficornio; abundant Llco S'oamnc sporlince, SpartIna polens, some Heliotrpoatm,
Baf,s, Dsfchhs a few UcG burrows bu rrows prairie grosses
Sand withi Mud with Sand with mud drape Sand and muddy sand washed from Surface sediment is sand and muddy sand
ripple-bedl rippled spoil
for sand

Spoil mound

Z 50' Intracoastal canal
315' wide
'@Iil Ali
First appearance of Sporfmc sportince, Prosopis, shrubs, Indian gross, No veget it
sorqhostrum exottit
SL --and shrubs

Surface sediment is sand, shell, Gravel-size slabs of mud and sand.,
sandstone and coliche fragments shell of bay species abundant

Profile 7 5

Spoil mound Wash from
si@olr mound
5-1,"-- 1 .__ _ -, @.11 - rtt, Salt marsh (wash from spoil mound) Pond
Intracoastal canal \Sw.h zon
Boy @Berm Salt Uca :U ..... .
margin marsh Su I
frface veneer
st-1- pi-, "o o terrige, .1 1 1
u- sand no coliche\ ly-
Mixed sand and mud /Vereer of coich-e-- Surface sediment is sand
n gravel over spoil
bay, rock frogmen's a'no 5
sheti on the berm, salt
marsh underlain by spoil
50

Profile 76
ound
aorsn

L Spoil
5 - I

gross,
@IrbslndM.@. No @@,e,.r.
"of lpo,',n,

DATE DUE

GAYLORD No. 2333 PRINTED M U SA

36668 1