U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2014–5048

Prepared in cooperation with the San Antonio River Authority and 

the Texas Water Development Board

Sediment Characteristics in the San Antonio River Basin 

Downstream From San Antonio, Texas, and at a Site on the 

Guadalupe River Downstream from the San Antonio River 

Basin, 1966–2013
Front cover: Collection of a suspended-sediment sample by hydrologic technicians at U.S. Geological 

Survey station 08185000 Cibolo Creek at Selma, Texas, on May 25, 2013. The technician in the 

foreground is using a smart phone to check peak discharge at the site during the flood by accessing 

http://m.waterdata.usgs.gov (http://waterdata.usgs.gov/nwis on the Web). Photograph by Chiquita Lopez, 

U.S. Geological Survey.

Back cover:

Top, Collection of a bedload sample at U.S. Geological Survey station 08186500 Ecleto Creek near 

Runge, Texas. Photograph by Chiquita Lopez, U.S. Geological Survey.

Bottom, Collection of a suspended-sediment sample from the bridge at U.S. Geological Survey station 

08181800 San Antonio River near Elmendorf, Texas. Photograph by Michael Nyman, U.S. Geological 

Survey. 
Sediment Characteristics in the San 

Antonio River Basin Downstream From 

San Antonio, Texas, and at a Site on the 

Guadalupe River Downstream from the 

San Antonio River Basin, 1966–2013

By Cassi L. Crow, J. Ryan Banta, and Stephen P. Opsahl

Prepared in cooperation with the San Antonio River Authority and 

the Texas Water Development Board

Scientific Investigations Report 2014–5048

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

Suggested citation:

Crow, C.L., Banta, J.R., and Opsahl, S.P., 2014, Sediment characteristics in the San Antonio River Basin downstream 

from San Antonio, Texas, and at a site on the Guadalupe River downstream from the San Antonio River Basin, 

1966–2013: U.S. Geological Survey Scientific Investigations Report 2014–5048, 33 p., http://dx.doi.org/10.3133/

sir20145048. 

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3779-4
iii

Contents

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

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

Purpose and Scope .............................................................................................................................2

Description of the Study Area ...........................................................................................................2

Methods ..........................................................................................................................................................5

Sample Collection and Analysis ........................................................................................................5

Suspended-Sediment Sample Collection and Laboratory Analysis ...................................5

Bedload Sample Collection and Laboratory Analysis ..........................................................9

Quality Control .............................................................................................................................9

Turbidity Measurements ...................................................................................................................12

Instantaneous Suspended-Sediment Load, Bedload, and Total Sediment Load ....................12

Estimating Suspended-Sediment Loads from Streamflow .........................................................13

Limitations in Estimating Suspended-Sediment Load from Streamflow ..................................13

Estimating Suspended-Sediment Loads from Turbidity ...............................................................14

Sediment Characteristics ..........................................................................................................................14

Suspended-Sediment Concentration .............................................................................................15

Bedload ................................................................................................................................................22

Sediment Loads ...........................................................................................................................................22

Instantaneous Suspended-Sediment Loads, Bedloads, and Total Loads ................................22

Estimated Daily Suspended-Sediment Loads from Streamflow ................................................26

Estimated Daily Suspended-Sediment Loads from Turbidity ......................................................29

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

References ...................................................................................................................................................31

Appendixes

1. Streamflow, suspended-sediment concentrations, and instantaneous suspendedsediment loads at the sample collection sites in the San Antonio River Basin 

downstream from San Antonio, Texas, and at a site on the Guadalupe River 

downstream from the San Antonio River Basin, including data prior to 2011.

2. Daily mean streamflow and estimated suspended-sediment loads at sample 

collection sites in San Antonio River Basin downstream from San Antonio, Texas, 

and at a site on the Guadalupe River downstream from the San Antonio River 

Basin, 2011–12.

3. Daily mean turbidity at U.S. Geological Survey station 08181800 San Antonio River 

near Elmendorf, Texas (SAR Elmendorf), 2010–13.

