of Change in Sand Transport in 

the Brazos River, Texas

Water-Resources Investigations Report 01–4057

U.S. Department of the Interior

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

U.S. Geological Survey

Indications and Potential Sources 

of Change in Sand Transport in 

the Brazos River, Texas

By David D. Dunn and Timothy H. Raines

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01–4057

In cooperation with the Texas Parks and Wildlife Department

and the University of Texas Bureau of Economic Geology

Austin, Texas

2001
ii

U.S. DEPARTMENT OF THE INTERIOR

Gale A. Norton, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names is for 

descriptive purposes only and does not imply 

endorsement by the U.S. Government.

For additional information write to

District Chief

U.S. Geological Survey

8027 Exchange Dr.

Austin, TX 78754–4733

E-mail: dc_tx@usgs.gov

Copies of this report can be purchased from

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225–0286

E-mail: infoservices@usgs.gov
CONTENTS iii

CONTENTS

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

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

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

Description of the Brazos River Basin ..................................................................................................................... 3

Suspended-Sediment Data-Collection Methods ....................................................................................................... 6

Previous Studies of Sediment Transport in the Brazos River ................................................................................... 8

Capacity of the Brazos River to Transport Sand ...................................................................................................... 9

Indications of Change in Sand Transport ............................................................................................................................. 12

Changes in Channel Geometry ................................................................................................................................. 12

Changes in Suspended-Sediment Characteristics ..................................................................................................... 15

Changes in Discharge ............................................................................................................................................... 21

Potential Sources of Change in Sand Transport ................................................................................................................... 25

Reservoir Construction ............................................................................................................................................. 25

Changes in Land Use ................................................................................................................................................ 26

Instream Sand and Gravel Mining ............................................................................................................................ 26

Summary .............................................................................................................................................................................. 30

References Cited .................................................................................................................................................................. 31

FIGURES

1. Map showing Texas Gulf of Mexico shoreline ..................................................................................................... 2

2. Map showing Brazos River Basin, Texas ............................................................................................................. 3

3. Photograph showing typical cut bank on the Brazos River, Texas ....................................................................... 4

4. Map showing location of selected streamflow-gaging stations in the lower Brazos River Basin, Texas ............. 5

5–7. Graphs showing: 

5. Historical development of reservoir conservation storage capacity in the Brazos River Basin, 

Texas .......................................................................................................................................................... 6

6. Particle-size distributions of bed material samples near streamflow-gaging station 08114000 

Brazos River at Richmond, Texas .............................................................................................................. 10

7. Flow-duration curve of daily mean discharge for streamflow-gaging station 08114000 Brazos 

River at Richmond, Texas, 1903–95 (excluding 1907–21) ....................................................................... 11

8–10. Graphs showing stage-discharge relation at streamflow-gaging station: 

8. 08111500 Brazos River near Hempstead, Texas ....................................................................................... 13

9. 08114000 Brazos River at Richmond, Texas ............................................................................................. 14

10. 08116650 Brazos River near Rosharon, Texas .......................................................................................... 14

11–13. Graphs showing water-surface altitudes at a discharge of 5,000 cubic feet per second at streamflowgaging station:

11. 08111500 Brazos River near Hempstead, Texas ....................................................................................... 15

12. 08114000 Brazos River at Richmond, Texas ............................................................................................. 16

13. 08116650 Brazos River near Rosharon, Texas .......................................................................................... 16
iv 

14–18. Graphs showing: 

14. Suspended-sand concentrations in samples at streamflow-gaging station 08114000 Brazos River 

at Richmond, Texas, 1969–95 .................................................................................................................... 17

15. Sampled discharges at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 

1969–95 ...................................................................................................................................................... 18

16. Relation of suspended-sand concentration to discharge at streamflow-gaging station 08114000 

Brazos River at Richmond, Texas, 1969–95 .............................................................................................. 19

17. Relation of percent sand in suspended-sediment samples to discharge at streamflow-gaging 

station 08114000 Brazos River at Richmond, Texas, 1969–95 ................................................................. 19

18. Relation of mean channel velocity to discharge at streamflow-gaging station 08114000 Brazos 

River at Richmond, Texas, 1969–95 .......................................................................................................... 20

19–20. Graphs showing flow-duration curves of daily mean discharge for streamflow-gaging station: 

19. 08111500 Brazos River near Hempstead, Texas, 1940–68 and 1969–95 .................................................. 21

20. 08114000 Brazos River at Richmond, Texas, pre-1940, 1940–68, and 1969–95 ...................................... 22

21–23. Graphs showing: 

21. Suspended-sand transport curve for streamflow-gaging station 08114000 Brazos River at 

Richmond, Texas ........................................................................................................................................ 24

22. Annual suspended-sand loads at streamflow-gaging station 08114000 Brazos River at Richmond, 

Texas .......................................................................................................................................................... 24

23. Total acres harvested in the 27 counties in the lower Brazos River Basin, Texas ...................................... 27

24. Map showing location of selected sand and gravel production sites in the lower Brazos River Basin, 

Texas ..................................................................................................................................................................... 28

25–26. Graphs showing: 

25. Total annual weight of sand extracted and annual loads of suspended sand transported in the 

Brazos River between Hempstead and Rosharon, Texas, 1979–95 ........................................................... 29

26. Cumulative annual weight of sand extracted and annual loads of suspended sand transported in 

the Brazos River between Hempstead and Rosharon, Texas, 1979–95 ..................................................... 29

TABLES

1. Selected characteristics of streamflow-gaging stations on the main stem of the lower Brazos River, Texas ....... 5

2. Selected characteristics of the 13 largest reservoirs in the Brazos River Basin, Texas ........................................ 7

3. Ratios of shear stress to critical shear stress required for initiation of motion at streamflow-gaging station 

08114000 Brazos River at Richmond, Texas ........................................................................................................ 11

4. Summary of statistical tests for differences in annually computed daily flow percentiles between the 

pre-1940 period and the 1969–95 period at streamflow-gaging station 08114000 Brazos River at 

Richmond, Texas ................................................................................................................................................... 23
Abstract 1

Indications and Potential Sources 

of Change in Sand Transport in 

the Brazos River, Texas

By David D. Dunn and Timothy H. Raines

Abstract

Changes in the capacity of the Brazos River 

to transport sand can be identified within the 

context of Lane’s relation through changes in 

channel geometry, changes in the characteristics 

of suspended loads, and changes in discharge. 

The Brazos River channel has been undergoing 

continual adjustment since the 1940s. For a discharge of 5,000 cubic feet per second, the watersurface altitude has decreased 2 to 4 feet at the 

Hempstead and Richmond streamflow-gaging 

stations between 1940 and 1995. The characteristics of suspended-sediment samples at the 

Richmond streamflow-gaging station have 

changed between the periods 1969–81 and 1982–

95. The amount of sand-size sediment transported 

in suspension has decreased. The distribution of 

both daily and annual-peak discharges has 

changed. However, the computed annual loads of 

suspended sand indicate no statistically significant 

change in the median annual load.

The transport of sand in the Brazos River 

depends on a complex set of factors, most of 

which are continually changing. Potential sources 

of change in sand transport in the Brazos River 

include the effects of reservoir construction, 

changes in land use, and instream sand and gravel 

mining. Extensive reservoir construction in the 

Brazos River Basin has reduced sand transport by 

trapping sediment and by reducing the magnitude 

of peak discharges. However, reductions in sand 

transport associated with reservoir construction 

apparently are compensated for by increases associated with tributary sediment inflow and localized 

bank erosion. The total area of harvested acres of 

non-hay crops in the lower Brazos River Basin 

during 1924–92 decreased more than 75 percent 

from about 32 percent to about 8 percent of the 

total area. Correspondingly, erosion potential has 

decreased substantially. Several sand and gravel 

mining sites are located on the Brazos River 

between Hempstead and Rosharon. The quantity of 

sediment extracted by instream sand and gravel 

mining operations could represent from 11 to 25 

percent of the total sand transported by the Brazos 

River. The effects of mining on sand transport 

could not be quantified. 

INTRODUCTION

The sand that constitutes much of the Texas 

shoreline along the Gulf of Mexico (fig. 1) is in continual motion. Tides, storm surges, wave action, wind, 

river processes, and longshore (littoral) drift act upon 

the sand and transport it along the shoreline to form and 

erode beaches, barrier islands, and peninsulas. Some of 

the sand supplied to this process originates in upland 

areas and is transported to the Gulf of Mexico by rivers 

and streams. Most of these rivers and streams discharge 

into bays or estuaries, which eventually will fill with 

sediment. Three of the largest rivers in Texas, the 

Brazos River, the Colorado River, and the Rio Grande, 

have filled their respective estuaries and have propagated deltas seaward (McGowen and others, 1977).

Longshore drift transports sand from the river 

deltas to other locations along the coast. The longshore 

drift generally moves material in a southwesterly direction along the coast from Port Arthur to south of Corpus 

Christi. An opposing current moves material in a northerly direction from the Mexico coastline toward Corpus 

Christi. A zone of convergence, also called a zone of 

stagnation, occurs at a latitude of about 27 degrees 

north. The exact location varies with prevailing tidal and 

weather patterns.
2 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

The Brazos River, Colorado River, Rio Grande, 

and the smaller San Bernard River are the only sources 

of additional sand supplied to the Texas Gulf beaches 

from outside the coastal system (McGowen and others, 

1977). Sargent Beach, located south of the Brazos River 

mouth, has undergone more than 1,500 feet (ft) of 

inland erosion since the mid 1800s, or an average of 

about 12 feet per year (ft/yr). Short-term erosion rates of 

as much as 36 ft/yr have been documented by Morton 

and Pieper (1975) and Seelig and Sorenson (1973). 

Stauble and others (1994) state that Sargent Beach is 

eroding at a faster rate than any other Texas beach, 

and that the rate of erosion is due to a lack of sand-size 

material in the system. The lack of sand-size material in 

LOCATION MAP

TEXAS

Longshore drift

Sargent Beach Longshore drift

GULF OF MEXICO

SABINE

MEXICO

UNITED STATES

COLORADO

RIVER

RIVER

BRAZOS

TRINITY

RIVER

RIO GRANDE

SAN

BERNARD

RIVER

GUADALUPE

RIVER

RIVER

RIVER

SAN ANTONIO

NUECES

RIVER

CORPUS

 CHRISTI

HOUSTON

BEAUMONT

PORT

 ARTHUR

98o 97o

96o

95o 94o

26o

27o

28o

29o

30o

0 20 40 60 MILES

Figure 1. Texas Gulf of Mexico shoreline.
INTRODUCTION 3

the system has been attributed to a decreased supply of 

sand from the Brazos River by several researchers 

(Mathewson and Minter, 1976; Seelig and Sorenson, 

1973; Stauble and others, 1994).

