U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5067

In cooperation with the Texas Water Development Board

Characterization of Geomorphic Units in the Alluvial Valleys 

and Channels of Gulf Coastal Plain Rivers in Texas, with 

Examples from the Brazos, Sabine, and Trinity Rivers, 2010

Sand wedge

Oxbow lake

Tie channel

Flow direction

Brazos River

29°16'

29°15'45"

29°15'30"

95°34'30" 95°34'15" 95°34' 95°33'45"

Neck cutoff

BEAUREGARD PARISH

NEWTON 

COUNTY

Flow direction

Sabine River

LOUISIANA

TEXAS

30°31'30"

30°31'

93°43' 93°42'
Front cover:

Left, Example of an oxbow lake with a tie channel and sand wedge on the Brazos River near Otey, 

Texas.

Right, Example of a neck cutoff at a meander on the Sabine River near Salem, Texas.

Back cover:

Top, Example of an unvegetated cutbank on the outside of a meander bend along the Trinity River near 

Massey Lake, Texas (photograph courtesy of Webster Mangham, Trinity River Authority, 2010).

Middle, Example of a bank failure on the cutbank of the Trinity River near Massey Lake, Texas 

(photograph courtesy of Webster Mangham, Trinity River Authority, 2010).

Bottom, Simplified representation of bench deposits and an erosional ledge in a river channel.
Characterization of Geomorphic Units 

in the Alluvial Valleys and Channels of 

Gulf Coastal Plain Rivers in Texas, with 

Examples from the Brazos, Sabine, and 

Trinity Rivers, 2010

By David K. Coffman, Greg Malstaff, and Franklin T. Heitmuller

In cooperation with the Texas Water Development Board

Scientific Investigations Report 2011–5067

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Coffman, D.K., Malstaff, Greg, and Heitmuller, F.T., 2011, Characterization of geomorphic units in the alluvial valleys 

and channels of Gulf Coastal Plain rivers in Texas, with examples from the Brazos, Sabine, and Trinity Rivers, 2010: 

U.S. Geological Survey Scientific Investigations Report 2011–5067, 31 p.
iii

Acknowledgments

The authors express gratitude to Mark Wentzel, Texas Water Development Board, and 

Jonathan Phillips, University of Kentucky, for support and technical guidance during the course 

of this project.
 
v

Contents

Acknowledgments .......................................................................................................................................iii

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................3

Previous Studies ..................................................................................................................................3

Description of Study Area ..................................................................................................................4

Methods .................................................................................................................................................4

Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples 

from the Brazos, Sabine, and Trinity Rivers ................................................................................6

Valley Geomorphic Units in Lowland, Coastal Plain Environments .............................................7

Terraces ........................................................................................................................................8

Flood Plains ..................................................................................................................................9

Crevasses and Crevasse Splays ..............................................................................................9

Flood-Plain Depressions and Tie Channels ..........................................................................11

Tributaries ..................................................................................................................................11

Paleochannels ...........................................................................................................................11

Anabranches .............................................................................................................................11

Distributaries .............................................................................................................................14

Natural Levees ..........................................................................................................................14

Neck Cutoffs and Oxbow Lakes .............................................................................................15

Constructed Channels ..............................................................................................................15

Channel Geomorphic Units ..............................................................................................................15

Channel Banks ...........................................................................................................................15

Benches and Ledges ................................................................................................................18

Bank Failures .............................................................................................................................20

Point Bars and Cross-Bar Channels ......................................................................................21

Other Channel Bars ..................................................................................................................21

Exposed Bedrock ......................................................................................................................22

Pools, Runs, and Crossovers ..................................................................................................26

Summary .......................................................................................................................................................28

References Cited .........................................................................................................................................28

Figures

1. Diagram showing hierarchical relation of the mapping categories in the River 

Styles framework .........................................................................................................................2

2. Map showing Texas Gulf Coastal Plain physiographic province, including major 

river basins ...................................................................................................................................5

3. Schematic illustrating the three main components of a river system: drainage 

basin, alluvial valley, and deltaic plain .....................................................................................6

4. Subdivisions of geomorphic units in Texas Gulf Coastal Plain rivers .................................6

5. Example of fluvial terraces mapped adjacent to the flood plain and lowlands of the 

Brazos River near Courtney, Texas ...........................................................................................9
vi

6. Example of a well-developed crevasse splay deposit along the concave side of 

the northernmost meander bend of the Brazos River deltaic plain near Rosharon, 

Texas ............................................................................................................................................10

7. Example of a tie channel that conveys water and sediment from the main channel 

of the Brazos River to a flood-plain depression near Calvert, Texas ................................12

8. Example of the sinuous course of a Texas Gulf Coastal Plain river and its 

paleochannel (Brazos River near the confluence of the Brazos and Navasota 

Rivers) near Navasota, Texas ..................................................................................................13

9. Example of an anabranch on a Texas Gulf Coastal Plain river from the Brazos River 

near Marlin, Texas .....................................................................................................................14

10. Example of a neck cutoff on a Texas Gulf Coastal Plain river at a meander on the 

Sabine River near Salem, Texas ..............................................................................................16

11. Example of an oxbow lake with a tie channel and sand wedge on a Texas Gulf 

Coastal Plain river from the oxbow lake on the Brazos River near Otey,Texas ..............17

12. Example of an older, partially filled oxbow swamp north of a more recent oxbow 

lake on a Texas Gulf Coastal Plain river from the sequence of oxbow lakes on the 

Brazos River near Chappell Hill, Texas ..................................................................................18

13. Example of a constructed channel conveying flow from the main channel of the 

Brazos River to Jones Creek near Fulshear, Texas ..............................................................19

14–15. Photographs showing: 

14. Example of a depositional bank of the Trinity River with a vegetated upper 

bank and a point-bar surface below the bankfull elevation near St. Paul, 

Texas ...................................................................................................................................20

15. Example of an unvegetated cutbank on the outside of a meander bend along 

the Trinity River near Massey Lake, Texas ...................................................................20

16. Simplified representation of bench deposits and an erosional ledge. .............................20

17. Photograph showing example of a bank failure on the cutbank of the Trinity River 

near Massey Lake, Texas .........................................................................................................21

18. Example of a large point bar on the inside of a meander bend of the Brazos River 

near Satin, Texas ........................................................................................................................22

19. Example of a cross-bar channel formed across a point bar on the inside of a 

meander bend of the Brazos River near Gause, Texas .......................................................23

20. Example of a vegetated, stabilized midchannel longitudinal bar that has developed 

into an island in the channel of the Brazos River near Robinson, Texas .........................24

21. Example of a transverse (linguoid) bar and a tributary mouth bar formed 

downstream from the confluence of Bull Hide Creek and the Brazos River near 

Robinson, Texas .........................................................................................................................25

22. Exposed bedrock of Hildago Falls on the Brazos River between Washington 

County and Brazos County near Navasota, Texas ...............................................................26

23. Example of a pool-run-crossover sequence on the Brazos River near Cedar 

Springs, Texas ............................................................................................................................27

Tables

1. Valley geomorphic units of Texas Gulf Coastal Plain rivers .................................................7

2. Channel geomorphic units of Texas Gulf Coastal Plain rivers .............................................8
vii

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
 
Abstract

The U.S. Geological Survey, in cooperation with the 

Texas Water Development Board, described and characterized 

examples of geomorphic units within the channels and alluvial 

valleys of Texas Gulf Coastal Plain rivers using a geomorphic 

unit classification scale that differentiates geomorphic units 

on the basis of their location either outside or inside the river 

channel. The geomorphic properties of a river system determine the distribution and type of potential habitat both within 

and adjacent to the channel. This report characterizes the 

geomorphic units contained in the river channels and alluvial 

valleys of Texas Gulf Coastal Plain rivers in the context of 

the River Styles framework. This report is intended to help 

Texas Instream Flow Program practitioners, river managers, 

ecologists and biologists, and others interested in the geomorphology and the physical processes of the rivers of the Texas 

Gulf Coastal Plain (1) gain insights into how geomorphic 

units develop and adjust spatially and temporally, and (2) be 

able to recognize common geomorphic units from the examples cataloged in this report. Recent aerial imagery (highresolution digital orthoimagery) collected in 2008 and 2009 

were inspected by using geographic information system 

software to identify representative examples of the types of 

geomorphic units that occurred in the study area. Geomorphic 

units outside the channels of Texas Gulf Coastal Plain rivers are called “valley geomorphic units” in this report. Valley 

geomorphic units for the Texas Gulf Coastal Plain rivers 

described in this report are terraces, flood plains, crevasses and 

crevasse splays, flood-plain depressions, tie channels, tributaries, paleochannels, anabranches, distributaries, natural levees, 

neck cutoffs, oxbow lakes, and constructed channels. Channel 

geomorphic units occur in the river channel and are subject to 

frequent stresses associated with flowing water and sediment 

transport; they adjust (change) relatively quickly in response 

to short-term variations in flow. Channel geomorphic units 

described in this report are channel banks, benches and ledges, 

bank failures, point bars, cross-bar channels, channel bars, 

exposed bedrock, pools, runs, and crossovers. 