Figures

1. Map showing data collection sites in the San Antonio River Basin downstream 

from San Antonio, Texas, and at a site on the Guadalupe River downstream from 

the San Antonio River Basin, January 2011 through May 2013 ...........................................3

2. Graphs showing daily mean streamflow with sediment sample collection dates 

and time-series suspended-sediment load estimated from daily mean streamflow 

and daily suspended-sediment concentration for selected sites in the San Antonio 

River Basin downstream from San Antonio, Texas, and at a site on the Guadalupe 

River downstream from the San Antonio River Basin, January 2011–May 2013 ..............6
iv

3. Photographs showing samplers used to collect suspended-sediment and 

bedload samples at sites in the San Antonio River Basin downstream from San 

Antonio, Texas, and at a site on the Guadalupe River downstream from the San 

Antonio River Basin, January 2011–May 2013 ........................................................................8

4. Graph showing distribution of suspended-sediment concentrations at sites in the 

San Antonio River Basin downstream from San Antonio, Texas, and at a site on 

the Guadalupe River downstream from the San Antonio River Basin, 1966–2013 ..........18

5. Graphs showing relations between suspended-sediment concentration and 

streamflow at sites A, SAR Elmendorf site (U.S. Geological Survey [USGS] 

station 08181800, San Antonio River near Elmendorf, Texas); B, SAR Falls City 

site (USGS station 08183500 San Antonio River near Falls City, Tex.); C, Cibolo 

Falls City site (USGS station 08186000, Cibolo Creek near Falls City, Tex.); 

D, Ecleto Runge site (USGS station 08186500, Ecleto Creek near Runge, Tex.); 

and E, SAR Goliad site (USGS station 08188500 San Antonio River at Goliad, Tex.) .......27

6. Graph showing daily mean turbidity from January 2011 through September 2013 

at SAR Elmendorf site (U.S. Geological Survey station 08181800, San Antonio 

River near Elmendorf, Texas), in formazin nephelometric units (FNU) .............................29

7. Graphs showing relation between suspended-sediment concentrations and 

turbidity at A, SAR Elemendorf site (U.S. Geological Survey [USGS] station 

08181800 San Antonio River near Elmendorf); B, SAR Goliad site (USGS station 

08188500 San Antonio River at Goliad); and C, GR Tivoli site (USGS station 

08188800 Guadalupe River near Tivoli, Texas) .......................................................................30

Tables

1. The number of historical and 2011–13 sediment samples collected at sites in the 

San Antonio River Basin downstream from San Antonio, Texas, and at a site on 

the Guadalupe River downstream from the San Antonio River Basin ................................4

2. Relative percent difference for replicate suspended-sediment data from sites 

sampled in the San Antonio River Basin downstream from San Antonio, Texas, 

and at a site on the Guadalupe River downstream from the San Antonio River 

Basin, January 2011–May 2013 ...............................................................................................10

3. Suspended-sediment data collected at sites in the San Antonio River Basin 

downstream from San Antonio, Texas, and at a site on the Guadalupe River 

downstream from the San Antonio River Basin, January 2011–May 2013, 

showing the percentage of sand-sized and fine-sized particles in each sample ..........15

4. Suspended-sediment data collected at sites in the San Antonio River Basin 

downstream from San Antonio, Texas, and at a site on the Guadalupe River 

downstream from the San Antonio River Basin, January 2011–May 2013, 

showing complete distribution of particle size for each sample........................................19

5. Bedload mass and instantaneous bedload transport for samples collected at 

sites in the San Antonio River Basin downstream from San Antonio, Texas, and 

at a site on the Guadalupe River downstream from the San Antonio River Basin, 

January 2011–May 2013 ...........................................................................................................23

6. Bedload particle-size distribution for samples collected at sites in the San 

Antonio River Basin downstream from San Antonio, Texas, and at a site on the 

Guadalupe River downstream from the San Antonio River Basin, January 2011–

May 2013 .....................................................................................................................................25

7. Regression equations, average estimated suspended-sediment load, and 

summary statistics for suspended-sediment concentration regressions .......................28
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

ton per day (ton/d) 0.9072 metric ton per day

ton per day (ton/d) 0.9072 megagram per day (Mg/d)

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

Volume

liter (L) 1.0567 quart (qt.)