In 1995, the U.S. Geological Survey (USGS), in 

cooperation with the Texas Parks and Wildlife Department (TPWD) and the University of Texas Bureau of 

Economic Geology, began a study of sand transport in 

the Brazos River. 

Purpose and Scope

This report describes the transport of sand-size 

sediment in the Brazos River and describes apparent 

changes in this sand transport with several indicators. 

The apparent changes are compared to potential 

causative sources, including reservoir construction, 

land-use changes, and instream sand and gravel mining. 

Sand-size sediment in this report refers to sediment 

particles having a sieve diameter larger than 0.062 

millimeter (mm). Available stage, discharge, and 

suspended-sediment data collected at three Brazos 

River streamflow-gaging stations operated by the 

USGS, and bed-material data collected near Richmond, 

Tex., are analyzed for the period 1903–95 (excluding 

1907–21).

Description of the Brazos River Basin

The Brazos River Basin (fig. 2) originates in eastern New Mexico and extends about 640 miles (mi) 

southeasterly across Texas to the Gulf of Mexico south 

RIVER

BRAZOS

LAKE

 WHITNEY

NEW MEXICO

TEXAS

LOCATION MAP

0 50 100 150 MILES

100o

98o

96o

102o

30o

32o

34o

RICHMOND

WACO

FORT WORTH

HOUSTON

Figure 2. Brazos River Basin, Texas.
4 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas

of Houston. The upper basin is about 70 mi wide in the

northwestern part of Texas, expands to a maximum

width of 110 mi near Waco, and constricts to about

10 mi wide near Richmond in the lower basin. The

Brazos River and its tributaries drain an area of about

45,600 square miles (mi

2

). About 9,570 mi

2 of the

upper part of the basin, including all of the area in New

Mexico, does not contribute to downstream flows.

The Brazos River can be characterized as an

incised, meandering, sand-bed channel with unstable

banks. Frequent, near-vertical cut banks 20 to 35 ft high

are prominent along much of its length (fig. 3). In the

lower 300 river miles of the Brazos River, the channel

slopes 0.7 foot per mile (ft/mi). At streamflow-gaging

station 08114000 Brazos River at Richmond, Tex., the

channel is about 300 ft wide and the median daily discharge is 3,000 cubic feet per second (ft

3

/s).

The USGS operates several streamflow-gaging

stations on the main stem of the Brazos River. Three of

these stations are located in the lower reach of the river

downstream from the 13 largest reservoirs: 08111500

Brazos River near Hempstead, Tex.; 08114000 Brazos

River near Richmond, Tex.; and 08116650 Brazos River

near Rosharon, Tex. (fig. 4).

There is very little difference in drainage area

between the three stations (table 1) because the

basin narrows substantially in its lower reaches. The

Richmond streamflow-gaging station has been active

the longest and can be used to characterize total flows

from the basin. All discharges recorded at the gaging

stations since about 1940 are affected substantially by

reservoir regulation. The mean annual discharge at the

Richmond streamflow-gaging station for the period

1941–95 was 7,600 ft

3

/s.

Reservoir construction has taken place in the

Brazos River Basin since the early 1900s. In 1986,

1,178 reservoirsin the Brazos River Basin were listed in

the dam inventory maintained by the Texas Natural

Resource Conservation Commission (Wurbs and

others, 1988). Reservoirs listed in the inventory meet at

Figure 3. Typical cut bank on the Brazos River, Texas.
INTRODUCTION 5

Table 1. Selected characteristics of streamflow-gaging stations on the main stem of the lower Brazos River, Texas 

1 Computed for the period 1941–95.

Station

number Station name Drainage area(square miles)

Period of

record

 Mean annual discharge

(cubic feet per second)

08111500 Brazos River at Hempstead, Texas 43,880 1939–95 6,900

08114000 Brazos River at Richmond, Texas 45,007 1903–06

1922–95

17,600

08116650 Brazos River at Rosharon, Texas 45,339 1967–95 8,660

BRAZOS

RIVER

SOMERVILLE

 LAKE

WASHINGTON

 COUNTY

AUSTIN

 COUNTY

WALLER

 COUNTY

FORT

 BEND

 COUNTY

BRAZORIA

 COUNTY

HEMPSTEAD

96o

96o30'

95o

30'

30o

29o30'

29o

0 10 20 30 MILES

RICHMOND

ROSHARON

08111500

08114000

08116650

Streamflow-gaging station

and number

EXPLANATION

08111500

Basin boundary

Figure 4. Selected streamflow-gaging stations in the lower Brazos River Basin, Texas.
6 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

least one of the following two criteria: (1) storage 

capacity of 15 acre-feet (acre-ft) or greater and dam 

height of 25 ft or greater or (2) storage capacity of 

50 acre-ft or greater and dam height of 6 ft or greater 

(Wurbs and others, 1988). Of 1,178 reservoirs, 13 contain 88 percent of the controlled storage in the Brazos 

River Basin. Nine of those 13 reservoirs are operated by 

the U.S. Army Corps of Engineers for flood control and 

contain all of the designated flood-control storage in the 

basin. Table 2 shows a summary of reservoir storage for 

the 13 largest reservoirs. Three of the reservoirs, including the largest, Lake Whitney, are located on the main 

stem of the Brazos River. 

The rate of reservoir construction increased 

substantially during 1940–69 (fig. 5). Three periods can 

be identified: early (pre-1940), middle (1940–68), and 

late (1969–95). The early period reflects a time when 

relatively little reservoir construction occurred. The 

middle period reflects a time when reservoir conservation storage in the basin increased from 258,675 to 

3,589,014 acre-ft (more than 1,300 percent), and the late 

period reflects a time when reservoir conservation 

storage increased at a slower rate to 4,613,800 acre-ft 

(about 29 percent).

Land use in the Brazos River Basin is predominantly agricultural and open rangeland. The dominant 

crops produced in the basin are cotton, corn, and 

sorghum (U.S. Department of Agriculture, 1997).

Suspended-Sediment Data-Collection 

Methods

The Texas Water Development Board (TWDB) 

and the USGS are the primary agencies that have 

collected sediment data from the Brazos River. The different sampling methods used by the agencies make the 

comparison of data difficult, particularly when analyzing the transport of sand-size particles. Only data collected by the USGS were analyzed in this study. The 

rationale for this decision is presented in the following 

discussion.

The TWDB has collected periodic suspendedsediment samples from streams in Texas, including the 

Brazos River, since the early 1900s. These data have 

been published in several compilation reports such as 

that by Dougherty (1979). The samples were collected 

using a device known as the “Texas sampler.” The Texas 

sampler consists of a 15-inch (in.) hanger bar to which 

a sheet-metal bottle holder is fastened. Below the bottle 

Figure 5. Historical development of reservoir conservation storage capacity in the Brazos River Basin, Texas 

(R.M. Slade, Jr., U.S. Geological Survey, written commun., 1999).

5,000,000

4,500,000

4,000,000

3,500,000

3,000,000

2,500,000

2,000,000

1,500,000

1,000,000

500,000

0

CUMULATIVE RESERVOIR STORAGE CAPACITY,

IN ACRE-FEET

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

YEAR
INTRODUCTION 7

holder, a 15-pound current-meter weight is attached. An 

8-ounce small-mouth bottle is placed vertically in the 

bottle holder. The bottle is lowered about 1 ft below the 

water surface and brought to the surface when air bubbles cease emanating from the bottle. The mean sediment concentration of the stream is computed as the 

average concentration of three samples taken at points 

one-fifth, one-half, and five-sixths of the distance across 

the river (Mathewson and Minter, 1976; Welborn, 

1967), or from the concentration of a single sample 

taken at midstream (Dougherty, 1979).

 The USGS has collected periodic and daily 

suspended-sediment data at the Brazos River at the 

Richmond gaging station since 1957. These data were 

collected using depth-integrating samplers as described 

by Edwards and Glysson (1999) and developed through 

the Federal Interagency Sedimentation project. The 

depth-integrating samplers consist of a streamlined 

body with a nozzle that leads to a sample bottle placed 

inside the sampler body. An exhaust port releases air as 

it is replaced with the water-sediment mixture. Vanes on 

the sampler body orient the nose of the sampler into the 

current. The sampler is lowered from the water surface 

to the streambed and returned at a constant rate. The 

sampler continuously collects the water-sediment 

mixture over the entire vertical water column, except for 

the bottom 0.3 ft. The sampler nozzle and exhaust ports 

are designed so that samples are taken isokinetically—

that is, the velocity of the water inside the sampler 

nozzle is the same as that of the surrounding water. This 

ensures that the sample is representative. Samples are 

obtained from multiple points across the stream by 

either the equal-width-increment (EWI) or the equaldischarge-increment (EDI) method (Edwards and 

Glysson, 1999). The mean sediment concentration of 

the stream is determined by the composite concentration of EWI samples or by discharge-weighting the concentrations of the individual EDI samples.

Suspended-sediment concentration varies with 

depth. Generally, silts and clays are uniformly distributed with depth, but the larger sand-size particles are 

present in greater concentration near the streambed 

because greater turbulence and shear stress are required 

to suspend them. For this reason, a sample collected 

near the water surface will not be representative of the 

entire water column and will consist primarily of wash 

load. In many instances, particularly when sand is not a 

substantial part of the sediment load, the mean concentration can be adjusted satisfactorily with a coefficient. 

The TWDB has recognized this and thus multiplies the 

Table 2. Selected characteristics of the 13 largest reservoirs in the Brazos River Basin, Texas (Wurbs and others, 

1988) 

Reservoir Stream

impounded

Year of initial

impoundment

Drainage

area

(square

miles)

 Storage capacity

(acre-feet)

Conservation

pool

Flood

control

Total

Possum Kingdom Lake Brazos River 1941 23,596 570,240 154,460 724,700

Lake Whitney Brazos River 1951 27,189 627,100 1,372,400 1,999,500

Belton Lake Leon River 1954 3,531 457,600 633,720 1,091,320

Hubbard Creek Reservoir Hubbard Creek 1962 1,085 317,750. 198,050 515,800

Proctor Lake Leon River 1963 1,259 59,400 314,800 374,200

Lake Waco Bosque River 1965 1,652 152,500 573,900 726,400

Somerville Lake Yegua Creek 1967 1,007 160,100 347,400 507,500

Stillhouse Hollow Lake Lampasas River 1968 1,313 235,700 394,700 630,400

Lake Granbury Brazos River 1969 25,679 153,490 87,150 240,640

Lake Limestone Navasota River 1978 675 225,400 92,700 318,100

Lake Georgetown San Gabriel River 1980 247 37,100 93,700 130,800

Granger Lake San Gabriel River 1980 709 65,500 178,700 244,200

Aquilla Reservoir Aquilla Creek 1983 252 52,400 93,600 146,000
8 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

mean sample concentration by a coefficient of 1.102 to 

estimate the mean concentration in the stream from 

the Texas sampler. Welborn (1967) compared paired 

sediment concentrations obtained using the two 

sampling methods and found the ratio of concentrations 

of suspended-sediment samples taken with depthintegrating samplers to samples taken with the Texas 

sampler ranged from 0.96 to 3.41. Welborn (1967) 

found that the ratio was largest in streams containing 

larger percentages of sand in the suspended load and 

that neither a single coefficient nor a set of coefficients 

could be computed to adjust data collected with the 

Texas sampler. The ratios for the Brazos River at 

Richmond ranged from 0.98 to 1.50, and there is no 

relation between the coefficient and discharge.