Introduction

The Texas Instream Flow Program (TIFP) is a river health 

monitoring program. It was enacted by the Texas Legislature 

in 2001 to assess the amount of water needed in rivers to maintain a sound ecological environment (Texas Instream Flow 

Program, 2008). Thompson and others (2001, p. 374–375) 

provide the following description of the role of geomorphic 

assessments in river health monitoring programs such as the 

TIFP:

In river health monitoring programs, habitat assessments provide information about the proximal 

causes of biotic impairment or early warnings of 

as yet undetected biotic impairment, particularly 

if biological monitoring is restricted to limited 

taxonomic groups. * * * Without a means of grouping rivers of equivalent physical form and function, 

habitat assessments [might] be based on erroneous 

assumptions about what constitutes a good physical 

condition. 

Geomorphological river characterizations provide a 

potential means of overcoming these shortcomings, 

and as such are integral to the TIFP. As geomorphic 

processes that operate over an array of scales determine the physical structure of rivers, geomorphological principles form a logical basis for characterizing and assessing river habitat. In the short term, 

spatial and temporal variability of instream habitat 

is determined by the interaction of channel morphology and substrate characteristics with discharge. 

Flow hydraulics are strongly influenced by channel cross-section shape, clast size, bed roughness 

and bed slope. Channel morphology and substrate 

Characterization of Geomorphic Units in the Alluvial 

Valleys and Channels of Gulf Coastal Plain Rivers in Texas, 

with Examples from the Brazos, Sabine, and Trinity Rivers, 

2010

By David K. Coffman, Greg Malstaff,1 and Franklin T. Heitmuller

1

 Texas Water Development Board.
2 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

character are in turn dependent on longer-term and 

larger-scale processes, including sediment supply 

and flood history. Geomorphic processes combined 

with valley geometry also determine the type and 

extent of floodplain habitats, such as * * * [the 

area of a floodplain between the top of the channel 

and the edge of the floodplain], wetlands and * * * 

[oxbow lakes], and the connectivity between channel 

and floodplain habitats. * * *

* * * hierarchical geomorphological schemes 

provide a framework for developing multi-scalar, 

process-based habitat surveys that allow appropriate 

comparisons between functionally similar systems. 

* * *Without geomorphological context of river 

character and behavior, habitat assessment procedures cannot meaningfully be interpreted. 

Geomorphic assessments can be done in the context of 

the River Styles framework, which was developed as a river 

management tool and utilizes concepts of fluvial geomorphology (the study of the interactions between river forms and processes over a range of space and time scales [Charlton, 2008]) 

to classify river systems by using a hierarchical structure that 

starts with the watershed and scales down to microhabitat features, such as logs and individual rocks (fig. 1) (Brierley and 

Fryirs, 2005). River styles, one of the hierarchical categories 

of the River Styles framework classification system, contains 

unique assemblages of geomorphic units, which often determine physical habitat type within the river valley and channel (Oregon State University, 2010). Thompson and others 

(2001) provide the following description of the River Styles 

framework:

The River Styles framework (Brierley and Fryirs, 

2000) is an open ended, generic geomorphological 

framework that has been applied to several coastal 

river systems in New South Wales (Brierley and others, 1999; Brierley and Fryirs, 2000). The procedure 

entails interpretation of river character and behavior 

at four interrelated scales: catchments, landscape 

units, River Styles and geomorphic units. Linkage of 

reach-scale geomorphic structure to local hydraulic 

habitat provides a basis for habitat assessment at 

smaller scales. The critical stage of analysis of River 

Styles lies in the interpretation of form-process associations of geomorphic units (pools, riffles, chute 

channels, levees, benches, bar types, backswamps, 

paleochannels, among others) which are the fundamental building blocks of river systems (Brierley, 

1996). Because associations between individual 

landforms and their formative processes are specific, 

analysis of assemblages of geomorphic units along 

a reach provides a basis to interpret river behavior 

(Brierley and Fryirs, 2000).

The River Styles framework differentiates between geomorphic units on the basis of their location either outside or 

Figure 1. Hierarchical relation of the mapping categories in the 

River Styles framework. 

LARGE SCALE DRAINAGE BASIN

LANDSCAPE UNIT

RIVER STYLE

GEOMORPHIC UNIT

HYDROLOGIC UNIT

FINE SCALE MICROHABITAT
Introduction 3

Purpose and Scope

This report characterizes the geomorphic units in the river 

channels and alluvial valleys of Texas Gulf Coastal Plain rivers using methods modified from the River Styles framework 

(Brierley and Fryirs, 2005). Examples of geomorphic units 

that exist in the alluvial valleys of the Gulf Coastal Plain rivers 

in Texas are described by using a geomorphic unit classification scheme that differentiates geomorphic units on the basis 

of their location either outside or inside the river channel. 

Geomorphic units existing outside the river channel are further 

designated as areal or linear on the basis of their general 

shape and extent. Detailed descriptions and example images 

of geomorphic units are included to help Texas Instream Flow 

Program (TIFP) practitioners, river managers, ecologists and 

biologists, and others interested in the geomorphology and 

the physical processes of the rivers of the Texas Gulf Coastal 

Plain (1) gain insights into how geomorphic units develop and 

adjust spatially and temporally, and (2) be able to recognize 

common geomorphic units from the examples cataloged in this 

report. This discussion of geomorphic units is applicable only 

to the alluvial valleys of Gulf Coastal Plain river systems in 

Texas, although the geomorphic units presented here can also 

occur within the upper watershed or deltaic plain of a Texas 

river system. Because of the relatively small scale of channel geomorphic units, few characteristics and dimensions of 

channel geomorphic units could be determined using the aerial 

imagery methods relied upon for this report.

Previous Studies

Phillips (2006, 2007a) assessed the geomorphic context, constraints, and change of the Lower Brazos River and 

Navasota River between Bryan, Tex., and the Gulf of Mexico 

and collected field data in support of a geomorphic classification scheme. Phillips (2006) discussed the importance of classifying river systems, along with the benefits of using geomorphology as a basis for classification, which he patterned after 

Parsons and others (2002). 

Phillips (2006) segmented the Brazos and Navasota 

Rivers into river reaches relevant to an instream flow assessment on the basis of the geomorphic characteristics of alluvial 

valleys and channels of the rivers. Geomorphic transition 

zones separating individual reaches were determined by using 

publicly available digital resources, including geologic maps, 

digital elevation models (DEMs), USGS topographic quadrangles, tectonic maps, soil maps, and a land resources map. 

Specific transition zone indicators included changes in surface 

geology, valley width, valley confinement, network characteristics, sinuosity, slope, paleomeanders, avulsions, and point 

bars (Phillips, 2007a). Locations along the river course that 

contained multiple transition zone indicators were considered 

important transition zones and divided reaches displaying different geomorphic character. 

Phillips (2006, 2007a) adapted the geomorphology-based 

classification scheme described in the River Styles framework 

inside the river channel—those outside the river channel are 

classified as flood-plain geomorphic units, and those inside 

the river channel are classified as channel geomorphic units. 

A similar classification scheme is used by the Oregon Department of Fish and Wildlife during stream habitat surveys for 

the State’s Aquatic Inventories Project (Moore and others, 

2010). 

Geomorphic units are defined by Brierley and Fryirs 

(2005, p. 19) as “landform-scale features” that “reflect 

formative processes that determine river structure and function.” Formed and adjusted by depositional and erosional 

processes, geomorphic units are related to streamflow, sediment transport, and other geological processes, and are 

associated with biological habitats that interactively characterize the instream and riparian ecological environment. 

Geomorphic units within a river channel (channel geomorphic 

units) adjust differently to changes in streamflow and sediment 

load compared to those located above the channel within the 

alluvial valley. As part of the physical template for aquatic 

and riparian ecosystems, the characteristics and hydrologic 

connectivity of channel and flood-plain geomorphic units 

determine structure, ecological functions, and levels of biotic 

activity of certain species (Winemiller and others, 2000; 

Li and Gelwick, 2005; Zeug and others, 2005; Dauwalter 

and others, 2007; Zeug and Winemiller, 2007). Channel 

geomorphic units are erosional or depositional features that 

occur between the banks of the river channel and typically 

undergo relatively rapid adjustment in response to short-term 

variations in flow conditions. Flood-plain geomorphic units 

are relatively large in comparison to channel geomorphic 

units, and they are in contact with the river only during infrequent, overbanking high-flow events. Flood-plain units are 

differentiated primarily by the general shape (areal or linear) 

and spatial extent. Channel geomorphic units are grouped as 

bank-attached or midchannel on the basis of their location in 

the channel during base-flow conditions (Brierley and Fryirs, 

2005). 

The fluvial geomorphic features of the Texas Gulf 

Coastal Plain river systems form the habitats to which all 

living organisms in the river ecosystem have adapted and 

become dependent (Texas Instream Flow Program, 2008). 

Identifying examples of flood-plain and channel geomorphic units allows for further insight into the patterns of 

habitat structure and the past and present processes responsible for the formation of a river’s current geomorphic setting. 

Cataloging examples of the geomorphic features of a river 

is one of the first steps in assessing the ecological status of 

the system and can provide insight toward the system’s 

expected future condition (Brierley and Fryirs, 2005; Phillips, 

2006, 2007a). Accordingly, the U.S. Geological Survey 

(USGS), in cooperation with the Texas Water Development 

Board, documented examples of geomorphic units in the 

channels and alluvial valleys of Texas Gulf Coastal Plain 

rivers. The alluvial valley is the principal region of water and 

sediment transport in a river system (Coleman and Wright, 

1971).
4 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

(Brierley and Fryirs, 2005) to define and map distinct geomorphic units in the lower Brazos River. The majority of geomorphic units listed in Phillips (2006, 2007a) were discernible 

from digital orthoimagery. In large rivers like the Brazos, 

some smaller channel geomorphic units are visible on highresolution digital orthoimagery. 