Mass

gram (g) 0.03527 ounce (oz.)

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

°F=(1.8×°C)+32

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

Concentrations of suspended sediment in water are reported in milligrams per liter (mg/L) .

Sediment Characteristics in the San Antonio River Basin 

Downstream From San Antonio, Texas, and at a Site on the 

Guadalupe River Downstream from the San Antonio River 

Basin, 1966–2013

By Cassi L. Crow, J. Ryan Banta, and Stephen P. Opsahl

Abstract

San Antonio and surrounding municipalities in Bexar 

County, Texas, are in a rapidly urbanizing region in the 

San Antonio River Basin. The U.S. Geological Survey, 

in cooperation with the San Antonio River Authority and 

the Texas Water Development Board, compiled historical 

sediment data collected between 1996 and 2004 and 

collected suspended-sediment and bedload samples over 

a range of hydrologic conditions in the San Antonio River 

Basin downstream from San Antonio, Tex., and at a site on 

the Guadalupe River downstream from the San Antonio 

River Basin during 2011–13. In the suspended-sediment 

samples collected during 2011–13, an average of about 

94 percent of the particles was less than 0.0625 millimeter 

(silt and clay sized particles); the 50 samples for which a 

complete sediment-size analysis was performed indicated 

that an average of about 69 percent of the particles was less 

than 0.002 millimeter. In the bedload samples collected 

during 2011–13, an average of 51 percent of sediment 

particles was sand-sized particles in the 0.25–0.5 millimetersize range. In general, the loads calculated from the samples 

indicated that bedload typically composed less than 1 

percent of the total sediment load. A least-squares log-linear 

regression was developed between suspended-sediment 

concentration and instantaneous streamflow and was used to 

estimate daily mean suspended-sediment loads based on daily 

mean streamflow. The daily mean suspended-sediment loads 

computed for each of the sites indicated that during 2011–12, 

the majority of the suspended-sediment loads originated 

upstream from the streamflow-gaging station on the San 

Antonio River near Elmendorf, Tex. A linear regression 

relation was developed between turbidity and suspendedsediment concentration data collected at the San Antonio 

River near Elmendorf site because the high-resolution data 

can facilitate understanding of the complex suspendedsediment dynamics over time and throughout the river 

basin.

Introduction

Sediment characteristics including suspended sediment, 

bedload, particle-size distribution, and turbidity in a river 

system are important to characterize because data pertaining 

to sediment characteristics are needed to understand the 

magnitude and type of sediment transported in a river 

basin. The magnitude and type of sediment transported in 

a river basin can affect biological communities (Wood and 

Armitage, 1997), the concentration and movement of natural 

constituents and anthropogenic contaminants (Moran and 

others, 2012; Kemble and others, 2013; Kolpin and others, 

2013), and the amount of sediment deposition in coastal 

environments (Milliman and Meade, 1983). The processes 

related to sediment transport are complex and dependent 

on many factors including (1) basin geology; (2) the 

frequency, magnitude, and duration of storm runoff events; 

(3) the presence of man-made impoundments or channel 

modifications; and (4) urban and rural land use in the basin 

(Leopold, 1997). Sediment transport in streams generally can 

be categorized as suspended-sediment transport (sediment 

transported within the water column) and bedload transport 

(sediment transported along the bottom of the streambed). 

The complex nature of sediment transport poses challenges in 

collecting quality sediment data (Edwards and Glysson, 1999). 

Sediment properties are specific to individual river basins, 

including the temporal and spatial variability of sediment 

concentrations and loads, and the particle-size distribution 

of suspended and bedload sediment. Historical patterns of 

sediment transport are important for understanding river 

channel geomorphology and interactions with the flood plain 

(Leopold, 1997). 