Andrews (1989) presents mean monthly and 

annual suspended-sediment loads at the Brazos River at 

the Richmond gaging station for the 1966–86 water 

years (October 1–September 30). These loads were 

computed from daily suspended-sediment records 

collected by the USGS. The TWDB also operated a 

suspended-sediment sampling station at this location. 

Andrews (1989) compared 14 years of coincident 

data and found that mean annual loads computed 

from samples collected using the Texas sampler and 

depth-integrating samplers compared favorably. However, the loads computed by the USGS generally were 

larger than those computed by the TWDB. The difference in sampling methods used by the TWDB and the 

USGS could explain some of this difference. The USGS 

method is more likely to capture sand-size particles, 

which generally are found in greater concentration in 

the lower part of the water column than near the surface. 

This will tend to increase individual sample concentrations and, therefore, the loads computed using those 

concentrations.

Much of the analysis presented herein depends on 

analyses of individual sediment samples. Although the 

method used by the TWDB appears sufficient to compute total annual loads of suspended sediment (composed of both sand and silt sizes), the differences in 

data-collection methods become crucial when analyzing only the sand fraction of the suspended load.

Previous Studies of Sediment Transport in the 

Brazos River

Mathewson and Minter (1976) analyzed 

suspended-sediment data collected by the TWDB and 

streamflow data collected by the USGS to estimate the 

effect of water-resource development (reservoir construction) on coastal erosion in the vicinity of the 

Brazos River. They found from an analysis of daily 

mean discharges during 1923–74 that construction of 

both Possum Kingdom Lake (east of Fort Worth) and 

Lake Whitney has slightly reduced the frequency of 

larger discharges at the Richmond streamflow-gaging 

station, but the mean annual discharge at Richmond has 

not been affected by reservoir construction. They also 

found from an analysis of TWDB data for 1924–70 that 

suspended-sediment loads at Richmond have decreased 

substantially because of reduced concentrations of 

suspended sediment. Mathewson and Minter (1976) 

found that reduced sediment transported from the 

Brazos River to the Gulf can be attributed to increased 

storage of sediment in actively building bar formations 

downstream from Waco. The frequency of these bar 

formations decreases with proximity to Richmond, and 

Mathewson and Minter postulate that the reduced 

frequency of large discharges has reduced the capacity 

of the river to transport sediment through Richmond to 

the Gulf. They also attribute much of the coastal erosion 

in the vicinity of Sargent Beach (fig. 1) to sediment 

trapped in reservoirs and indicate that sand transport 

throughout the Brazos River has been reduced enough 

to account for the entire amount of sand lost in the 

coastal zone since at least 1937.

Seelig and Sorenson (1973) investigated 

shoreline changes at Sargent Beach and state that beach 

erosion is further aggravated by decreased sand input 

to the coast from the Brazos River because of alterations 

to the river and its drainage basin since the 1940s. They 

analyzed sand loads computed from TWDB and 

USGS samples at the Richmond streamflow-gaging 

station for water years 1922–65. Using mean annual 

concentrations of sediment load, they identified three 

periods with distinctly different suspended-sediment 

concentrations: (1) 1922–40, concentrations averaged 

about 5,000 parts per million (ppm), (2) 1941–50, concentrations declined steadily from 5,000 to 2,000 ppm, 

and (3) 1951–65, concentrations declined slightly but 

remained in the 1,000 to 3,000 ppm range. They used 

methods developed by Colby (1957) and modified 

by C.T. Welborn (U.S. Geological Survey, written 

commun., 1970) to compute total loads of sand transported by the Brazos River to the coast. They identified 

a decline in sand transported to the coast and attributed 

this decline to the completion of a series of dams beginning with Possum Kingdom, improved soil conservation, and widespread transition from cotton production 
INTRODUCTION 9

to grains and grazing in the Brazos Valley during World 

War II. They estimated that the construction of the dams 

in the 1940s reduced the rate of sand input to the coast 

to one-third of the rate before the 1940s.

Gillespie and Giardino (1997) describe and 

explain causative factors for the Brazos River channel 

migration. They hypothesize that reservoir construction 

immediately upstream of three study reaches has 

reduced the frequency of channel-forming discharges 

and has reduced sediment loads in the Brazos River 

as measured at the Richmond streamflow-gaging 

station (08114000). They analyzed total monthly and 

maximum daily discharge statistics at three USGS 

streamflow-gaging stations on the Brazos River (Waco 

[08096500], Bryan [08109000], and Hempstead 

[08111500]) and suspended-sediment concentrations 

measured by the TWDB at the Richmond streamflowgaging station. They divided the records into two parts: 

pre-1939 (pre-dam) and post-1939 (post-dam). Using 

Analysis of Variance (ANOVA), they found that predam discharge and sediment-concentration statistics are 

significantly greater than the post-1939 statistics. 

Gillespie and Giardino state that the results are consistent with a slowed rate of channel migration exhibited 

by the Brazos River in the post-dam period.

Capacity of the Brazos River to Transport 

Sand

Sediment is transported in a river as suspended 

load, as bed load by rolling and sliding along the bed, or 

both. Movement of sediment as suspended or bed load 

depends on the nature of the sediment (size, specific 

gravity, and shape) and the hydraulic characteristics 

of the flow. Generally, smaller particles such as silts 

and clays will move in suspension and larger sand-size 

particles will move both in suspension and as bed load. 

High-gradient streams have the capacity to transport 

appreciable quantities of sand- and gravel-sized 

particles in suspension, whereas low-gradient streams, 

transport most sand- and gravel-sized particles as bed 

load.

Velocity, depth, turbulence, and energy slope are 

the hydraulic characteristics that define the capacity of 

a river to transport sand-size sediment. For a given river, 

these characteristics will vary with the magnitude of 

flow; larger discharges will have a larger velocity, 

depth, turbulence, and energy slope than smaller discharges. Generally, only larger discharges develop the 

hydraulic characteristics necessary to transport appreciable quantities of sand-size or larger sediment in a 

river. The capacity of a river to transport sand depends 

on the frequency of discharges that develop the requisite 

hydraulic characteristics. As much as 90 percent of total 

sediment load in many streams is transported by flows 

that are equaled or exceeded about 10 percent of the 

time (Richardson and others, 1990). 

The sediment load of a river can be divided into 

wash-load and bed-load fractions. The wash load is 

defined as those smaller particles, generally silt and 

clay, that remain in suspension over a wide range of discharges and are found in minimal quantities in the bed 

material. The wash load originates as wash-off from 

storm events within the watershed. The wash load 

depends primarily on the rate and source of supply; it is 

generally not correlated with the hydraulic characteristics of the channel. The bed load consists of particles in 

the bed material and is usually correlated strongly with 

hydraulic characteristics, and thus, discharge (Chang, 

1988). The bed material of the Brazos River consists 

primarily of sand-size particles and gravel.

Incipient motion analysis can provide a rough 

indication of the capacity of a river to transport sand. 

When the hydraulic characteristics of a river are minimally sufficient to initiate movement of sand-size 

particles, the particles are moved along the streambed 

by the force of the flowing water. This force is characterized by the bed shear stress and can be computed for 

a given channel cross section as

, (1)

where 

is shear stress, in pounds per square foot; 

is unit weight of water, in pounds per cubic foot; 

is hydraulic depth (area divided by top width), in 

feet; and

is bed slope, in feet per foot.

For a given sediment grain size, the critical shear 

stress necessary to initiate motion can be determined 

using the Shields diagram (Vanoni, 1975). The shear 

stress, , resulting from a given discharge can be 

compared to the critical shear stress, , for incipient 

motion to determine if the given particle size will be 

transported. When the shear stress is less than the critical shear stress, the flow does not have the capacity to 

transport sediment of the representative size; when the 

shear stress is greater than the critical shear stress, the 

given bed material will be transported. The median sediment size (D50) represents the mesh size of a sieve 

τo = γDS

τo

γ

D

S

τo

τc
10 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

through which 50 percent, by weight, of the material 

passes and commonly is used as the representative size 

of bed material for tractive-force and bed-material 

transport computations (Stelczer, 1981).

Bed-material samples were obtained from the 

main channel at a daily mean discharge of about 

3,500 ft3

/s at the US 59 bridge 7 river miles downstream 

from the Richmond streamflow-gaging station during 

August 1991 for an unrelated study. Two additional 

samples were obtained from the main channel 3 river 

miles downstream from the Richmond streamflowgaging station in December 1996 and February 1997 at 

instantaneous discharges of 11,600 and 29,500 ft3/s, 

respectively. The samples were obtained using a 

US BMH–60 sampler, which obtains a 175-cubiccentimeter sample of bed material with a spring-loaded 

scoop. The scoop activates when the sampler contacts 

the bed and penetrates into the bed 1.7 in. (Guy and 

Norman, 1970). The shallow penetration of the sampler 

allows the capture of bed material that is most readily 

available for transport. Five to 10 individual samples 

were collected from the main part of the channel and 

were combined into a composite sample. The Shields 

diagram was used to compute the shear stress ratio 

for the D50 size for bed-material samples collected at 

three discharges. The particle-size distributions of the 

composite samples (fig. 6) were obtained using sieve 

analyses. The bed material coarsens substantially as discharge increases; the D50 increases from 0.34 to 

1.79 mm over the range of discharges sampled.

The instantaneous discharges sampled (3,500, 

11,600, and 29,500 ft3

/s) correspond to daily mean 

discharges with 46-, 20-, and 5-percent exceedance 

probabilities, respectively, for the 1903–95 period 

(excluding 1907–21) at the Richmond streamflowgaging station (fig. 7). Each sampled discharge has 

adequate capacity to initiate motion of the bed material 

on the basis of Shields diagram analysis (table 3). 