The TIFP (Texas Instream Flow Program, 2008, p. 10) 

describes the purpose of the Texas Instream Flows Program: 

“[to] perform scientific and engineering studies to determine 

flow conditions necessary to support a sound ecological 

environment in the river basins of Texas,” as established under 

the authority of Texas Senate Bill 2, enacted by the 77th Texas 

legislature in 2001. The TIFP has decided to apply a hierarchical approach to geomorphological river classification and 

characterization, adapted from the River Styles framework 

(Brierley and Fryirs, 2005), to separate study watersheds 

into mapped units of manageable scales for ecological field 

investigations. Phillips (2006; 2007a, b, d; 2008) and Phillips 

and Slattery (2007) have developed methods to define and map 

river styles in Texas Gulf Coastal Plain rivers. 

Description of Study Area

The Gulf Coastal Plain physiographic province of 

Texas includes three subprovinces: the Coastal Prairies, the 

Interior Coastal Plains, and the Blackland Prairies. The Gulf 

Coastal Plain physiographic province covers approximately 

one-third of the land surface of Texas, stretching from the 

Texas-Louisiana state line southwest along the Gulf of Mexico 

coast to the Rio Grande River (fig. 2). The climate of the 

Gulf Coastal Plain is characterized as subtropical to humid 

subtropical, with average annual rainfall exceeding 56 inches 

near the Texas-Louisiana state border, ranging to semiarid 

or arid, with approximately 20 inches annual rainfall in the 

Rio Grande River valley (Texas Forest Service, 2008). Most 

rainfall results from low-pressure systems with circulating air 

patterns bringing moisture northward from the Gulf of Mexico 

or from convective thunderstorms that form frequently in the 

afternoons between May and September. Hurricanes, tropical 

storms, and tropical depressions occasionally make landfall 

along the Texas coast and bring large amounts of rainfall, 

resulting in localized or widespread flooding (Bomar, 2011). 

Vegetation of the Interior Coastal Plains ranges from pine and 

hardwood forests in the east to chaparral brush and grasses 

in the southwest. The Coastal Prairies are grass-covered 

expanses of low-slope terrain. The Blackland Prairies contain 

rich, black clay soils that have weathered from underlying 

Cretaceous sandstone, limestone, and marl (loose, earthy 

deposits of clay and calcium carbonate [Neuendorf and others, 

2005]). Since the settlement of Texas, the Blackland Prairies 

have been cleared of most native vegetation for agricultural 

purposes. The sand, silt, and clay substrate in this province 

is almost entirely deltaic in origin and was transported by 

rivers as suspended and bedload sediment toward the coast 

from the elevated Texas interior over geologic time (Wermund, 

1996). 

The Gulf Coastal Plain of Texas contains several major 

rivers between the Texas-Mexico border and the TexasLouisiana state border; from west to east, they are the Nueces, 

San Antonio, Guadalupe, Lavaca, Colorado, Brazos, San 

Jacinto, Trinity, Neches, and Sabine (fig. 2). The rivers of the 

Texas Gulf Coastal Plain are alluvial systems; alluvial river 

systems continually adjust their geometry according to differential rates of streamflow (discharge) and sediment load, 

and these changes in geometry are expressed in a variety of 

fluvial features which can be grouped as geomorphic units 

(Heitmuller and Greene, 2009; Knighton, 1998). Each of the 

large rivers of the Texas Gulf Coastal Plain has a drainage 

basin, alluvial valley, deltaic plain, river delta, and receiving basin, in one form or another. The alluvial valley is the 

principal region of water and sediment transport in the fluvial 

system (Coleman and Wright, 1971). The alluvial valley 

accepts flow and sediment discharges from the upper drainage basin and tributary watersheds and transports them to the 

deltaic plain, river delta, and receiving basin (fig. 3). 

The large rivers of the Texas Gulf Coastal Plain occupy 

broad alluvial valleys shaped by the rivers over tens of millions of years; they contain a variety of fluvial geomorphic 

features that reflect historic and contemporary hydrologic conditions (Phillips, 2006, 2007a). Human activities in the past 

120 years have altered the amount of sediment and water in 

all the large rivers of the Texas Gulf Coastal Plain, resulting in 

accelerated changes in the geomorphic characteristics of these 

rivers (Heitmuller and Green, 2009; Phillips, 2007a, c). 

Methods

A modified version of the geomorphic unit scale of the 

River Styles framework (Brierley and Fryirs, 2005) was used 

to distinguish geomorphic units on the basis of their location 

either outside or inside the channel. The River Styles framework differentiates geomorphic units as either flood-plain 

geomorphic units (geomorphic units found outside the channel) or channel geomorphic units (geomorphic units that occur 

inside the channel). During this study, the authors determined 

that geomorphic units outside the channel did not always 

occur on the flood plain, but occasionally at elevations higher 

than the flood-plain surface yet still within the alluvial valley. 

For this reason, geomorphic units that are located outside the 

channels of Texas Gulf Coastal Plain rivers are called “valley geomorphic units” in this report. Valley geomorphic units 

outside the channel are further classified as areal or linear on 

the basis of their general shape and extent. Channel geomorphic units can be further classified as midchannel or bankattached (fig. 4), but this designation can only be made on a 

case-by-case basis under base-flow conditions. Information 

gained from literature reviews was used to help describe the 

various geomorphic units. Descriptions are provided of how 

each geomorphic unit develops and adjusts spatially and 

temporally. Visual analysis of the occurrence of geomorphic 

units in the alluvial valleys and channels of Gulf Coastal 

Plain rivers in Texas was done by using recent aerial imagery 
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Trinity

Basin

Brazos

Basin

Neches

Basin

Sabine

Basin

Colorado

Basin

San Jacinto

Basin

Lavaca

Basin

Guadalupe

Basin San 

Antonio

Basin

Nueces

Basin

GULF OF MEXICO

LOUISIANA

UNITED STATES

MEXICO

TEXAS

ARKANSAS

OKLAHOMA

Robinson

Salem

Hill

Chappell

Fulshear

Navasota

Courtney

Cedar

Springs

Bryan

Washington

Otey

Gause

Tomlinson Hill

Satin

Millican

Calvert

Massey 

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St. Paul

Marlin

Rosharon

Sabine

River

Sabine River

River

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San

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River Brazos

River Colorado

River

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River Trinity

River

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River

San

Nueces

River Lavaca

River

Medina

G C O A A L P

Base from U.S. Geological Survey digital data,

Albers Equal-Area projection

North American Datum of 1983

EXPLANATION

Physiographic subprovince

Blackland Prairies

Interior Coastal Plains

Coastal Prairies

Basin boundary

Geomorphic unit

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94°

96°

98°

100°

34°

32°

30°

28°

26°

0 5 25 0 75 100 MILES

0 5 25 0 75 100 KILOMETERS

TEXAS

Study area Gulf Coastal Plain

OKLAHOMA ARKANSAS

MEXICO

UNITED 

STATES

LOUISIANA

GULF OF 

MEXICO

Physiographic Subprovinces of Texas from 

Bureau of Economic Geology, 1996.

Basins modified from Texas Water

Development Board, 2002

Introduction 5

Figure 2. Texas Gulf Coastal Plain physiographic province, including major river basins. 
6 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

(high-resolution digital orthoimagery) collected in 2008 and 

2009 through the National Agriculture Imagery Program (U.S. 

Department of Agriculture, Farm Service Agency, 2009). Both 

natural color and color infrared imagery were used at 0.5- to 

1-meter resolution. The aerial photographs were thoroughly 

inspected by using geographic information system (GIS) software to identify representative examples for geomorphic units 

that occurred in the study area. 

Characterization of Geomorphic Units 

in Texas Gulf Coastal Plain Rivers, with 

Examples from the Brazos, Sabine, and 

Trinity Rivers

Fluvial geomorphic units of the alluvial Texas Gulf 

Coastal Plain rivers occur either outside or inside the river 

channel and are separated into two categories: valley geomorphic units and channel geomorphic units. Flow-related disturbances occur less often for those units found above bankfull 

stage (valley geomorphic units) than for those found below 

bankfull stage (channel geomorphic units). The bankfull stage 

of a river often is difficult to determine (Leopold and others, 

1964). It is identified on the basis of a reach-scale investigation, because localized variability in channel bank conditions 

and natural levee elevations will determine the stage at which 

water inundates the flood plain (Leopold, 1994).

Relatively large valley geomorphic units (size measured 

in miles) exist in flood-plain and valley environments. They 

typically require considerable time to form and adjust, on the 

order of decades to hundreds of years, and only during overbanking high-flow events (Brierley and Fryirs, 2005). Valley 

geomorphic units were grouped into two broad types on the 

basis of their shape when viewed on aerial imagery: areal or 

linear (table 1). Areal valley geomorphic units covered large 

areas of the valley floor, and linear valley geomorphic units 

were those that follow active or abandoned channel courses. 

Relatively small channel geomorphic units (size measured in feet) exist in the channel. Channel geomorphic units 

DRAINAGE

BASIN

UPSTREAM

DOWNSTREAM

ALLUVIAL

VALLEY

DELTAIC

PLAIN

RECEIVING

BASIN

Figure 3. Schematic illustrating the three main components of 

a river system: drainage basin, alluvial valley, and deltaic plain 

(modified from Coleman and Wright, 1971).