The sediment characteristics of the San Antonio River 

Basin downstream from San Antonio, Tex., are not well 

understood because relatively little sediment data have 

been collected. Some historical data were collected prior to 

2004. Changes in land use in the study area such as urban 

development in the greater San Antonio area and development 
2 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

activities downstream from San Antonio might increase runoff 

into streams during storm events. Increases in runoff during 

storm events can potentially increase the amount and timing 

of sediment transported into the drainage network, affecting 

water quality and sediment loads throughout the San Antonio 

River Basin downstream from San Antonio (Ockerman and 

McNamara, 2003). 

To better understand sediment characteristics in the 

San Antonio River Basin downstream from San Antonio, 

the U.S. Geological Survey (USGS), in cooperation with 

the San Antonio River Authority (SARA) and the Texas 

Water Development Board (TWDB), collected and analyzed 

suspended-sediment, bedload, particle-size distribution, and 

turbidity data over a wide range of hydrologic conditions at 

sampling sites in the San Antonio River Basin downstream 

from San Antonio, Tex., and at one site on the Guadalupe 

River downstream from the San Antonio River Basin from 

January 2011 through May 2013 and combined that data with 

the available historical sediment data collected at the same 

sampling sites. During 2011–13, most sediment samples 

were collected at USGS streamflow-gaging stations. At 

two locations, sampling sites were established downstream 

from streamflow-gaging stations because the bridge where 

the gaging station was located was deemed unsuitable for 

sediment sampling (fig. 1). 

Turbidity data were collected to develop a basinwide 

understanding of sediment characteristics and to provide 

insight into sediment sources and transport dynamics. Because 

turbidity data are simpler to collect than sediment data, the 

establishment of the relation between suspended-sediment 

concentration (SSC) and turbidity in the study area could 

prove useful for additional investigations.

The presence of dissolved and suspended material 

(clays, silt, fine organic matter, and other material) in stream 

water typically results in more turbid (less clear) water (ASTM 

International, 2007). Turbidity, a measure of water clarity, 

is often correlated to discrete measurements of SSC and the 

resulting relation is used as a surrogate for SSC (Rasmussen 

and others, 2009). Previous studies have demonstrated that 

during stormflow events, sediment loads often peak prior to 

the streamflow hydrograph peak (Wood, 1977; VanSickle 

and Beschta, 1983; Glysson, 1987; Clark and others, 2013). 

As a result, the suspended-sediment loads (SSL) associated 

with a certain streamflow value observed during the rising 

limb of a hydrograph might be different than the loads 

associated with the same streamflow value observed during 

the receding limb of the hydrograph. For this reason, 

turbidity has been theorized to be a better surrogate for 

predicting SSC than streamflow (Lewis, 1996). Once a 

strong relation between SSC and turbidity is established for 

a given river basin, real-time turbidity data can be used to 

estimate SSC, and high-resolution data from selected sites 

can be collected to facilitate understanding of the complex 

suspended-sediment dynamics over time and throughout the 

river basin. 

Purpose and Scope

Sediment properties in the San Antonio River Basin 

downstream from San Antonio were characterized by using 

historical sediment data collected during 1966–2004 and 

recent sediment data collected during 2011–13. Suspendedsediment and bedload were characterized at 10 sites in the 

San Antonio River Basin and at 1 site on the Guadalupe River 

downstream from the San Antonio River Basin. Sixty-seven 

suspended-sediment samples and 22 bedload samples were 

collected from January 2011 through May 2013 (hereinafter 

referred to as 2011–13 samples), and data from these samples 

were compared with historical suspended-sediment data 

collected before 2011 (hereinafter referred to as historical 

samples) at 5 sites sampled during 2011–13. Daily SSLs were 

estimated during 2011–12; the SSLs were estimated based 

on regression equations developed between streamflow and 

SSCs at five sites by using the historical and 2011–13 data. In 

addition, regression equations between turbidity measurements 

and suspended-sediment samples were developed at three sites 

in the San Antonio River Basin. 