For the sample with D50 equal to 1.79 mm, the ratio 

is greater than unity for all discharges larger than 

Figure 6. Particle-size distributions of bed material samples near streamflow-gaging station 08114000 Brazos 

River at Richmond, Texas.

2 5 10 20 30 40 50 60 70 80 90 95

PERCENT FINER

0.1

0.2

0.3

0.4

0.5

0.7

1

2

3

4

5

7

10

PARTICLE-SIZE DIAMETER, IN MILLIMETERS

Sample discharges, in

cubic feet per second

29,500

3,500

11,600
INTRODUCTION 11

1,000 ft3/s (82-percent exceedance probability). This 

indicates that the Brazos River has sufficient capacity to 

initiate motion of the bed material (as characterized by 

the D50 size) at least 82 percent of the time (fig. 7).

The motion analysis was repeated using the D85 

size from each sampled discharge to examine the 

characteristics of the coarser fraction of bed material 

(table 3). For the sample with D85 equal to 5.98 mm, the 

ratio is greater than unity for all discharges larger than 

about 17,000 ft3

/s (11-percent exceedance probability). 

This indicates that the Brazos River has sufficient 

capacity to initiate motion of the coarser bed material 

(as characterized by the D85) at least 11 percent of the 

time (fig. 7).

1,000,000

100,000

10,000

1,000

100

10

DAILY MEAN DISCHARGE, IN CUBIC FEET PER SECOND

0.01 0.1 1 2 5 10 20 50 90 95 98 99 99.9 99.99

EXCEEDANCE PROBABILITY, IN PERCENT

Figure 7. Flow-duration curve of daily mean discharge for streamflow-gaging station 08114000 Brazos River at 

Richmond, Texas, 1903–95 (excluding 1907–21).

Table 3. Ratios of shear stress to critical shear stress required for initiation of motion at streamflow-gaging station 

08114000 Brazos River at Richmond, Texas

[D50 and D85, particle size for which 50 and 85 percent of the material is finer]

1At assumed temperature of 15 degrees Celsius.

Discharge

(cubic

feet per

second)

Hydraulic

radius

(feet)

Shear

stress

(pounds

per square

foot)

D50

(millimeters)

Critical

shear

stress1

(pounds

per square

foot)

Ratio of

shear

stress to

critical

shear

stress

D85

(millimeters)

Critical

shear

stress1

(pounds

per square

foot)

Ratio of

shear

stress to

critical

shear

stress

3,500 6.10 0.0506 0.34 0.0043 11.8 0.47 0.0057 8.9

11,600 11.47 .0952 .52 .0058 16.4 2.95 .0458 2.1

29,500 18.70 .1552 1.79 .0242 6.4 5.98 .1172 1.3
12 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

INDICATIONS OF CHANGE IN SAND 

TRANSPORT

The transport of sediment in most rivers is 

affected by multiple interdependent factors that 

include land-use changes in the basin, natural erosion 

processes along the main stem and tributaries, basin 

hydrology (particularly extreme events), water-resource 

development, and human activities in the main channel. 

River morphology and sediment transport change continually as a river adjusts in response to changes in these 

and other factors.

River channels will adjust to minimize energy 

dissipation, and these adjustments affect the capacity to 

transport the available sediment with the available water 

discharge. Channel adjustment processes can be characterized by the qualitative relation set forth by Lane 

(1955),

, (2)

where

Q is river discharge;

S is river slope;

is bed-sediment discharge; and

D is a characteristic particle-size diameter.

Lane’s relation states that the product of river discharge 

and river slope is proportional to the product of the 

bed-sediment discharge and a characteristic particlesize diameter. The river slope can refer to either the 

channel slope or the slope of the energy-grade line of 

the river but is usually taken as the latter.

If one or more of these four variables are modified, then one or more of the remaining variables will 

change to maintain the proportional relation. These 

changes can occur rather suddenly, as in response to 

abrupt disturbances such as channel shortening or 

straightening (Robbins and Simon, 1983); but often 

the changes will occur over long periods of time in 

response to more subtle disturbances such as land-use 

changes. Observed changes in channel characteristics 

might be remnant responses to disturbances that 

occurred decades earlier. Present-day channel responses 

usually cannot be attributed to one causative factor but 

to a combination of recent and historical factors.

Using the concepts described by Lanes’s relation, 

if the size of sediment available for transport and the 

dominant discharge (discharge with about a 1 to 2.5-

year recurrence interval) remain constant, then construction of a dam will decrease the energy slope of the 

river upstream from the dam. The sediment discharge 

also will decrease because of the reduced transport 

capacity of the river. The river will agrade (sediment 

will accumulate in the channel), and the size of the 

sediment transported likely will decrease. Downstream 

from the dam, if the dam traps the majority of the bed 

material and does not affect the dominant discharge, 

the channel would be expected to adjust the slope to 

maintain the proportional relation. The channel likely 

would erode downward (down-cut) to reduce the 

slope. In this case, the bed material eroded from downcutting—and bank material from subsequent bank failures—is made available for transport, and the sediment 

supply increases somewhat, at least temporarily. This 

illustrates the complexity of the river-adjustment 

phenomenon, which has been referred to by Thornes 

(1977) as having “indeterminacy” and “equifinality;” 

meaning, respectively, that a channel can respond to 

changes in many different ways, and a given response 

can be caused by several different factors. In actuality, 

all of the variables are changing at any given time, and 

a river remains in a state of continual adjustment toward 

dynamic equilibrium. If, however, one of the variables 

undergoes consistent long-term change, then the river 

can be expected to exhibit corresponding long-term 

change in accordance with Lanes’s relation.

Changes in the capacity of a river to transport 

sand can be identified within the context of Lane’s 

relation through changes in slope indicated by changes 

in channel geometry, changes in the characteristics 

of suspended loads, and changes in discharge. Total 

sediment-discharge data can be analyzed directly if 

available, but total sediment discharge rarely is 

measured because of the expense and difficulty of 

collecting the required bed-load data. However, 

suspended-sediment data are collected routinely by the 

USGS and other agencies and can be used as an indicator of changes in the sediment-transport characteristics 

of a river. Changes in suspended-sand load, if particle 

sizes of the suspended load have been determined, can 

be used to infer changes in the total sand load. All of 

these indicators, used conjunctively, provide evidence 

for concluding whether change in the sand-transport 

characteristics of a river has taken place.

Changes in Channel Geometry

Adjustment of river slope in response to 

changes in sediment supply and discharge often 

manifests as changes in channel geometry such as 

QS Qs ∼ D

Qs
INDICATIONS OF CHANGE IN SAND TRANSPORT 13

channel agradation or incision and channel widening. 

Streamflow-gaging-station records and surveyed channel geometry are sources of information from which to 

measure geometric adjustments of a river. 

Discharge measurements made at streamflowgaging stations over time can be used to identify 

changes in the channel characteristics. The Richmond 

streamflow-gaging station has been operated continually since 1925 at its present location and had 902 

measurements available at the time of this study; the 

Hempstead streamflow-gaging station since 1938 with 

746 measurements; and the Rosharon streamflowgaging station since 1967 with 241 measurements. 

Occasionally the streamflow-gaging-station datum is 

changed, so all stages have been converted to altitude 

above sea level.

The stage-discharge relations for the Hempstead, 

Richmond, and Rosharon streamflow-gaging stations 

are shown in figures 8–10. The measurements are 

differentiated by time period—pre-1940 (except 

Rosharon), 1940–68, and 1969–95. Long-term changes 

in the stage-discharge relations are indicated: At discharges below about 30,000 ft3/s, the water-surface 

altitude has declined several feet. At discharges above 

30,000 ft3/s, this trend is not apparent.

An analysis was done to estimate the rate of 

change of the water-surface altitude for a particular 

discharge. Water-surface altitude was regressed with 

discharge for all of the discharge measurements in a 

moving 5-year window for the entire period of each 

station’s record using a quadratic function of the form 

, (3)

where

A is water-surface altitude; 

, , and are regression coefficients; and

Q is discharge.

The regression equation for a given year was based on 

the discharge measurements made during the 5-year 

period starting 2 years before and ending 2 years after 

the given year. The water-surface altitude corresponding to a discharge of 5,000 ft3

/s was computed using 

the individual regressions for each year for which the 

full 5-year window of measurements was available. 

The water-surface altitudes at a discharge of 

5,000 ft3

/s for the three streamflow-gaging stations are 

A B1 B2Q + B3Q2 = +

B1 B2 B3

165

160

155

150

145

140

135

130

125

120

115

110

WATER-SURFACE ALTITUDE, IN FEET ABOVE SEA LEVEL

100 1,000 10,000

DISCHARGE, IN CUBIC FEET PER SECOND

100,000 200,000

Pre-1940

1940–68

1969–95

Figure 8. Stage-discharge relation at streamflow-gaging station 08111500 Brazos River near Hempstead, Texas.
14 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

55

50

45

40

35

30

25

20

15

10

5

0

WATER-SURFACE ALTITUDE, IN FEET ABOVE SEA LEVEL

10 100 1,000 10,000 100,000 200,000

DISCHARGE, IN CUBIC FEET PER SECOND

1940–68

1969–95

Figure 10. Stage-discharge relation at streamflow-gaging station 08116650 Brazos River near Rosharon, Texas.

90

85

80

75

70

65

60

55

50

45

40

35

WATER-SURFACE ALTITUDE, IN FEET ABOVE SEA LEVEL

10 100 1,000 10,000 100,000 200,000

DISCHARGE, IN CUBIC FEET PER SECOND

Pre-1940

1940–68

1969–95

Figure 9. Stage-discharge relation at streamflow-gaging station 08114000 Brazos River at Richmond, Texas.
INDICATIONS OF CHANGE IN SAND TRANSPORT 15

shown in figures 11–13. A non-parametric KendallTheil line (Helsel and Hirsch, 1992) was fit to the data 

to determine the long-term slope. The Kendall-Theil 

line defines the monotonic relation between two 

variables, in this case water-surface altitude and time. 

The analysis indicates that the long-term decline in 

water-surface altitude for a discharge of 5,000 ft3

/s has 

been about 0.028 ft/yr for the Hempstead streamflowgaging station, about 0.056 ft/yr for the Richmond 

streamflow-gaging station, and 0.066 ft/yr for the 

Rosharon streamflow-gaging station. The slopes of the 

lines are statistically significant (p <0.00003). 

The data at the Hempstead and Richmond 

streamflow-gaging stations also demonstrate cyclical 

patterns in the monotonic trends that match very closely 

from 1940 to about 1965 (figs. 11, 12). The overall 

decreases in the water-surface altitude at the three 

streamflow-gaging stations indicate that the transport 

characteristics of the river changed, which is consistent 

with a decreased supply of sand-size sediment in the 

river.