GEOMORPHIC UNIT

VALLEY GEOMORPHIC UNIT CHANNEL GEOMORPHIC UNIT

Bankattached Areal Linear Midchannel

Figure 4. Subdivisions of geomorphic units in Texas Gulf Coastal Plain rivers. 
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 7

were grouped into two types on the basis of their location in 

the channel: bank-attached or midchannel (table 2). Channel 

geomorphic units adjust during a wide range of in-channel 

flows and, therefore, adjust more frequently than valley geomorphic units (Brierley and Fryirs, 2005). 

Valley Geomorphic Units in Lowland, Coastal 

Plain Environments

Valley geomorphic units of lowland, coastal plain rivers are typically larger than channel geomorphic units, and 

their typical lateral dimensions occur on the order of 1,000 

to 10,000 feet. Many valley geomorphic units can be identified and mapped using aerial imagery and later verified in the 

field, although this becomes increasingly difficult in forested 

environments. For lowland, coastal plain rivers, GIS applications and digital aerial imagery can be used to describe the 

characteristics and measure the dimensions of valley geomorphic units. Field investigations of valley geomorphic units are 

useful for accomplishing several important tasks including, for 

example, accurately delineating geomorphic units, determining unit composition (for example, particle-size distribution 

and composition), identifying points of hydrologic connectivity to the main channel, and assessing localized variations in 

geomorphic unit form and vegetation characteristics. 

Valley geomorphic units do not always occur on the 

hydrologic flood plain. The hydrologic flood plain, also 

known as the active flood plain, is a level area adjacent to a 

river channel, constructed by the river in the present climate 

and overflowed during moderate flow events (Leopold, 1994). 

Fluvial terraces, for example, are typically located at higher 

elevations than the topographic flood plain. The topographic 

flood plain refers to the area of land inundated by a flood of a 

specific magnitude and frequency, such as the 100-year flood 

(Federal Interagency Stream Restoration Working Group, 

1998). A 100-year flood has 1 percent chance of occurring in 

a given year, referred to as the 1-percent annual exceedance 

probability (Holmes and Dinicola, 2010). Valley geomorphic 

units proximal to the channel are typically hydrologically connected to the channel at lower flood stages than are those units 

farther from the channel. The authors determined that valley 

geomorphic units could be further classified as “active” or 

“inactive” on the basis of their frequency of connectivity to the 

main river channel by flowing water. Classifying valley geomorphic units as “active” or “inactive” was beyond the scope 

of this report. This differentiation would be made on a caseby-case basis and would depend on the location of the specific 

valley geomorphic unit being described. The frequency of 

inundation of a valley geomorphic unit and, therefore, its 

activity (“active” or “inactive”), is dependent on its elevation 

relative to the top of the channel bank (“high” or “low”) and 

Table 1. Valley geomorphic units of Texas Gulf Coastal Plain rivers.

Unit name Shape Description

Fluvial terrace Areal Abandoned flood-plain surface, activity dependent on distance from and elevation above top of 

channel banks.

Topographic flood plain Areal Land surface inundated by a flood of a specific magnitude and frequency (100-year flood1, for 

example).

Hydrologic flood plain Areal Level area adjacent to a river channel, constructed by the river in the present climate and overflowed 

during moderate flow events.

Crevasse and crevasse splay Areal Narrow channel scoured though natural levees and subsequent deposits.

Flood-plain depression Areal Lowest point on valley floor; activity dependent on distance from and elevation above top of 

channel banks.

Tie channel Linear Conveys water and sediment from the main channel to floodplain depressions during high-flow 

events; flow direction in tie channel can reverse as flood flows in the main channel recede.

Tributary Linear Supplies water and sediment to the main channel and might contain its own channel geomorphic 

units. 

Paloechannel Linear Abandoned channel course; occasionally occupied by tributaries, anabranches, or distributaries.

Anabranch Linear Conveys water and sediment away from main channel and back to main channel downvalley.

Distributary Linear Conveys water and sediment away from the channel to distal parts of the valley floor.

Natural levee Linear Natural deposit of coarse-grained material deposited adjacent to channel.

Neck cutoff Linear Intersection of two meanders, shortens channel, increases slope.

Oxbow lake Linear Result of a neck cutoff. If the arms of the cut-off meander fill with sediment, an oxbow lake can 

form.

Constructed channel Linear Conveys water from channel for irrigation or water supply or to the channel as drainage.

1

 A flood with a 1-percent annual exceedence probability (Holmes and Dinicola, 2010). 
8 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

its distance from the top of the channel bank (“proximal” or 

“distal”). For the Texas Gulf Coastal Plain rivers, valley geomorphic units at elevations closest to the top-of-bank elevation 

and those located least distant from the top of the channel 

bank are subject to the most frequent fluvial stresses associated with flowing water and sediment transport.

Terraces 

Sumner (2000, p. E-26), describing other investigators’ 

findings, noted that “terraces are defined as a part of the fluvial system that no longer actively experiences fluvial modification (Ruhe, 1975), [that is] no longer aggrading.” Terraces 

are commonly encountered in the alluvial valleys of Texas 

Gulf Coastal Plain rivers and often take the form of abandoned 

flood plains representative of previous hydrologic and sedimentary environments. Terraces indicate periods of relative 

vertical stability, allowing flood-plain formation, followed by 

a period of channel incision and flood-plain abandonment. 

The incised channel eventually forms a new flood plain, with 

the abandoned flood plain becoming a terrace, and the cycle 

continues (Leopold and others, 1964). Abandoned flood 

plains formed over relatively long geomorphic timescales, 

on the order of hundreds to thousands of years, might have 

been abandoned comparatively rapidly (on the order of years 

or decades) as a result of channel incision (Leopold, 1994). 

Factors controlling the amount of channel incision (downcutting or deepening of the channel resulting from erosion of 

the channel bottom) include tectonic uplift, base level (sea 

level) fluctuation, or changes in flow regime resulting from 

climate variability or human activities (Phillips, 2007a, c). 

As topographic features, terraces generally are flat to gently 

sloping and are bounded by an elevated valley or terrace 

wall and a terrace bank that slopes down to the lower active 

flood plain or a lower terrace. In some cases, lower terraces 

are partially buried by modern fluvial sediment deposits. As 

a sedimentary body, terraces are typically characterized by a 

fining-upward sequence of deposited sediment capped by finer 

overbank sediment. Some terraces display no evidence of fine 

overbank sediment (Blum and Straffin, 2001). Terraces commonly are oriented parallel with the river valley but also can 

exist as isolated “islands” surrounded by the active flood plain 

or river channel (Leopold and others, 1964). Large terraces 

near the coastal margin often extend laterally considerable 

distances away from the flood plain. Although not considered 

part of the flood plain, terraces can be subject to inundation 

and overbank sedimentation during infrequent high-magnitude 

floods. 

Fluvial terraces are composed of fluvial sediment 

deposits of different ages and can sometimes be distinguished 

by soil development. In tilled agricultural areas, fluvial terraces can be distinguished from active flood plains in the 

Texas Gulf Coastal Plain on the basis of their appearance in 

aerial photographs (for example, reddish color associated 

with prolonged oxidation). Fluvial terrace deposits commonly 

occur at various elevations above the active river channel, 

and changes in vegetation can indicate a terrace boundary 

(fig. 5). High-resolution DEMs derived from ground surveys 

or airborne-lidar data can be used successfully to identify 

fluvial terraces; soil survey maps are also helpful for this purpose. Precise identification of fluvial terrace deposits requires 

field verification, including topographic surveys and collection of sediment samples for compositional analysis and age 

determination (Blum, 1994).

Table 2. Channel geomorphic units of Texas Gulf Coastal Plain rivers.

Unit name Location Description

Channel bank Bank-attached Erosional or depositional indicated by presence or absence of vegetation.

Bench Bank-attached Depositional feature within the channel occuring below the bankfull stage, 

commonly exists on convex side of meander bend.

Ledge Bank-attached Erosional scarp within the channel occuring below the bankfull stage, composed of 

erosion-resistant materials.

Bank failure Bank-attached Indicates bank instability and potential channel migration.

Point bar Bank-attached Depositional feature on inside of meander bend.

Cross-bar channel Bank-attached Bifurcates point bar during high flows.

Channel bar Bank-attached and/or midchannel Depositional feature on channel bottoms, includes longitudinal bars, transverse 

bars (also referred to as linguoid bars), tributary mouth bars, and so forth. 

Exposed bedrock Bank-attached and/or midchannel Exposure of erosion-resistant bedrock alters flow conditions.

Pool Midchannel Section of channel with deepest water and slowest flow velocities. Commonly 

occurs on the cutbank side of a meander bend.

Run Midchannel Moderate depth and velocity connecting pools and crossovers.

Crossover Midchannel Shallow, fast-flow pool, typically at meander inflection point.
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 9

Flood Plains 

In some instances, the alluvial valley floor does not 

contain a specific geomorphic unit; in these instances, we refer 

to the alluvial valley floor as the flood plain. There are two 

different types of flood plains. The topographic flood plain is 

the area of land inundated by a flood of a specific magnitude 

and frequency, such as the 100-year flood (Federal Interagency 

Stream Restoration Working Group, 1998). The topographic 

flood plain is of primary interest to engineers and flood-plain 

managers concerned with flooding in developed areas. 