Description of the Study Area

The study area consists of 2,150 square miles (mi2) of the 

San Antonio River Basin downstream from San Antonio, Tex., 

and about 2 mi2 of the Guadalupe River Basin downstream 

from the San Antonio River Basin (fig. 1). The upstream 

boundary of the study area includes the San Antonio River 

near Elmendorf, Tex., site (SAR Elmendorf) and the Cibolo 

Creek at Selma, Tex., site (Cibolo Selma) (map identifiers 1 

and 4, respectively, fig. 1 and table 1). The downstream 

boundary of the study is the Guadalupe River near Rivoli, 

Tex., site (GR Tivoli) on the Guadalupe River about 1 mile 

(mi) downstream from the confluence of the San Antonio and 

Guadalupe Rivers. Cibolo Creek and Ecleto Creek are part of 

the study area, which encompasses parts of Bexar, Guadalupe, 

Wilson, Karnes, DeWitt, Goliad, Victoria, and Refugio 

Counties (fig. 1). The San Antonio River extends about 190 

river mi from Elmendorf, Tex., to the confluence of the San 

Antonio and Guadalupe Rivers. From the upstream boundary 

of the study area near Selma, Tex., Cibolo Creek extends about 

75 river mi downstream to the confluence of Cibolo Creek 

and the San Antonio River in Karnes County. Ecleto Creek 

extends about 55 river mi from northern Wilson County to 

the confluence of Ecleto Creek and the San Antonio River in 

Karnes County. 
Figure 1. Data collection sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a site on the Guadalupe River downstreaRiver Basin, January 2011 through May 2013. 

COASTAL LOWLANDS

AQUIFER SYSTEM

TEXAS COASTAL UPLANDS

AQUIFER SYSTEM

EDWARDS-TRINITY

AQUIFER SYSTEM

EXPLANATI

Study areaAquifer system boundary ( Kasmarek and RobinsoU.S. Geological Survey str map identifier

—See tab

U Sediment sampling site an

8 C

3

UC

UC

UC

UC

UC

U

U

C

UC

UC

10 

4

9

6

7

8

5 3

2

1

97°00'

98°00'

29°30'

29°00'

28°30'

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUN

DEWITT

COUNTY

WILSON COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

KARNES COUNTY

REFUGIO

COUNTY

GOLIAD

COUNTY

GOLIAD

COUNTY

COMAL

COUNTY

87

183

281

87

281

77

90

59

77

281

181

77

181

37

35

35

10

35

410

35

37

10

410

Poth

Falls 

City

Floresville

Stockdale

Nixon

Cibolo

San

Antonio

Goliad

McF

Kenedy

Runge

Karnes

City

Yorktown

St. Hedwig

Elmendorf

Selma

Sutherland

Springs

TEXAS

San Antonio

River Basin

Study area

GULF OF 

MEXICO

Guadalupe River

Basin

Base modified from U.S. Geological Survey digital data, 1:24,000Universal Transverse Mercator, zone 14North American Datum of 1983

Modified from Lizarraga and Ockerman 

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

Medin

 River

a

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez

Calaveras Lake

San Antonio

CiboloCreek

 River

Cibolo Creek

San Antonio River

Cabeza Creek

Creek

H

1For analysis, data collected at USGS statio

were combined and reported under site ide

2For analysis, data collected at USGS statio

were combined and reported under site ide

123456

SAR Elmendorf

SAR Falls City1

SAR Falls City1

Cibolo Selma

Cibolo Falls City

Ecleto Runge2

Short name

(table 1)

Map

identifier

id
Table 1. The number of historical and 2011–13 sediment samples collected at sites in the San Antonio River Basin downstream from San Antonio, TeGuadalupe River downstream from the San Antonio River Basin. [SSC, suspended-sediment concentration; SSF, suspended-sediment sand-fine break; SSP, suspended-sediment particle-size distribution; BLM, bedload mass; BLP, bedload partavailable; USGS, U.S. Geological Survey] Map identifier (fig. 1) U.S. Geological Survey station number U.S. Geological Survey station name Short name for sampling site Period of record (historical samples) Period of record (2011–13 samples) Data type and num