Changes in Suspended-Sediment 

Characteristics

During 1969–95, 215 periodic samples for 

measurement of suspended-sediment concentration 

were taken at the Richmond streamflow-gaging station 

over a range of discharge from 445 to 88,100 ft3

/s. 

The percent fines (generally silt and clay, with size 

less than 0.062 mm) was determined for 208 of the 

samples. The percent of sand in each sample was 

computed by subtracting the percent fines from 100 

percent. Four samples seemed to have erroneous 

concentrations and were excluded from the analysis. 

The percent sand in each sample was multiplied by 

the suspended-sediment concentration to compute the 

suspended-sand concentration (fig. 14). 

The suspended-sand concentrations apparently 

decreased, starting about 1982. Suspended-sand 

concentrations typically will decrease as discharge 

decreases, so decreases in suspended-sand concentrations could be attributed to systematically smaller 

sampled discharges. Such changes in sampled discharge 

over time could have a substantial temporal effect 

127

126

125

124

123

WATER-SURFACE ALTITUDE, IN FEET ABOVE SEA LEVEL

1221930

1935194019451950195519601965197019751980198519901995

YEAR

Water-surface altitude

Kendall-Theil line

Figure 11. Water-surface altitudes at a discharge of 5,000 cubic feet per second at streamflow-gaging station 

08111500 Brazos River near Hempstead, Texas.
16 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

19901995

YEAR

15

14

13

12

11

WATER-SURFACE ALTITUDE, IN FEET ABOVE SEA LEVEL

10

Water-surface altitude

Kendall-Theil line

Figure 13. Water-surface altitudes at a discharge of 5,000 cubic feet per second at streamflow-gaging station 

08116650 Brazos River near Rosharon, Texas.

48.0

47.5

47.0

46.5

46.0

45.5

45.0

44.5

44.0

43.5

WATER-SURFACE ALTITUDE, IN FEET ABOVE SEA LEVEL

43.01930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

YEAR

Water-surface altitude

Kendall-Theil line

Figure 12. Water-surface altitudes at a discharge of 5,000 cubic feet per second at streamflow-gaging station 

08114000 Brazos River at Richmond, Texas.
INDICATIONS OF CHANGE IN SAND TRANSPORT 17

on suspended-sand concentrations. The discharges 

sampled, which ranged from 445 to 88,100 ft3/s are 

shown in figure 15. The Wilcoxon rank-sum test (Helsel 

and Hirsch, 1992) was applied to test for differences 

between the median minimum and median maximum 

discharges sampled during 1969–81 and 1982–95. The 

median of the yearly minimum sampled discharges for 

1969–81 (2,649 ft3

/s) is significantly larger (p = 0.011) 

than the median for 1982–95 (1,145 ft3

/s). No change 

was detected in the maximum discharges sampled 

each year (p = 0.41) nor in the range of discharges sampled (p = 0.45). These tests indicate that more samples 

were collected at generally smaller discharges during 

1982–95 but that the maximum discharge sampled each 

year did not change significantly. About equal numbers 

of samples were taken above 2,000 ft3

/s (median daily 

discharge) for both periods (81 samples during 1969–81 

and 86 during 1982–95)—in other words, median 

sampled discharge was about the same for both periods. 

The apparent change in the suspended-sand concentrations cannot be attributed to a change in the discharges 

sampled.

LOcally WEighted Scatterplot Smoothing 

(LOWESS) lines (Cleveland, 1979) were fit to 1969–81 

and 1982–95 suspended-sand concentration as a 

function of discharge (fig. 16). For discharges larger 

than 1,000 ft3/s, the suspended-sand concentrations of 

samples taken during 1969–81 were greater than those 

taken during 1982–95 for similarly sampled discharges. 

The LOWESS lines provide a graphical comparison 

between the two periods but do not assign a statistical 

significance to the difference.

A LOWESS line was fit to the suspended-sand 

versus discharge graph with no temporal differentiation, 

and residuals from that line were computed (residual 

equals LOWESS minus measured). The residuals represent concentration, adjusted to remove any relation to 

discharge. Residuals thus computed can be identified as 

“flow-adjusted residual concentrations” (Helsel and 

Hirsch, 1992). A Wilcoxon rank-sum test indicates that 

the median residual concentration during 1969–81 

is significantly less than the median residual concentration during 1982–95 (p <0.00005), which provides 

strong statistical evidence that the suspended-sand 

1,400

1,200

1,000

800

600

400

200

0

SUSPENDED-SAND CONCENTRATION, IN

MILLIGRAMS PER LITER

YEAR

19691971197319751977197919811983198519871989199119931995

Figure 14. Suspended-sand concentrations in samples at streamflow-gaging station 08114000 Brazos River at 

Richmond, Texas, 1969–95.
18 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

concentration at a given discharge decreased during 

1982–95.

The suspended-sediment samples contained less 

than 35-percent sand-size particles. A LOWESS line 

fit to a graph of percent sand versus discharge (fig. 17) 

indicates that the percent sand increased at discharges 

larger than about 5,000 ft3

/s. Residuals computed 

from this LOWESS line represent the percent sand 

independent of discharge. A Wilcoxon rank-sum test 

indicates that the median residual percentage of sand 

during 1969–81 is significantly smaller than the median 

during 1982–95 (p = 0.084), which indicates that the 

percent sand at a given discharge also decreased during 

1982–95.

These analyses indicate that suspended-sand 

concentrations in the Brazos River at Richmond for a 

given discharge decreased during 1982–95. Both 

suspended-sand concentrations and the percentage of 

sand in suspension decreased. These decreases could be 

caused by a change in the hydraulics of the flow at the 

measurement cross section. A decrease in the channel 

velocity would cause more of the sediment to be transported as bed load, which would not be measured by a 

suspended-sediment sampler. However, a graph of discharge and mean velocity for all measurements during 

1969–95 (fig. 18) indicates that mean velocities have 

not decreased. LOWESS lines were fit to this graph for 

all discharge measurements made from the US 90–A 

bridge. The median residuals for the two periods are not 

significantly different (p = 0.24). However, this p-value 

does indicate some weak evidence that the mean velocity has changed.

Velocity varies across a cross section, and the suspension of sand will vary accordingly. Only a small 

fraction of the cross section can sustain velocities and 

shear stresses capable of transporting sand in suspension; as the velocity in this part of the cross section 

decreases, the transport of sand in suspension likely will 

decrease. If the mean velocity remains constant, the 

corresponding increase in velocity over the remainder 

of the cross section might be insufficient to increase the 

transport of suspended sand. For this reason, the mean 

velocity might not be a sufficient indicator of the 

hydraulic capacity of a stream to transport sand in 

suspension. The maximum velocity measured in the 

cross section during each discharge measurement might 

100,000

10,000

1,000

100

 DISCHARGE, IN CUBIC FEET PER SECOND

196919711973197519771979198119831985

YEAR

19871989199119931995

Figure 15. Sampled discharges at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 

1969–95.
INDICATIONS OF CHANGE IN SAND TRANSPORT 19

32

28

24

20

16

12

8

4

0

PERCENT SAND

100 1,000 10,000 100,000

DISCHARGE, IN CUBIC FEET PER SECOND

LOWESS

36

Figure 17. Relation of percent sand in suspended-sediment samples to discharge at streamflow-gaging station 

08114000 Brazos River at Richmond, Texas, 1969–95.

10,000

1,000

100

10

0.1

SUSPENDED-SAND CONCENTRATION, IN MILLIGRAMS PER LITER

100 1,000 10,000 100,000

DISCHARGE, IN CUBIC FEET PER SECOND

LOWESS, 1969–81

LOWESS, 1982–95

1969–81

1982–95

1

Figure 16. Relation of suspended-sand concentration to discharge at streamflow-gaging station 08114000 Brazos 

River at Richmond, Texas, 1969–95.
20 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

provide a better indication of reduced transport 

capacity. A decrease in the maximum velocity in the 

cross section for a given discharge would provide 

some explanation for the decrease in suspended sand 

measured. A LOWESS line was fit to a graph of discharge versus the maximum velocity in the cross section 

for the measurements made during 1969–95. The residuals from that LOWESS line do not indicate that the 

maximum velocity in the cross section decreased for a 

given discharge during 1982–95 (p = 0.440). 

 The depth at the cross-section location at which 

the maximum velocity was measured was derived from 

the discharge measurements. A LOWESS line was fit to 

the graph of mean velocity versus this depth, which 

closely approximates the maximum depth in the 

cross section. The residuals from this LOWESS line 

indicate that for a given mean velocity, the depth of the 

cross-section location of maximum velocity was about 

3.6 ft deeper during 1969–81 than during 1982–95 

(p <0.00005). The mean channel depth and channel 

width were similarly analyzed, and the results indicate 

that the mean channel depth was greater and the channel 

width was narrower during 1969–81 than during 1982–

95 (p <0.00005). However, no change in the crosssectional area of the channel was detected. 

Although the maximum velocity in the cross 

section has not changed at a level near statistical significance, the change in mean velocity could be considered 

weakly significant (p = 0.24). Coupled with changes 

in other characteristics known to affect the capacity 

of the channel to transport sediment, a subtle change in 

the mean velocity might affect a channel’s transport 

capacity. 

Shear stress developed in a channel (eq. 1) is 

an indicator of the capacity of a channel to transport 

sediment. A change in shear stress can therefore reflect 

a change in the capacity of a channel to transport 

sediment. Assuming that the overall bed slope of the 

channel and the unit weight of water remain constant, 

a change in the hydraulic depth will signal a change 

in shear stress. A LOWESS line was fit to a hydraulic 

depth versus discharge graph. The residuals from 

the LOWESS line changed significantly over time 

(p <0.01), indicating that the channel has a reduced 

6.0

DISCHARGE, IN CUBIC FEET PER SECOND

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

MEAN VELOCITY, IN FEET PER SECOND

500 1,000 10,000 100,000

LOWESS, 1969–81

LOWESS, 1982–95

1969–81

1982–95

Figure 18. Relation of mean channel velocity to discharge at streamflow-gaging station 08114000 Brazos River at 

Richmond, Texas, 1969–95.
INDICATIONS OF CHANGE IN SAND TRANSPORT 21

capacity to transport sand-size sediment because of a 

reduction in shear stress developed by the channel. 

In summary, no statistically significant change 

was detected in the mean or maximum channel velocity, 

but some evidence exists that suggests the mean channel 

velocity decreased during 1982–95. The channel depth 

has decreased as the channel width has increased; the 

resulting water-surface altitude has decreased, and 

the hydraulic depth related to shear stress has decreased. 