The second type of flood plain is the hydrologic flood 

plain. Schumm and Lichty (1963) and Burkham (1972) 

suggest that hydrologic flood plains are present only along 

natural channels not affected by human activity. Hydrologic 

flood plains are not present in all reaches of some Texas Gulf 

Coastal Plain rivers. The incised nature of many of the Texas 

Gulf Coastal Plain rivers, whether naturally incised or incised 

in response to human activity in the watershed, allows connectivity with the flood plain only during relatively high-flow 

events.

Crevasses and Crevasse Splays

A crevasse is a local breach in a natural levee. Crevasses 

can result in crevasse channels, crevasse splays, or a combination of the two. Narrow flood-scoured crevasse channels 

are capable of transmitting flow and sediment away from the 

main river channel at high-flow stages (Brierley and Fryirs, 

2005). Crevasse splay deposits have been found along the 

outside of meander bends in Texas Gulf Coastal Plain rivers (fig. 6). Crevasse channels commonly cut through natural 

Figure 5. Example of fluvial terraces mapped adjacent to the flood plain and lowlands of the Brazos River near Courtney, Texas.

Terrace

Terrace

Flood plain and lowland

Courtney

WASHINGTON

COUNTY

WALLER COUNTY

GRIMES COUNTY

Brazos River

WALLER COUNTY

Flow 

 direction

96°6' 96°4'

30°15'15"

30°13'30"

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2009, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

0 0.25 0.75 0.5 1 MILE

0 0.5 0.25 0.75 1 KILOMETER
10 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

levees on the outside of meander bends, and can result in a 

considerable amount of flood-plain deposition. Small, shallow 

channels sometimes radiate away from the crevasse location 

(Saucier, 1994), transporting flow and sediment during floods. 

Deposition results when the sediment-laden water leaves the 

main channel and spreads out in a fan-like pattern on the distal 

side of the levee. Crevasse channels can be distinguished 

from local tributaries and gullies by their downward slope 

away from and perpendicular to the levee. Persistent crevasse 

channels can become somewhat permanent distributaries 

(Jackson, 1997).

Crevasse splays are accumulations of overbank sediment 

(sand and silt) that are the result of the rapid deposition from 

laterally spreading and decelerating flows (fig. 6) and exhibit 

a fining-outward (away from the levee) sequence. Farrell 

(1987) shows that crevasse splay sedimentary characteristics 

are mostly associated with sheet flow, and sequential deposits 

typically reflect the following progression: (1) the onset of 

Figure 6. Example of a well-developed crevasse splay deposit along the concave side of the northernmost meander bend of the 

Brazos River deltaic plain near Rosharon, Texas (from Heitmuller and Greene, 2009). 

Crevasse splay

Flow 

 direction
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 11

crevassing, (2) full development, and (3) crevasse abandonment. Crevasses and crevasse splays have the potential to 

occur along Texas Gulf Coastal Plain rivers anywhere a natural 

levee is present. 

Flood-Plain Depressions and Tie Channels

Flood-plain depressions are the lowest areas of the valley floor, located between elevated natural levees and the 

valley wall or terrace, where water and sediment are stored 

during and after overbanking flow events. Water and sediment 

are sometimes transferred to flood-plain depressions from the 

main river channel during relatively high-flow events through 

tie channels (fig. 7). High rates of sediment deposition are 

typical in flood-plain depressions (Schenk and Hupp, 2010; 

Walling and He, 1997). When floodwater moves away from 

the main channel into the depression, it loses energy and is 

only capable of carrying a suspended load. As water in the 

depression infiltrates to the subsurface or evaporates, fine sediment and organic matter are deposited (Walling and He, 1997).

A wide variety of topographic lows occur in alluvial flood 

plains, but two are particularly important in Texas Coastal 

Plain rivers. One type of topographic low is prominent meander-scar depressions that occur in conjunction with the fluvial 

terraces, particularly in the Trinity, Neches, and Sabine River 

systems. These reflect previous higher discharge regimes and 

are appreciably larger than active or abandoned meanders 

associated with the modern river. These meander-scar depressions are often evident in crescentic scallops in the valley wall 

(similar in shape and form to present-day cutbanks) despite 

being distal from the channel, and are valley geomorphic 

units which are frequently flooded (sometimes by local runoff 

or tributaries). Swales, another type of topographic low, are 

somewhat linear depressions that occur between old, elevated 

point-bar deposits as a result of lateral channel migration, in 

a pattern typically called ridge-and-swale topography (Smith, 

1996; Brierley and Fryirs, 2005). 

Low-energy environments such as the flood-plain depressions found in parts of the Texas Gulf Coastal Plain rivers can 

become colonized by wetland and swamp vegetation that traps 

additional fine- grained sediments delivered by flood waters. 

In addition, flood-plain depressions can be associated with 

abandoned channels (discussed in the Paleochannels section of 

this report), artificial impoundments, and borrow or gravel pits 

(Wilcox, 1993). 

Tributaries

In Texas Gulf Coastal Plain rivers, tributaries draining 

upland areas adjacent to the main river valley can extend for 

considerable distances through the valley before joining the 

main river at a confluence. The upland tributaries thus sustain 

their own channel geomorphic units, as well as some of their 

own valley geomorphic units, within the alluvial valleys 

of the larger rivers. In lowland alluvial valleys, tributaries 

can run parallel to the main river for considerable distances 

where they occupy former channels of the main river. Phillips 

(2006, 2007a) noted this occurrence in the Brazos alluvial 

valley where the Navasota River occupies a Brazos River 

paleochannel. 

Meander migration occasionally allows main river 

channels to capture tributary channels. This truncation can 

result in rapid changes in the lower tributary and at the tributary confluence caused by the increased hydraulic slope of 

the tributary channel. Tributary confluences are typically 

areas of high geomorphic and hydraulic unit diversity (Rice 

and others, 2001; Torgersen and others, 2008; Best, 1986, 

1988; Rhoads, 1987; Richards, 1980; Benda and others, 2003, 

2004a, 4b). 

Paleochannels

Abandoned channel courses, or paleochannels, are former 

positions of the main river that extend over one meander wavelength in distance (Brierley and Fryirs, 2005) and occasionally 

extend for considerable lengths, sometimes tens of miles in 

the alluvial valleys of Texas Gulf Coastal Plain rivers (fig. 8). 

The exception is cutoffs of individual bends or meanders, 

discussed separately below. For meandering rivers, paleochannels typically include multiple meander bends. Paleochannels 

either convey flood flow and sediment load down the valley, 

or are infilled with overbank sediment in a fining-upward 

sequence and found on the land surface or preserved as part 

of the geologic record (Saucier, 1994). Similar to the main 

river, abandoned channels can intercept and convey drainage 

and sediment load from upland-draining tributaries and local 

flood-plain sources. In some cases, older paleochannels of the 

Brazos River are reoccupied by tributaries (Phillips, 2006).

Paleochannels can exist in several different states or conditions, with several different varieties sometimes occurring 

along the same length of abandoned channel (Phillips, 2008). 

Common paleochannel types in the Gulf Coastal Plain include 

the following:

•	 anabranches and distributaries (discussed separately 

below)

•	 sloughs—these contain water at all times (except perhaps during extreme droughts) but are not connected 

to the active channel and do not convey flow except 

during floods 

•	 semiactive anabranches or flood channels—these 

channels do not convey flow under normal or low-flow 

conditions, but are activated at high flows, often at subbankfull levels

•	 infilled paleochannels—these are shallow linear 

depressions inundated only during floods

Anabranches

Anabranches are channels that connect to the main river 

at their upstream and downstream end and are separated from 
12 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Figure 7. Example of a tie channel that conveys water and sediment from the main channel of the Brazos River to a flood-plain 

depression near Calvert, Texas.

ROBERTSON 

COUNTY

MILAM 

COUNTY

Flood-plain 

depression

Tie channel Flow 

 direction

Brazos River

96°46' 96°45' 96°44'

31°2'

31°1'

31°

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

0 0.25 0.5 0.75 1 KILOMETER

0 0.25 0.5 0.75 1 MILE

Brazos

 River

Calvert

ROBERTSON 

COUNTY

MILAM 

COUNTY

LOCATION MAP
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 13

the main river channel(s) and one another by vegetated floodplain islands (fig. 9). They are typically active at all but the 

lowest flows and commonly are formed by avulsion processes 

that are promoted by aggradation and/or benching of sediment 

in the main channel (Makaske, 2001; Phillips, 2009). Wende 

and Nanson (1998, p. 205) provide the following description 

of anabranching rivers:

Anabranching rivers form a highly diverse group of 

streams that, despite their occurrence worldwide, 

remain the last major category of alluvial system to 

be thoroughly described and explained (Nanson and 

Knighton, 1996). They consist of multiple channels separated by vegetated semipermanent alluvial 

islands or ridges, and provide a marked contrast to 

most rivers that flow in single channels in order to 

maintain an efficient conveyance of water and sediment. It is well understood that alluvial rivers generally respond to changes in environmental conditions 

by adjusting their hydraulic geometry and planform 

with the result that river morphology can vary across 

a wide spectrum. Anabranching rivers are no exception and as a consequence they add greatly to the 

diversity of river systems.