(samples collecte

samples collected 

SSC SSF SSP 

1 08181800 San Antonio River near Elmendorf, Tex. SAR Elmendorf 1996–2004 2011–12 95/10 1/3 0/7 2 08183500 San Antonio River near Falls City, Tex. SAR Falls City1 1966–75 na 48/0 2/0 7/0 3 08183550 San Antonio River at Highway 181 near Falls City, Tex. SAR Falls City1 na 2011–12 0/9 0/2 0/7 4 08185000 Cibolo Creek at Selma, Tex. Cibolo Selma na 2012–13 0/3 0/1 0/2 5 08186000 Cibolo Creek near Falls City, Tex. Cibolo Falls City 1967–75 2011–12 36/7 1/2 6/5 6 08186500 Ecleto Creek near Runge, Tex. Ecleto Runge2 1966–75 2011–12 31/3 0/0 10/3 7 08186550 Ecleto Creek at County Road 326 near Runge, Tex. Ecleto Runge2 na 2012 0/2 0/0 0/2 8 08188060 San Antonio River at State Highway 72 near Runge, Tex. SAR 72 na 2011–12 0/5 0/0 0/5 9 08188500 San Antonio River at Goliad, Tex. SAR Goliad 1974–94 2011–12 162/8 163/2 0/6 

10 08188570 San Antonio River near McFaddin, Tex. SAR McFaddin na 2011–12 0/6 0/3 0/3 

11 08188800 Guadalupe River near Tivoli, Tex. GR Tivoli na 2011–12 0/14 0/4 0/10 

1For analysis, data collected at USGS station numbers 08183500 and 08183550 were combined and reported under short name “SAR Falls City.”2For analysis, data collected at USGS station numbers 08186500 and 08186550 were combined and reported under short name “Ecleto Runge.”
Methods 5

The northern part of the study area overlies the EdwardsTrinity aquifer system. The remainder of the study area 

overlies the Texas Coastal Uplands and Coastal Lowlands 

aquifer systems (Ryder, 1996) (fig. 1). The Cretaceous-age 

rocks of the Edwards-Trinity aquifer system primarily consist 

of limestone and sandstone (Ashworth and Hopkins, 1995). 

The Texas Coastal Uplands aquifer system is composed 

of formations of Paleocene and Oligocene age, with the 

sediments (in order of dominance) consisting mostly of sand, 

silt, and clay, distributed as relatively uniform sequences of 

predominantly fine- or coarse-grained material (Ryder, 1996). 

The Texas Coastal Lowlands aquifer system is composed 

of younger formations from Oligocene through Holocene 

age that dip and thicken towards the Gulf of Mexico, with 

sediments that exist in complex, overlapping mixtures of sand, 

silt, and clay as a result of numerous oscillations of ancient 

shorelines (Kasmarek and Robinson, 2004; Lizárraga and 

Ockerman, 2010). 

Topography, land cover, precipitation, and population 

can affect sediment characteristics in the study area. The study 

area is composed of gently sloping, rolling terrain; the coastal 

uplands are somewhat more dissected and rolling compared 

to the coastal lowlands (Ryder, 1996). The land cover 

consists mostly of brush and grassland (Multi-Resolution 

Land Characteristics Consortium, 2013), and average annual 

precipitation ranges from about 30 inches in the northern 

sections of the basin to about 40 inches in the southern 

sections of the basin near the coast (National Oceanic and 

Atmospheric Administration, 2013). San Antonio, the seventh 

most populous city in the Nation (U.S. Census Bureau, 2012), 

was founded on the headwaters of the San Antonio River. The 

population of San Antonio grew by 35 percent from 1990 to 

2010 (U.S. Census Bureau, 2012). 

Methods

Historical suspended-sediment data collected between 

1966 and 2004 were compiled for use with the 2011–13 

suspended-sediment and bedload data. Historical data were 

collected for various periods at each site ranging from 

about 10 to 21 years. The historical and 2011–13 data were 

collected by the USGS at sites in the study area (fig. 1; 

table 1). In this report, each site is referred to by its map 

identifier and short name. All sampling sites were located 

at USGS stream-gaging stations with the exception of two 

sites sampled during 2011–13—USGS stations 08183550 

and 08186550. For analysis, data collected at USGS 

stations 08183500 and 08183550 were combined and reported 

under the short name “SAR Falls City.” Similarly, data 

collected at USGS stations 08186500 and 08186550 were 

combined and reported under the short name “Ecleto Runge” 

(table 1). 