These changes in channel geometry might be responsible for the apparent changes in suspended-sand 

transport. The apparent changes in suspended-sediment 

transport cannot be attributed to changes in the sampling or laboratory methods, which have remained 

constant since the start of the sampling at the Richmond 

station. In the late 1980s, the bridge from which all 

measurements had been taken was removed, and a new 

bridge was constructed at the same location.

Changes in Discharge

Sand discharge in a river varies greatly with 

water discharge. Sand will not be transported in substantial quantities until the channel develops sufficient 

hydraulic characteristics, which usually occur during 

larger (above-average) flows. Changes in the magnitude 

and frequency of larger discharges can be caused by 

changes in land use, such as rapid urbanization; longterm climatic changes; diversion for flood control; and 

reservoir construction, both for flood control and water 

supply. Of these, the most likely to influence large flows 

in the Brazos River is reservoir construction, particularly those reservoirs constructed for flood control. 

Flows in the Brazos River are unlikely to be influenced 

by urbanization, simply because the size of the basin is 

large relative to the size of the urban areas within. The 

effects of long-term climatic changes will be more gradual than the effects of reservoir construction, considering the extent of reservoir construction in the basin.

Reservoirs can affect the distribution of both daily 

and annual-peak discharges. Flow-duration curves of 

daily mean discharge at the Hempstead and Richmond 

streamflow-gaging stations are shown in figures 19 and 

20. The records have been divided into two (for Hempstead) or three (for Richmond) periods: early (pre1940), middle (1940–68), and late (1969–95). These 

periods roughly correspond to periods in which the rate 

200,000

100,000

10,000

1,000

100

DAILY MEAN DISCHARGE, IN CUBIC FEET PER SECOND

0.01 0.1 1 2 5 10 20 50 90 95 98 99 99.9 99.99

NONEXCEEDANCE PROBABILITY, IN PERCENT

1940–68

1969–95

Figure 19. Flow-duration curves of daily mean discharge for streamflow-gaging station 08111500 Brazos River 

near Hempstead, Texas, 1940–68 and 1969–95.
22 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

of increase in reservoir storage was distinctly different. 

The differences in the distributions are typical of 

changes in discharge downstream from reservoirs—the 

lower-tail discharges are increased as a result of controlled releases, and the upper-tail discharges are 

reduced as a result of regulation for flood control.

The upper parts of the curves at both streamflowgaging stations do not differ substantially until about the 

4-percent exceedance probability (which indicates 

the discharge that was equaled or exceeded during 

4 percent of the days in the period—about 30,000 ft3/s). 

At 4-percent and greater exceedance probabilities, 

the magnitudes of discharge at given frequencies of 

occurrence are substantially different. The 1-percent 

discharge at Hempstead changed from about 61,000 

ft3/s during 1940–68 to 50,000 ft3/s during 1969–95. 

The 1-percent discharge at Richmond changed little 

from pre-1940 to 1940–68 (67,000 ft3

/s) but changed to 

55,700 ft3/s during 1969–95. 

Comparing flow-duration curves gives only a preliminary indication that the distribution of daily flows at 

the streamflow-gaging stations has changed. Many of 

the discharges in the upper tails of the distributions 

likely are from the same hydrologic events and thus are 

not independent. Enough daily discharges exist that several independent events are included in the upper tails of 

the distributions; however, the distributions only imply 

that differences in flows between periods exist without 

assigning a level of statistical significance to support the 

implication.

The distributions of daily discharge at the 

Richmond streamflow-gaging station were investigated 

on an annual basis by computing flow percentiles for 

each annual series of daily discharge. The daily discharges composing the distribution in each year likely 

are not independent, but the distributions from year 

to year can be considered independent. Upper-tail 

percentiles (50, 75, 80, 83.3, 87.5, 90, 95, 98, and 99) 

were computed for each year of both periods. The early 

period contains 21 complete years of daily streamflow 

record for which flow percentiles were computed. The 

late period contains 27 years, so for each flow percentile, the 27 values were converted to an equivalent 21-

year series using order statistics (David, 1981; Kaigh 

and Driscoll, 1987). The order statistics preserve the 

underlying statistical distribution of a sample series 

0.01 0.1 1 2 5 10 20 50 90 95 98 99 99.9 99.99

200,000

100,000

10,000

1,000

100

10

Pre-1940

1940–68

1969–95

DAILY MEAN DISCHARGE, IN CUBIC FEET PER SECOND

NONEXCEEDANCE PROBABILITY, IN PERCENT

Figure 20. Flow-duration curves of daily mean discharge for streamflow-gaging station 08114000 Brazos River at 

Richmond, Texas, pre-1940, 1940–68, and 1969–95.
INDICATIONS OF CHANGE IN SAND TRANSPORT 23

while compressing its size and are ideal for comparing 

the distributions of samples of unequal sizes.

For each flow percentile, two tests were done to 

identify differences between the periods. Wilcoxon 

rank-sum tests were done to test for differences in the 

medians of each of the percentiles computed. Then, the 

percentiles from each year in both periods were ranked, 

the differences between the ranked data were computed, 

and Student’s t-tests (Helsel and Hirsch, 1992) were 

done on the differences. The rank-sum tests indicate 

no statistically significant differences in the medians 

of any flow percentile; however, the p-values for 

discharges larger than the 83.3 percentile are all less 

than 0.20—evidence for differences, but not strong 

enough evidence to declare the differences “significant.” The tests on the differences in the ranked percentiles indicate that the daily discharges for the early and 

late periods are significantly different for the 75 percentile and larger flows. The p-values associated with these 

tests are summarized in table 4. Similar analyses were 

not done for the Hempstead streamflow-gaging station 

data because continuous daily discharge for the early 

period was not available; however, the distributions of 

daily flows are similar between the two sites. 

The preceding analyses indicate that the magnitude of discharges with larger recurrence intervals 

(90 percentile and larger daily flows) has decreased 

since the early 1940s. The reduction in the upper tails of 

the distributions of the daily discharges are consistent 

with what would be expected downstream from major 

reservoirs (Collier and others, 1996). The larger discharges likely transport a large part of the Brazos River 

sand load.

Changes in transport capacity because of reductions in the frequency and magnitude of large discharges 

can be identified by estimating loads using the historical 

record of daily discharge and a functional relation 

between sand transport and discharge. This functional 

relation can be from a regression analysis relating 

measured transport rates with discharge, a sedimenttransport curve relating transport rates with discharge, 

or one of the many sediment-transport equations published in the literature.

A sediment-transport curve was drawn relating 

suspended-sand load to water discharge at the Richmond streamflow-gaging station (fig. 21) to test 

whether the capacity of the Brazos River to transport 

sediment has been reduced significantly solely because 

of reductions in large discharges. The sediment samples 

taken before 1982 and after 1981 were not differentiated 

because the intent was to determine, given a representative transport relation, whether reductions in the frequency and magnitude of large discharges could reduce 

the capacity of the Brazos River to transport sand-size 

sediment. Load-discharge pairs were read from the 

rating curve and used to build a rating table. The rating 

table was applied to the historical record of daily discharge at the Richmond streamflow-gaging station 

using log-log interpolation between the rating table 

values to compute daily values of suspended-sand load.

The daily values of suspended-sand load were 

then summed to annual values, as shown in figure 22. 

Table 4. Summary of statistical tests for differences in annually computed daily flow percentiles between the pre1940 period and the 1969–95 period at streamflow-gaging station 08114000 Brazos River at Richmond, Texas 

Percentile

Median discharge

(cubic feet per second) p-value

Pre-1940 1969–95

Test for difference

in median percentile

Test for difference

in ranked percentile

50 8,583 7,594 0.450 0.332

75 17,427 15,190 .265 .013

80 20,615 17,717 .241 .039

83.3 23,212 19,811 .210 .039

87.5 26,836 23,284 .176 .013

90 29,650 26,105 .151 .004

95 38,465 34,439 .145 <.001

98 49,870 43,290 .134 <.001

99 57,923 48,679 .129 <.001
24 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

34,000

32,000

30,000

28,000

26,000

24,000

22,000

20,000

18,000

16,000

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0

ANNUAL SUSPENDED-SAND LOAD, IN TONS PER DAY

1900191019201930194019501960197019801990

YEAR

1997

No data

Pre-1940

1940–68

1969–95

Mean annual loads

Median annual loads

Figure 22. Annual suspended-sand loads at streamflow-gaging station 08114000 Brazos River at Richmond, 

Texas.

300,000

100,000

10,000

1,000

100

10

1

0.1

SUSPENDED-SAND LOAD, IN TONS PER DAY

100 1,000 10,000 100,000 200,000

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 21. Suspended-sand transport curve for streamflow-gaging station 08114000 Brazos River at Richmond, 

Texas.
POTENTIAL SOURCES OF CHANGE IN SAND TRANSPORT 25

There is no statistically significant difference in the 

median annual loads between the early, middle, and 

late periods. The median annual suspended-sand load 

was estimated to be 6,350 tons per day during the early 

period and 4,960 tons per day during the late period. 

Although the difference between the early and late 

periods seems appreciable, it is not statistically significant (p = 0.22), probably because of the large variance 

in the data and the relatively few datapoints available. 

The fact that the median annual suspended-sand loads 

were not significantly different between the early and 

late periods prevents a definitive conclusion that the 

frequency and magnitude of large discharges reduced 

the capacity of the Brazos River to transport sand-size 

sediment. 

POTENTIAL SOURCES OF CHANGE IN 

SAND TRANSPORT

Several indicators of change in the transport of 

sand in the Brazos River have been identified, and all 

relate within the context of Lane’s relation (eq. 3). The 

Brazos River was shown to develop sufficient shear 

stress to mobilize its bed material at least 82 percent of 

the time, so the capacity of the river to transport bed 

material is not in question. The transport of sand in the 

Brazos River depends on a complex set of factors, most 

of which are continually changing. Identification of a 

single causative factor as the sole source of change generally is impossible, unless that activity overwhelmingly influences the hydrology of the basin or the 

hydraulic characteristics of the river channel. All of the 

indicators of change identified provide only subtle 

inferences of change. They do not provide definitive 

evidence of a change in sand loads, but they do imply 

that sand loads are decreasing somewhat. Below, several 

possible sources of change are discussed in relation to 

the indicators previously identified.

Reservoir Construction

The extensive reservoir construction in the Brazos 

River Basin could substantially reduce sand transport to 

the Gulf by trapping sediment and by reducing the magnitude of upper-tail discharges. The entrapment of sand 

generally has the most critical effect immediately downstream from a reservoir. Water released from the reservoir contains little or no sand and removes material 

from the stream channel to satisfy the transport capacity. Subsequent channel down-cutting as great as 30 or 

40 ft has been observed in some rivers. Channel erosion 

continues downstream until instream sediment is resupplied by erosion from the bed and banks of the channel 

or by tributaries.