In fine-grained, lowland, coastal plain environments, 

a network of connected anabranches and the main river 

channel(s) is referred to as an anastomosing river system 

WASHINGTON COUNTY

BRAZOS

COUNTY

GRIMES

COUNTY

BRAZOS

COUNTY

GRIMES

COUNTY

Flow 

direction

Paleochannel

Flow direction

Flow direction

Navasota

Brazos River

NavasotaRiver

96°12'30" 96°10' 96°7'30" 30°27'

30°25'30"

30°24'

30°22'30"

30°21'

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

0 0.5 1 KILOMETER

0 0.5 1 MILE

Figure 8. Example of the sinuous course of a Texas Gulf Coastal Plain river and its paleochannel (Brazos River near the confluence of 

the Brazos and Navasota Rivers) near Navasota, Texas. 
14 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

(Nanson and Knighton, 1996). Phillips (2006, 2007a) 

describes the reach of the Navasota River between Lake 

Limestone and Navasota, Tex., as an anastomosing river with 

high channel-flood plain connectivity. 

Distributaries

Distributaries commonly occur in deltaic plain environments and can develop when a midchannel bar divides flow in 

an actively enlarging delta. A different type of distributary can 

exist in nondeltaic parts of the valley when a crevasse is persistent enough to scour a channel capable of conveying flow 

and sediment away from the main river during even moderate 

and low flows (Saucier, 1994). These are referred to as “tie 

channels” (fig. 7). Distributaries can range from very small to 

near the size of the main channel. When they become as large 

as the main river channel and can sustain a steeper slope than 

the main channel, a bar will form in the old main channel, 

causing it to convey a smaller proportion of flow and sediment 

load as the distributary channel takes over as the main channel. 

Natural Levees

Natural levees are slightly elevated ridges, displaying a 

fining-upward sequence of mostly sand and silt that gently 

slope away from the river channel to lower areas of the flood 

plain (Brierley and Fryirs, 2005). They occur as a result of 

overbank flooding and the general decrease in flow velocity and stream power with distance away from the channel, 

promoting deposition of suspended sediment. Natural levees 

are best developed along cutbanks and generally extend 

farther into the flood plain at meander bends of low curvature 

(Hudson and Heitmuller, 2003). Relatively coarse-grained 

natural levees tend to be steeper and narrower than relatively 

Figure 9. Example of an anabranch on a Texas Gulf Coastal Plain river from the Brazos River near Marlin, Texas. 

Anabranch occupying 

paleochannel Marlin

Flow direction

Brazos River

Flow 

direction

31°18'15"

31°16'30"

31°14'45"

96°58' 96°56' 96°54'

012 0.5 1.5 MILES

0 1 0.5 1.5 KILOMETERS 2

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 15

fine-grained natural levees (Saucier, 1994). As natural levees 

grow in height and width over time, they gradually establish 

the spatial limits over which the river migrates by meandering 

processes, referred to as a meander belt. 

Natural levees can be difficult to identify from aerial 

imagery for some Texas Gulf Coastal Plain rivers. Various 

clues can help the practitioner determine if natural levee 

deposits are well developed or underdeveloped. Because natural levees are coarse-grained relative to surrounding overbank 

deposits, they are less saturated by water and often appear 

lighter in tone on aerial imagery when vegetation is absent. 

When present, different types of vegetation might preferentially become established on the well-drained levee deposits 

when compared to the surrounding areas. Artificial levees are 

similar in form to natural levees but have been constructed 

along river courses by humans for flood protection purposes. 

They are easily distinguished from natural levees by their spatial extent, material composition, and uniform shape. 

Neck Cutoffs and Oxbow Lakes

Neck (meander) cutoffs are somewhat different from 

abandoned channel courses in that they only affect one 

meander bend instead of an entire length of channel. Neck 

cutoffs form when the cutbanks of two actively eroding 

meander bends intersect, initially leaving behind an oxbow 

(fig. 10). Neck cutoffs result in decreased sinuosity and 

increased channel slope (Brierley and Fryirs, 2005). The 

arms of a cutoff meander (oxbow) first begin to fill in with 

relatively coarse sand near the active river channel, termed a 

sand wedge (Saucier, 1994). This infilling typically occurs 

in the downstream arm of the oxbow. As the river migrates 

farther away from the oxbow, a small tie channel can form 

in the upstream arm of the oxbow and convey local drainage 

from the oxbow to the river and floodwater from the river to 

the oxbow (fig. 11). Over time, fine-grained overbank sediment settles into the tie channel, resulting in the formation of 

an oxbow lake. An oxbow becomes an oxbow lake once it is 

able to retain water after flood flows recede. If an oxbow lake 

becomes completely infilled with fine-grained sediments, it 

will serve as an erosion-resistant, cohesive clay unit that will 

slow channel migration if the river switches back to that location. The longevity of an oxbow lake depends on how rapidly 

and how far the active river migrates away; the farther the river 

is located from the oxbow, the less frequently it is inundated 

by flood flows and sediment, and the longer it will take to fill 

with sediment. Sediment transported to the oxbow from surrounding hill slopes by overland flow can continue to fill the 

lake until it is virtually indistinguishable from the surrounding 

terrain.

There are generally five stages in the lifecycle of an 

oxbow (Saucier, 1994). They are as follows:

1. Flow-through cutoff meander (oxbow) with both ends still 

connected to the main channel at all but the lowest flows

2. Formation of a sand wedge and tie channel

3. Filling of the tie channel and formation of an oxbow lake 

4. Filling of the oxbow lake resulting in an oxbow swamp

5. Completely filled oxbow swamp evident as a meander 

scar (infilled depression). 

Oxbows and oxbow lakes can be easily mapped from 

aerial imagery (fig. 12). For lowland, coastal plain, meandering rivers, oxbow lakes usually retain water, but their beds 

can be exposed during abnormally dry conditions. Neck cutoffs are commonly short-lived because they transition to the 

main river channel as sediment is deposited in both arms of the 

oxbow. 

Constructed Channels

A constructed channel is considered a valley geomorphic 

unit when it is used to link the main channel to another part of 

the alluvial surface (fig. 13). Constructed channels have been 

used for navigation, flood control, and agriculture (Brierley 

and Fryirs, 2005). Constructed channels have been used for 

various purposes on the major rivers of the Texas Gulf Coastal 

Plain. For example, along the Brazos River, constructed channels have been used to drain wetlands, so they could be used 

for agricultural purposes, and to speed the drainage of existing 

low-lying agricultural areas. Constructed channels have also 

been used to straighten river courses in attempts to reduce the 

affects of floods (U.S. Department of Agriculture, Natural 

Resources Conservation Service, 2007). Constructed channels 

can result in a number of negative effects, including increased 

bed erosion rates, bank instability and widening, loss of complexity of other geomorphic units, and loss of riverine habitat 

(Brierley and Fryirs, 2005). 

Channel Geomorphic Units 

Channel geomorphic units of alluvial, coastal plain rivers are typically smaller than valley geomorphic units, with 

dimensions on the order of 10 feet to less than 1 mile. Channel 

geomorphic units occur in the river channel and are subject to 

frequent stresses associated with flowing water and sediment 

transport; they adjust (change) relatively quickly in response 

to short-term variations in flow (Brierley and Fryirs, 2005). 

Relatively few characteristics of the channel geomorphic units 

can be determined or measured from aerial imagery. Field 

observations useful for interpreting channel geomorphic units 

include measurements of their length, width, height, thickness, 

composition (particle-size distribution and mineralogy), orientation, and vegetation characteristics, among others (Brierley 

and Fryirs, 2000, 2005). 

Channel Banks

The channel banks of Texas Gulf Coastal Plain rivers 

consist mostly of combinations of other channel geomorphic 

units and alluvial deposits. Alluvial channel banks of lowland, 
16 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

actively meandering rivers such as the rivers of the Texas 

Gulf Coastal Plain are generally categorized as depositional, 

erosional, or stable. Depositional channel banks typically 

consist of convex, gradually sloping upper banks and lowsloped lower banks leading to the water’s edge (fig. 14). 

Depending on flow conditions, depositional banks typically 

contain vegetation all or part of the year. Erosional channel banks (cutbanks) are identified by relatively steep concave faces and a lack of permanent vegetation and are most 

common on the outside of meander bends (fig. 15). Cutbanks 

typically indicate active channel erosion and meander migration. Stable channel banks are typically vegetated, with no 

evidence of ongoing deposition or erosion. Channel banks 

are categorized on the basis of convexity, concavity, irregularity, angle, and interpreted formative process. Brierley and 

Fryirs (2005) and Phillips (2008) provide detailed information 

regarding the categories of alluvial banks and their formative 

processes. 

Figure 10. Example of a neck cutoff on a Texas Gulf Coastal Plain river at a meander on the Sabine River near Salem, Texas.

Salem

BEAUREGARD PARISH

NEWTON 

COUNTY

LOUISIANA

Sabine

River

LOCATION MAP

TEXAS

Neck cutoff

BEAUREGARD PARISH

NEWTON 

COUNTY

Flow direction

Sabine River

LOUISIANA

TEXAS

30°31'30"

30°31'

93°43' 93°42'

0 0.125 0.25 0.375 0.5 MILE

0 0.125 0.375 0.25 0.5 KILOMETER

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2009, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 17

Figure 11. Example of an oxbow lake with a tie channel and sand wedge on a Texas Gulf Coastal Plain river from the oxbow lake on the 

Brazos River near Otey,Texas. 

Sand wedge

Oxbow lake

Tie channel

Flow direction

Brazos River

29°16'

29°15'45"

29°15'30"

95°34'30" 95°34'15" 95°34' 95°33'45"

0 0.125 0.25 MILE

0 0.125 0.25 KILOMETER

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

BRAZORIA

COUNTY

Otey

Brazos 

 River

LOCATION MAP
18 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Benches and Ledges

Benches are depositional geomorphic units occurring 

below bankfull stage (fig. 16). The flat surface at the top of 

the bench is sometimes referred to as an “inset flood plain” 

(Brierley and Fryirs, 2005). Benches serve as important 

sediment storage features in river channels and commonly 

exist on the inside of a meander bend. Heitmuller and Greene 

(2009) found evidence that benches have formed on point bars 

of the lower Brazos River during the last 50 years and, combined with vegetation encroachment, have reduced bankfull 

channel width. 