The amount of sediment trapped in the Brazos 

River reservoirs is difficult to determine. Initial 

reservoir-capacity surveys made during the times when 

most reservoirs were constructed were of limited accuracy. Relatively few updated surveys have been made in 

the Brazos River Basin, and until recently, those surveys 

also had accuracy limitations. The TWDB recently 

began using improved reservoir-capacity survey techniques that have been used for a few reservoirs in the 

Brazos River Basin. The accuracy of the new surveys is 

greatly improved over those made using traditional 

methods. However, the increase in accuracy limits the 

ability to make meaningful comparisons between the 

new surveys and the original surveys to determine reservoir sedimentation rates. One cannot make a comparison between the two with any degree of confidence 

(Scott Sullivan, Texas Water Development Board, oral 

commun., 1996). Any change in reservoir capacity 

could be attributed simply to differences in survey techniques. Several of the recent reservoir-capacity surveys 

made by the TWDB have shown larger reservoir capacities than those obtained during the original surveys.

Regardless, reservoir entrapment of sand probably has little effect on sand transport to the lower 

reaches of the Brazos River. About 350 river miles 

separate the farthest downstream main-channel reservoir (Lake Whitney) and the Richmond streamflowgaging station. The City of Waco operates a watersupply diversion dam on the main stem of the Brazos 

River, about 309 river miles upstream of the Richmond 

gaging station, which also has the potential to trap sand. 

About 40 tributaries drain to the Brazos River between 

Lake Whitney and Waco. Childress Creek is the largest 

unregulated tributary between Lake Whitney and Waco, 

with a drainage area of 87 mi2

. Allen and others (1989) 

documented about 4 to 5 ft of local channel agradation 

in this reach since construction of Lake Whitney in 

1951 and stated that the reduction of peak flows from 

Lake Whitney reduced the ability of the Brazos River 

to transport sediment discharged from tributaries, 
26 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

facilitating the growth of tributary delta systems downstream of Lake Whitney. Bank loss in this reach has 

ranged from 5 to 14 ft/yr, indicating that substantial 

quantities of sand are being introduced to the main 

channel. Apparently, sand trapped by the upstream 

reservoir is compensated for within a short distance 

downstream by the introduction of tributary sediment 

and localized bank erosion.

Reservoirs have reduced the magnitude of uppertail discharges in the lower Brazos River and reduced 

the capacity of the Brazos River to transport sand to the 

Gulf. However, during extended, severely dry periods 

such as occurred during the drought of the 1950s, regulated peak flows were smaller than natural peaks under 

drought conditions to maintain reservoir conservation 

storage. Seelig and Sorenson (1973) analyzed flows and 

sediment loads from the Brazos River to the Gulf from 

about 1930 to about 1965. They identify a substantial 

decrease (about 60 percent) in the transport of sand to 

the Gulf beginning about 1941 and attribute much of the 

decrease to reservoir construction. However, data collected since 1965 indicate that sand transport has not 

decreased as much as reported by Seelig and Sorenson 

(1973). The decreases identified by Seelig and Sorenson 

(1973) likely were caused more by the extended drought 

of the 1950s than by upstream reservoir construction. 

However, the presence of the reservoirs during this 

severe drought did reduce peak flows and, consequently, sediment transport. 

Changes in Land Use

Changes in land use in a river basin will alter the 

amount of sediment delivered to a stream by erosion 

processes. During the 19th and early parts of the 20th 

centuries, the amount of land in the Brazos River Basin 

under agricultural production increased substantially. 

This land, originally rangeland or forested woodland, 

was cleared of vegetation to grow crops such as cotton, 

corn, and sorghum. Modern soil-conservation practices 

in agriculture were not available, and the erosion potential of the cleared land greatly increased when the 

natural vegetation was removed. Although no records 

are available, it is recognized generally that substantial 

quantities of topsoil were lost. The topsoil was transported to the tributaries and main channel of the Brazos 

River by hillslope erosion processes. 

Although accurate determination of the quantity 

of soil transported to the Brazos River attributable to 

agricultural practices is not possible, documented historical amounts of land under agricultural production 

can be used to estimate qualitatively the erosion potential in the watershed at various times. The U.S. Department of Agriculture publishes by county the number of 

acres harvested every 5 years. The county figures are 

aggregated by individual crop types. The data for 1924–

92 were retrieved for total crops harvested and for total 

acres of hay for the 27 counties that are partly or 

completely within the lower one-third of the Brazos 

River Basin. The total area in these counties is about 

15,000,000 acres. Acres of hay were subtracted from 

the total acres harvested to estimate the amount of land 

under non-hay crop production. Land under production 

of non-hay crops generally has more erosion potential 

than land under hay production.

As shown in figure 23, the total area of harvested 

acres of non-hay crops in the lower basin during 1924–

92 decreased from about 32 percent to about 8 percent 

of the total area. Without consideration of improved 

soil-conservation practices, the erosion potential in the 

Brazos River Basin has decreased substantially since 

1924, simply on the basis of total acreage removed from 

agricultural production. Most of this land has been 

transformed to open rangeland for which the erosion 

potential is small. Substantial decreases in the erosion 

potential of a river basin generally are followed by a 

decrease in the quantity of sediment reaching the mainstem channel; the decrease generally is not immediate 

but will occur gradually as the supply from watershed 

erosion diminishes. Any decrease in the quantity of sediment transported by the Brazos River can be attributed 

partly to a decrease in the supply of sediment from the 

watershed, manifested first in a decrease in wash load, 

which tends to have a rapid response to changes in 

watershed erosion potential.

Instream Sand and Gravel Mining

Sand and gravel have been mined from the 

lower Brazos River channel for many years. The locations of sand and gravel mining operations, as permitted 

by the TPWD in 1995, in the reach between the 

Hempstead and Rosharon streamflow-gaging stations 

are shown in figure 24. The operations in this area 

account for 2 to 3 percent of the total statewide sand and 

gravel production (Gustavson and Bullen, 1996). 

Annual weight of sand extracted by the operators in the 

area as reported to the TPWD for 1979–95 is shown in 

figure 25. The total suspended sand transported past the 
POTENTIAL SOURCES OF CHANGE IN SAND TRANSPORT 27

Richmond streamflow-gaging station computed using 

the sediment-transport curve described previously also 

is shown in figure 25.

Cumulative weight of sand extracted and cumulative sand transported for 1979–95 are shown in figure 

26. During that period, sand extracted from the Brazos 

River channel totaled about 28 percent of the suspended 

sand transported past the Richmond streamflow-gaging 

station. The amount of sand extracted through instream 

mining represents a fraction of the total sediment transported by the Brazos River.

Depending on the configuration and size of operation, the dredged (excavated) part of the main channel 

has the potential to refill rather quickly by trapping sediment transported from upstream. Most dredged areas 

can be substantially refilled during a single, prolonged 

rise of the Brazos River (David Kurz, Sand Supply 

Corp., oral commun., 1996). Bed load is trapped, and 

larger particles in suspension also can settle out when 

velocities in the dredged area generally are slower than 

those in the undisturbed channel immediately adjacent.

A sudden decrease in the supply of sediment 

(such as potentially would occur downstream from a 

mining operation that traps substantial quantities of sediment) could cause bank erosion and general channel 

instability downstream. In Lane’s relation (eq. 3), a 

decrease in sediment supply, assuming no change in the 

dominant discharge or the characteristic size of the 

transported sediment, will result in a decrease in the 

energy slope. A decrease in the energy slope will result 

in either channel down-cutting, an increased rate of 

channel migration (meandering), or a combination of 

both.

Gustavson and Bullen (1996) investigated the 

wavelength and amplitude of meander bends in the 

Brazos River in the same area as this study and found 

no temporal changes in the general morphology of 

individual meander loops. They did find that the lateral 

migration of meander bends decreases through the 

study area and that the channel becomes progressively 

more incised in the downstream direction. However, 

Gustavson and Bullen (1996) attribute these findings 

more to variation in valley slope and lithology than to 

sand and gravel mining.

5,000,000

4,500,000

4,000,000

3,500,000

3,000,000

2,500,000

2,000,000

1,500,000

1,000,000

500,000

HARVESTED ACRES

1924

1929

1934

1940

1945

1950

1954

1959

1964

1969

1974

1978

1982

1987

1992

YEAR

Hay

Non-hay

Figure 23. Total acres harvested in the 27 counties in the lower Brazos River Basin, Texas.
28 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

The quantity of sediment extracted by instream 

sand and gravel mining operations represents about 28 

percent of the suspended sand transported by the Brazos 

River at Richmond. However, it represents a potentially 

smaller percentage of the total sand transported by the 

river in suspension and bed load. The bed load can range 

from 10 to 150 percent of the suspended load of a stream 

(Simons and Senturk, 1992). On the basis of these percentages, the quantity of sand extracted from the Brazos 

River could represent from 11 to 25 percent of the total 

sand transported. This extraction occurs over a distance 

of about 100 river miles, which probably lessens the 

overall effects. Consequently, the overall long-term 

effects of instream sand and gravel mining on sand 

transported from the Brazos River to the Gulf of Mexico 

could not be quantified.

BRAZOS

RIVER

SOMERVILLE

 LAKE

WASHINGTON

 COUNTY

AUSTIN

 COUNTY

WALLER

 COUNTY

FORT

 BEND

 COUNTY

BRAZORIA

 COUNTY

HEMPSTEAD

96o

96o30'

95o

30'

30o

29o30'

29o

0 10 20 30 MILES

RICHMOND

ROSHARON

08111500

08114000

08116650

Sand and gravel production

site

Streamflow-gaging station

and number

EXPLANATION

08111500

Basin boundary

Figure 24. Location of selected sand and gravel production sites in the lower Brazos River Basin, Texas.
POTENTIAL SOURCES OF CHANGE IN SAND TRANSPORT 29

45,000

40,000

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0 1979

1980198119821983198419851986198719881989199019911992199319941995

YEAR

THOUSANDS OF TONS

Cumulative weight of sand extracted

Cumulative load of suspended

 sand transported

Figure 26. Cumulative annual weight of sand extracted and annual loads of suspended sand transported in the 

Brazos River between Hempstead and Rosharon, Texas, 1979–95.

13,000

12,000

11,000

10,000

9,000

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

THOUSANDS OF TONS

19791980198119821983198419851986198719881989199019911992199319941995

YEAR

Annual weight of sand extracted

Annual load of suspended sand transported

Figure 25. Total annual weight of sand extracted and annual loads of suspended sand transported in the Brazos 

River between Hempstead and Rosharon, Texas, 1979–95.
30 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

SUMMARY

The Brazos River can be characterized as an 

incised, meandering, sand-bed channel with unstable 

banks. Frequent, near-vertical cut banks 20 to 35 ft high 

are prominent along much of its length. The channel 

slopes at about 0.7 ft/mi in the lower 300 river miles. 