Figure 12. Example of an older, partially filled oxbow swamp north of a more recent oxbow lake on a Texas Gulf Coastal Plain river from 

the sequence of oxbow lakes on the Brazos River near Chappell Hill, Texas. 

WALLER

WASHINGTON

COUNTY

COUNTY

Flow direction

Partially filled oxbow

Oxbow lake

Brazos

River

96°10'30" 96°9'45"

30°11'15"

30°10'30"

0 0.25 0.5 KILOMETER

0 0.25 0.5 MILE Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers equal-area projection

North American Datum of 1983

WALLER

COUNTY

WASHINGTON

COUNTY

Brazos

River

Chappell 

Hill

LOCATION MAP
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 19

Figure 13. Example of a constructed channel conveying flow from the main channel of the Brazos River to Jones Creek near 

Fulshear, Texas. 

Constructed

channel

Flow direction

Flow 

 direction

95°54' 95°53'

29°39'

29°38'30"

Base from National Agriculture Imagery 

Program (NAIP), U.S. Department of 

Agriculture (USDA)-Farm Service Agency 

(FSA) Aerial Photography Field Office, 2008, 

1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

0 0.25 0.5 0.75 1 MILE

0 0.25 0.5 0.75 1 KILOMETER

FORT BEND

COUNTY

Brazos

River

Brazos River

Jones

Creek

Jones

Creek

Fulshear

LOCATION MAP
20 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Ledges, although similar in form to benches, are 

erosional scarps occurring below the bankfull stage. Ledges 

indicate channel incision and/or expansion (Brierley and 

Fryirs, 2005) and often occur when bank erosion processes 

encounter resistant layers that erode less easily than overlying 

units. In Texas Gulf Coastal Plain alluvial rivers, ledges 

are typically composed of bedrock or cohesive clays, overlain by less-resistant, noncohesive sandy soils or alluvium 

(Phillips, 2008). Ledges also occur when a period of incision downcuts a narrow channel into the former channel bed. 

These types of ledges were documented by Phillips and others 

(2005) in the Trinity River. When present, benches and ledges 

form a series of risers from the river channel to the active 

flood plain.

Bank Failures

Bank failure, or mass failure, is a geomorphic process 

that is useful in determining bank type. Bank failure features 

and remnants are bank-attached channel geomorphic units. 

According to Brierley and Fryirs (2005, p. 99) “the susceptibility of banks to mass failure depends on their geometry, 

structure, and material properties”. In Texas Gulf Coastal 

Plain alluvial rivers, bank material weathering occurs in the 

form of soil desiccation, wetting/drying cycles, and to a lesser 

extent, freeze/thaw cycles. Thorne (1982) describes in detail 

how wetting and drying cycles weaken bank material, thus 

making it more susceptible to mass failure. Typical methods 

of mass failure in alluvial rivers include slab failure, beam 

Figure 14. Example of a depositional bank of the Trinity River 

with a vegetated upper bank and a point-bar surface below the 

bankfull elevation near St. Paul, Texas (photograph courtesy of 

Webster Mangham, Trinity River Authority, 2010). 

Figure 15. Example of an unvegetated cutbank on the outside 

of a meander bend along the Trinity River near Massey Lake, 

Texas (photograph courtesy of Webster Mangham, Trinity River 

Authority, 2010). 

High stage

Low stage

Bench Ledge

Figure 16. Simplified representation of bench deposits and an erosional ledge. 
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 21

failure, and rotational slip, or slumping (Brierley and Fryirs, 

2005). Undercutting of the channel banks increases their 

susceptibility to mass failure and is typical on cutbanks of 

alluvial, coastal plain rivers. Bank failure material and remnants that are not immediately entrained by flow can become 

vegetated and temporarily stable, providing habitat and armoring the bank from additional undercutting and subsequent 

failure. Bank failures are most easily located and described in 

the field (fig. 17), but large failures are sometimes visible in 

high-resolution aerial photography.

Point Bars and Cross-Bar Channels

A point bar is an arc-shaped deposit along the inside 

of a meander bend and is tilted upward away from the river 

channel (Brierley and Fryirs, 2005; fig. 18). Point bars are 

fairly common in Texas Gulf Coastal Plain rivers. They 

are frequently inundated, often more than once per year, 

and, therefore, much of their surface is sparsely vegetated. 

Secondary flow currents at a meander bend, perpendicular 

to the general downstream flow direction, erode sediment 

from the outside of the bend, forming a cutbank and pool, 

and transport and deposit bed sediment toward the inside of 

the bend, forming a point bar with a fining-upward sequence 

(Brierley and Fryirs, 2005). Higher distal sections of the point 

bar are capped with fine-grained (silt and clay) sediment. 

Point bars also exhibit a longitudinal grading, with relatively 

coarse-grained material deposited on the upstream end of the 

bar and relatively fine-grained sediment on the downstream 

end of the bar. Eventually, distal, older parts of the point bar 

merge with the elevation of surrounding flood-plain deposits. When overbank deposits comprise the majority of sediment along a point-bar surface, it can be considered a valley 

geomorphic unit, as opposed to a channel geomorphic unit. 

Portions of point bars, or sometimes entire bars, may become 

permanently or semipermanently vegetated either because of 

accretion above regular flow levels or during prolonged lowflow periods.

Cross-bar channels can be found on the inside meander 

bends on Texas Gulf Coastal Plain rivers and form when a low 

swale in a point bar is inundated during high-flow events and 

becomes the preferential flow path, thereby slightly shortening the channel and reducing sinuosity (fig. 19). The shortened 

channel length leads to higher flow energies and the reworking of the bar and other channel geomorphic units. Cross-bar 

channels are differentiated from neck cutoffs in that they do 

not circumvent an entire meander, but only the surface of the 

point bar.

Other Channel Bars

A variety of channel bar deposits can occur in Texas 

Gulf Coastal Plain rivers, including longitudinal, transverse, 

and tributary mouth bars. Longitudinal bars are elongated 

deposits of bed sediment oriented parallel to flow. They 

form when relatively coarse sediment deposited in the channel diverts flow and promotes further sediment deposition 

immediately downstream (Leopold and Wolman, 1957). They 

can occur in the middle of the channel or attached to the bank, 

with the latter situation being more common with increasing 

downstream distance (Church and Jones, 1982) and decreasing 

bed-material size. A midchannel longitudinal bar can develop 

into an island with an elevation exceeding bankfull stage 

if channel-bed degradation occurs, or the rate of sediment 

aggradation on the bar exceeds the rate of erosion, or sufficient vegetation is established that promotes vertical growth 

and stability (Brierley and Fryirs, 2005) (fig. 20). In contrast 

to longitudinal bars, transverse bars are oriented perpendicular to the flow and span across the river. Transverse bars 

specifically refer to midchannel bars oriented perpendicular 

to flow and occupying most of the channel width (also referred 

to as linguoid bars). They typically have a lobate shape with 

a steep face on the downstream end (fig. 21). They tend to 

occur where there is an increase in channel width that locally 

reduces flow velocity and promotes sediment deposition. 

Transverse channel bars will begin migrating downstream at 

varying flow rates depending on their grain-size distribution 

and composition (Cant and Walker, 1978). Tributary mouth 

bars form in the main river channel as a result of converging flow at a tributary confluence (fig. 21). Flow separation 

and secondary current generation allow for the formation of 

a backwater zone immediately downstream from the tributary mouth on the same side of the channel. The lower flow 

velocities present in the backwater zone promote deposition of 

larger particles, even during periods of high flows. Tributary 

mouth bars are typically composed of poorly sorted sediment 

and organic debris. Other types of channel bars occur in Texas 

Gulf Coastal Plain rivers, including lateral bars, diagonal bars, 

forced bars, and sand sheets. Phillips (2008) provides detailed 

discussions and photographs of the different types of channel 

bars.

Figure 17. Example of a bank failure on the cutbank of the Trinity 

River near Massey Lake, Texas (photograph courtesy of Webster 

Mangham, Trinity River Authority, 2010). 
22 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Exposed Bedrock

Resistant bedrock becomes exposed when overlying alluvial material is eroded and transported downstream. Bedrock 

exposures can influence local flow patterns and, thus, other 

channel geomorphic units. Exposed bedrock geomorphic 

units can be described by their position and orientation in the 

channel (Phillips, 2008). Hidalgo Falls is a unique geomorphic 

feature on the Brazos River that Phillips (2006, 2007a) characterizes as an individual river style (fig. 22). Here, the river 

falls approximately 11.5 vertical feet in 1,800 linear feet. This 

feature is one of the only places on the Brazos River alluvial 

Brazos River

Flow 

direction

96°59'30" 96°59'15" 96°59'

31°19'

31°18'45"

Point bar

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

0 0.1 0.2 0.3 0.4 MILE

0 0.1 0.2 0.3 0. 4 KILOMETER

FALLS

COUNTY

LOCATION MAP

Brazos

River

Satin

Figure 18. Example of a large point bar on the inside of a meander bend of the Brazos River near Satin, Texas. 
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 23

Brazos River

Cross-bar channel

Point bar 

MILAM

COUNTY

ROBERTSON

COUNTY

Flow 

 direction

96°39' 96°38'30"

30°47'45"

30°47'30"

30°47'15"

0 0.1 0.2 0.3 0.4 MILE

0 0.1 0.2 0.3 0.4 KILOMETER

MILAM

COUNTY

ROBERTSON

COUNTY

Gause Brazos

River

LOCATION MAP

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

Figure 19. Example of a cross-bar channel formed across a point bar on the inside of a meander bend of the Brazos River near Gause, 

Texas. 
24 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Figure 20. Example of a vegetated, stabilized midchannel longitudinal bar that has developed into an island in the channel of the 

Brazos River near Robinson, Texas. 

Brazos River

Flow 

 direction

97°0'10" 97° 96°59'50"

31°27'40"

31°27'30"

Vegetated, stabilized

midchannel

longitudinal bar

0 0.05 0.1 0.15 0.2 MILE

0 0.05 0.1 0.15 0.2 KILOMETER

MCLENNAN

COUNTY

Robinson

Brazos

River

LOCATION MAP

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 25

plain where the channel bed and banks consist of resistant, 

Tertiary sandstone bedrock (U.S. Department of Agriculture, 

Natural Resources Conservation Service, 2002). Other areas 

of exposed bedrock, both consolidated or unconsolidated, 

exist in the channels of Texas Gulf Coastal Plain rivers and are 

classified individually as channel geomorphic units (Phillips, 

2008). 

Another class of channel geomorphic unit forms as a 

result of exposed bedrock in Texas Gulf Coastal Plain rivers. 

Depending on flow conditions, bedrock rapids form as water 

flows across irregular surfaces of exposed bedrock. The flow 

conditions associated with rapids can cause increased erosion of a river’s bed and banks in the vicinity of the bedrock 

outcrop. 

Figure 21. Example of a transverse (linguoid) bar and a tributary mouth bar formed downstream from the confluence of Bull Hide Creek 

and the Brazos River near Robinson, Texas. 

Flow direction

Brazos

River

Tributary

Tributary 

mouth bar Transverse 

bar

97°1' 97°0'30"

31°23'15"

31°23'

31°22'45"

0 0.2 0.4 KILOMETER 0.1 0.3

0 0.1 0.2 0.3 0.4 MILE

MCLENNAN COUNTY

FALLS COUNTY

Robinson

Brazos 

River

Bull Hide Creek

LOCATION MAP

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-area projection

North American Datum of 1983
26 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Pools, Runs, and Crossovers

Pools are the sections of channel with the deepest water 

and slowest flow velocities, when compared to adjacent 

upstream and downstream sections. Alternating pool/riffle 

sequences refer to successive deep and shallow lengths of 

the river channel, respectively, and are commonly associated 

with gravel-bed rivers. In the sand-bed rivers of the Texas 

Gulf Coastal Plain, the shallow sections between pools with 

higher flow velocities are called crossovers, because there is 

not sufficient gravel present to form riffles. Runs are the sections of channels of moderate depth and velocity that connect 

pools and crossovers. Isolated riffles can form in the channels of Texas Gulf Coastal Plain rivers near locations where 

coarse gravels originating from upstream igneous intrusions 

or paleochannel deposits eroded from the banks. There is 

typically not enough gravel and other large clasts to form pool/

riffle sequences. 

Figure 22. Exposed bedrock of Hildago Falls on the Brazos River between Washington County and Brazos County near Navasota, 

Texas. 

BRAZOS

COUNTY

WASHINGTON COUNTY

Flow direction

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

Brazos River

96°11' 96°10'30" 96°10'

30°23'30"

30°23'

0 0.1 0.2 0.3 0.4 MILE

0 0.2 0.4 KILOMETER 0.1 0.3

GRIMES

COUNTY

BRAZOS

COUNTY

WASHINGTON

COUNTY

Navasota Navasota

River

Brazos

River

LOCATION MAP

Exposed bedrock

of Hidalgo Falls
Characterization of Geomorphic Units in Texas Gulf Coastal Plain Rivers, with Examples from the Brazos, Sabine, and Trinity Rivers 27

 In lowland, meandering river channels, a pool commonly is located on the cutbank side of a meander bend, and 

crossovers occur between meander bends (fig. 23). Pools at 

meander cutbanks are maintained by secondary flow circulation, which occurs when flow is deflected down towards the 

channel bed by the cutbank, enabling localized scour of the 

bed. Sediment scoured at cutbank pools can then be transported across the channel and downstream and deposited as 

various types of channel bars. Other types of pools occur on 

small scales. These include forced pools associated with flow 

obstructions, scour pools resulting from turbulent flow around 

obstructions such as bridge piers, and solution pools that are 

dissolved into bedrock (Brierley and Fryirs, 2005). Crossovers 

occur between pools and can manifest as various channel-bar 

forms (Phillips, 2008), including transverse bars. Channels 

tend to be wider at crossovers than at pools for a given straight 

reach of channel as a result of their need to accommodate 

streamflow in the shallow crossovers. For a detailed explanation of categories of pools and runs in Texas Gulf Coastal 

Plain rivers, see Phillips (2008). 

Figure 23. Example of a pool-run-crossover sequence on the Brazos River near Cedar Springs, Texas. 

Run

Pool

Crossover

Pool

Brazos River

96°54' 96°53'30"

31°12'15"

31°12'

0 0.1 0.2 MILE

0 0.1 0.2 KILOMETER

FALLS

COUNTY

Cedar Springs

Brazos

River

LOCATION MAP

Base from National Agriculture Imagery Program

(NAIP), U.S. Department of Agriculture (USDA)-Farm

Service Agency (FSA) Aerial Photography Field

Office, 2008, 1-Meter Compressed County Mosaic.

Albers Equal-Area projection

North American Datum of 1983

Flow direction
28 Characterization of Geomorphic Units in the Alluvial Valleys and Channels of Gulf Coastal Plain Rivers in Texas

Summary

The U.S. Geological Survey, in cooperation with the 

Texas Water Development Board, described and characterized examples of geomorphic units (pools, riffles, cross-bar 

channels, levees, benches, bar types, paleochannels, among 

others) within the channels and alluvial valleys of Texas Gulf 

Coastal Plain rivers by using a geomorphic unit classification 

scale that differentiates geomorphic units on the basis of their 

location either outside or inside the river channel. Geomorphic 

unit mapping is an integral part of the Texas Instream Flow 

Program (TIFP) designed to maintain a sound ecological environments in rivers. The River Styles framework was adapted 

by the TIFP to more directly address the needs of instream 

flow studies. The River Styles framework provides a method 

to geomorphologically characterize river systems using a 

hierarchical structure that starts with the watershed and scales 

down to microhabitat features. The geomorphic properties of 

a river system determine the distribution and type of potential 

habitat both within and adjacent to the channel. River styles, 

one of the hierarchical categories of the River Styles framework classification system, contain unique assemblages of 

geomorphic units, which often determine physical habitat type 

within the river valley and channel. This report characterizes 

the geomorphic units contained in the river channels and alluvial valleys of Texas Gulf Coastal Plain rivers in the context 

of the River Styles framework. Concepts of the River Styles 

framework are used to clearly differentiate between the different spatial scales of geomorphic units on the basis of their 

location either outside or inside the river channel. 

This report is intended to help TIFP practitioners, river 

managers, ecologists and biologists, and others interested in 

the geomorphology and the physical processes of the rivers 

of the Texas Gulf Coastal Plain (1) gain insights into how 

geomorphic units develop and adjust spatially and temporally, and (2) be able to recognize common geomorphic units 

from the examples cataloged in this report. This discussion of 

geomorphic units is applicable only to the alluvial valleys of 

Texas Gulf Coastal Plain rivers, although the geomorphic units 

presented here can also occur within the upper watershed or 

deltaic plain of a Texas river system.

Recent aerial imagery (high-resolution digital orthoimagery) collected in 2008 and 2009 were inspected by using 

geographic information system software to identify representative examples of the types of geomorphic units that occurred 

in the study area. Whereas the River Styles framework differentiates geomorphic units as flood-plain geomorphic units or 

channel geomorphic units on the basis of their location inside 

or outside of the channel, the authors determined that in the 

study area, geomorphic units did not always occur on the flood 

plain, but occasionally at elevations higher than the flood-plain 

surface yet still within the alluvial valley. For this reason, 

geomorphic units that are located outside the channels of 

Texas Gulf Coastal Plain rivers are called “valley geomorphic 

units” in this report. Valley geomorphic units are either areal 

or linear, depending on their general shape and extent; they 

are in contact with the river only during periodic flow events 

when channel-flood plain connectivity is achieved. Valley 

geomorphic units for the Texas Gulf Coastal Plain rivers 

described in this report are terraces, flood plains, crevasses and 

crevasse splays, flood-plain depressions, tie channels, tributaries, paleochannels, anabranches, distributaries, natural levees, 

neck cutoffs, oxbow lakes, and constructed channels. Channel 

geomorphic units occur in the river channel and are subject to 

frequent stresses associated with flowing water and sediment 

transport; they adjust (change) relatively quickly in response 

to short-term variations in flow. Channel geomorphic units 

can be grouped as bank-attached or midchannel, depending on 

their location within the channel under base-flow conditions. 

Channel geomorphic units described in this report are channel 

banks, benches and ledges, bank failures, point bars, crossbar channels, channel bars, exposed bedrock, pools, runs, and 

crossovers.

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