Extensive reservoir construction has taken place in the 

Brazos River Basin since the early 1940s. The reservoir 

storage in the basin increased from 258,675 acre-ft in 

1940 to 4,613,800 acre-ft in 1995. Land use in the 

Brazos River Basin predominantly is agricultural and 

open rangeland, with about 1,900,000 harvested acres in 

the 27 counties in the lower part of the basin (in 1992). 

The dominant crops produced in the basin are cotton, 

corn, and sorghum.

In a river, sediment is transported in suspension, 

as bed load by rolling and sliding along the bed, and 

conjunctively by suspension and bed load. This movement depends on the nature of the sediment (size, 

specific gravity, and shape) and the hydraulic characteristics of the flow. Generally, only larger discharges 

develop the hydraulic characteristics necessary to transport substantial quantities of sand-size sediment. The 

results of a tractive-force analysis at three sampled discharges for the D50 and D85 particle sizes indicate that 

each sampled discharge has adequate capacity to initiate 

motion of the bed material in the Brazos River.

The transport of sediment in most rivers is 

affected by multiple interdependent factors including land-use changes in the basin, natural erosion 

processes along the main stem and tributaries, basin 

hydrology (particularly extreme events), water-resource 

development, and human activity in the main channel. 

River morphology and sediment transport change continually as a river adjusts in response to changes in these 

and other factors. River channels will adjust to minimize energy dissipation, and these adjustments affect 

the capacity to transport the available sediment with the 

available discharge. Channel-adjustment processes can 

be characterized by Lane’s relation: The product of river 

discharge and river slope are proportional to the product 

of the bed-sediment discharge and a characteristic 

particle-size diameter. 

Changes in the capacity of a river to transport 

sand can be identified in the context of Lane’s relation 

through changes in channel geometry, changes in the 

characteristics of suspended loads, and changes in discharge. On the basis of inspection and analysis of historical stage-discharge relations, the Brazos River channel 

has been undergoing continual adjustment since the 

1940s. The expected water-surface altitude for a discharge of 5,000 ft3

/s has decreased about 2 to 4 ft at the 

Hempstead and Richmond gaging stations since the 

early 1940s. These decreases in water-surface altitude 

indicate channel adjustments consistent with a 

decreased supply of sand-size sediment. 

The amount of sand-size sediment in suspendedsediment samples at the Richmond streamflow-gaging 

station decreased between the periods 1969–81 and 

1982–95. This decrease cannot be attributed to any 

change in sampling strategy; although the number of 

smaller discharges sampled each year did increase, no 

identifiable change was detected in the larger sampled 

discharges for which sand concentrations typically are 

larger. These changes occurred during a relatively short 

time period (1969–95) compared with the length of 

record for stage-discharge measurements and give no 

insight into overall changes since the early 1940s. The 

distribution of both daily and annual-peak discharges 

at the Hempstead and Richmond streamflow-gaging 

stations has changed. Discharges larger than the 90-

percentile daily flow and most annual-peak discharges 

have decreased. The reduction in the largest discharges 

would tend to reduce the capacity of the Brazos River to 

transport sand. However, the computed annual loads of 

suspended sand indicate no statistically significant 

change in the median annual load.

The transport of sand in the Brazos River depends 

on a complex set of factors, most of which are continually changing. Potential sources of change in sand 

transport in the Brazos River include the effects from 

reservoir construction, changes in land use, and 

instream sand and gravel mining. Extensive reservoir 

construction in the Brazos River Basin has reduced sand 

transport downstream from reservoirs by trapping sediment and by reducing the magnitude of peak discharges. 

The reservoir entrapment of sand probably has little 

effect on sand transport to the lower reaches of the 

Brazos River because bank losses in the downstream 

reaches indicate that substantial quantities of sand are 

being introduced to the main channel. The sand that is 

trapped by the upstream reservoir apparently is compensated for downstream by the introduction of tributary 

sediment and localized bank erosion. 

Changes in land use in a river basin will alter the 

amount of sediment delivered to a stream by erosion 

processes. Substantial decreases in the erosion potential 

of a river basin generally are followed by a decrease 

in the quantity of sediment reaching the main-stem 
REFERENCES CITED 31

channel. The total area of harvested acres of non-hay 

crops in the lower Brazos River Basin during 1924–92 

decreased more than 75 percent from about 32 percent 

to about 8 percent of the total area. The removal of 

major acreage from agricultural production has substantially reduced erosion potential. 

Several sand and gravel mining sites are located 

on the Brazos River between Hempstead and Rosharon. 

The quantity of sediment extracted by instream sand 

and gravel mining operations could represent from 11 to 

25 percent of the total sand transported by the Brazos 

River. However, the extraction occurs over a distance of 

about 100 river miles, which probably lessons the overall effects. The effects of mining on sand transport could 

not be quantified. 

REFERENCES CITED

Allen, P.M., Hobbs, R., and Maier, N.D., 1989, Downstream 

impacts of a dam on a bedrock fluvial system, Brazos 

River, Central Texas: Bulletin of the Association of 

Engineering Geologists, v. XXVI, no. 2, p. 165–189.

Andrews, F.L., 1989, Monthly and annual suspendedsediment loads in the Brazos River at Richmond, Texas, 

1966–86 water years: U.S. Geological Survey WaterResources Investigations Report 88–4216, 20 p.

Chang, H.H., 1988, Fluvial processes in river engineering: 

New York, John Wiley, 432 p.

Cleveland, W.S., 1979, Robust locally weighted regression 

and smoothing scatterplots: Journal of the American 

Statistical Association, v. 74, p. 829–836.

Colby, B.R., 1957, Relationship of unmeasured sediment discharge to mean velocity: American Geophysical Union 

Transactions, v. 35, no. 5, p. 708–717.

Collier, M., Webb, R.H., and Schmidt, J.C., 1996, Dams and 

reservoirs, a primer on the downstream effects of dams: 

U.S. Geological Survey Circular 1126, 94 p.

David, H.A., 1981, Order statistics: New York, John Wiley, 

360 p.

Dougherty, J.P., 1979, Suspended-sediment load of Texas 

streams, compilation report, October 1971–September 

1975: Texas Department of Water Resources Report 

233, 83 p.

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 3, chap. C2, 89 p.

Gillespie, B.M., and Giardino, J.R., 1997, The nature of 

channel planform change—Brazos River, Texas: Texas 

Journal of Science, v. 49, no. 2, p. 108–142.

Gustavson, T.C., and Bullen, G.R., 1996, Sand and gravel 

mining in the Brazos River—An assessment of erosion 

potential and economic impact: University of Texas 

Bureau of Economic Geology, 28 p.

Guy, H.P., and Norman, V.W., 1970, Field methods for 

measurement of fluvial sediment: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 3, chap. C2, 59 p.

Helsel, D.R., and Hirsch, R.M., 1992, Studies in environmental science 49—Statistical methods in water resources: 

New York, Elsevier, 522 p.

Kaigh, W.D., and Driscoll, M.F., 1987, Numerical and graphical data summary using o-statistics: The American 

Statistician, v. 41, no. 1, p. 25–32.

Lane, E.W., 1955, The importance of fluvial geomorphology 

in hydraulic engineering: American Society of Civil 

Engineers Proceedings, v. 81, no. 745, p. 1–17.

Mathewson, C.C., and Minter, L.L., 1976, Impact of water 

resource development on coastal erosion, Brazos River, 

Texas: Texas Water Resources Institute, Texas A&M 

University, Technical Report 77, 85 p.

McGowen, J.H., Garner, L.E., and Wilkinson, B.H., 1977, 

The Gulf Shoreline of Texas—Processes, characteristics, and factors of use: University of Texas Bureau of 

Economic Geology, Geological Circular 77–3, 27 p.

Morton, R.A., and Pieper, M.J., 1975, Shoreline changes in 

the vicinity of the Brazos River Delta (San Luis Pass to 

Brown Cedar Cut), an analysis of historical changes of 

the Texas Gulf Shoreline: University of Texas Bureau of 

Economic Geology, Geological Circular 75–4, 47 p.

Richardson, E.V., Simons, D.B., and Julien, P.Y., 1990, 

Highways in the river environment: Participant Notebook, Federal Highway Administration, Publication 

FHWA–HI–90–016.

Robbins, C.H., and Simon, A., 1983, Man-induced channel 

adjustment in Tennessee streams: U.S. Geological 

Survey Open-File Report 83–043, 95 p.

Seelig, W.N., and Sorenson, R.M., 1973, Investigation of 

shoreline changes at Sargent Beach, Texas: Texas A&M 

University, C.O.E. Report 169, 153 p.

Simons, D.B., and Senturk, F., 1992, Sediment transport technology, water and sediment dynamics: Littleton, Colo., 

Water Resources Publications, 897 p.

Stauble, D.K., Hubertz, J.M., Hoban, R.J., Livingston, C.R., 

Pollock, C.E., and Levine, D.R., 1994, Coastal studies in 

support of the Sargent Beach, Texas, erosion control 

project: Coastal Engineering Research Center, U.S. 

Army Corps of Engineers Waterways Experiment 

Station, Miscellaneous Paper CERC–94–3, 94 p.

Stelczer, K., 1981, Bed-load transport, theory and transport: 

Littleton, Colo., Water Resources Publications, 295 p.

Thornes, J.B., 1977, Hydraulic geometry and channel 

changes, in Gregory, K.J., ed., River channel changes: 

New York, John Wiley, p. 91–100.
32 Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

U.S. Department of Agriculture, 1997, Texas’ 1992 

census of agriculture state and county highlights: 

National Agricultural Statistics Service, accessed 

November 27, 2000, at URL

http://www.nass.usda.gov:80/census/census92/atlas92/

html/tx.htm

Vanoni, V.A., ed., 1975, Sedimentation engineering—

Manuals and reports on engineering practice, no. 54: 

New York, American Society of Civil Engineers, 745 p.

Welborn, C.T., 1967, Comparative results of sediment sampling with the Texas sampler and the depth-integrating 

samplers: Texas Water Development Board Report 36, 

109 p.

Wurbs, R.A., Bergman, C.E., Carriere, P.E., and Walls, B.W., 

1988, Hydrologic and institutional water availability 

in the Brazos River Basin: Texas Water Resources 

Institute, Texas A&M University, Technical Report 144, 

238 p.
Dunn and Raines— Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas 

District Chief U.S. Geological Survey 8027 Exchange Dr.Austin, TX 78754–4733