Sample Collection and Analysis

During 2011–13, samples of suspended sediment were 

collected at 10 sites (map identifiers 1 and 3–11; fig. 1; 

table 1) for the analysis of SSC and particle-size distribution. 

In addition, samples of bedload material were collected at six 

sites (map identifiers 1, 5, 6, 7, 9, and 11; fig. 1; table 1) for 

the analysis of bedload mass and particle-size distribution. 

Suspended-sediment samples were collected over a range 

of streamflows ranging from a minimum instantaneous 

streamflow of 1.9 cubic feet per second (ft3/s) at the Cibolo 

Selma site (map identifier 4, fig. 1; table 1) on September 

17, 2012, to a maximum instantaneous streamflow of 10,600 

ft3/s at the same site on May 25, 2013 (fig. 2). Note that the 

streamflow plotted in figure 2 is the daily mean streamflow 

for each site in cubic feet per second, but the streamflow 

associated with each sample is the instantaneous streamflow 

recorded at the time the sample was collected. 

Suspended-Sediment Sample Collection and 

Laboratory Analysis

Suspended-sediment samples were collected during 

2011–13 following standard USGS methods described in 

Edwards and Glysson (1999). At each sampling site, samples 

were collected at a minimum of 10 equal-width increments 

across the stream by using samplers designed to allow water 

to enter the sampler with no change in velocity or direction as 

it entered the sampler from the stream, a method referred to as 

isokinetic sampling. When stream depths were shallow enough 

to be waded, samples were collected using a US DH–81 1-liter 

bottle sampler (Davis, 2005) attached to a wading rod (fig. 3). 

When the stream was too deep to be waded, samples were 

collected from a bridge using a US DH–2 1-liter collapsible 

bag sampler (Davis, 2005) attached to a reel and crane system 

(fig. 3). Samples collected using the US DH–81 sampler at 

streamflow velocities less than 1.5 feet per second (ft/s) and 

samples collected using the US DH–2 sampler at velocities 

less than 2.0 ft/s were labeled as grab samples because the 

samplers are unable to collect isokinetic samples at or less 

than those velocities. Samples were composited into a 14-liter 

polyethylene churn splitter, and representative suspendedsediment samples were dispensed from the churn splitter into 

3-liter polypropylene bottles. The churn splitter method can 

potentially bias results when sand-sized (greater than or equal 

to 0.0625 millimeter [mm]) particles are present in suspendedsediment samples because the heavier sand-sized particles 

might not remain uniformly suspended in the churn (Capel and 

Larson, 1995; Horowitz and others, 1997). This potential bias 

was not expected to substantially influence samples because 

historical data from sites in the study area indicate the system 

is dominated by suspended particle sizes much less than 

0.0625 mm, even during stormflow events.
6 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Figure 2. Daily mean streamflow with sediment sample collection dates and time-series suspended-sediment load estimated from 

daily mean streamflow and daily suspended-sediment concentration (from U.S. Geological Survey, 2013) for selected sites in the San 

Antonio River Basin downstream from San Antonio, Texas, and at a site on the Guadalupe River downstream from the San Antonio 

River Basin, January 2011–May 2013. A, SAR Elmendorf site (U.S. Geological Survey [USGS] station 08181800, San Antonio River near 

Elmendorf, Tex.); B, SAR Falls City site (USGS station 08183500 San Antonio River near Falls City, Tex.); C, Cibolo Selma site (USGS station 

08185000, Cibolo Creek at Selma, Tex.); D, Cibolo Falls City site (USGS station 08186000, Cibolo Creek near Falls City, Tex.); E, Ecleto Runge 

site (USGS station 08186500, Ecleto Creek near Runge, Tex.); F, SAR 72 site (USGS station 08188060, San Antonio River at State Highway 

72 near Runge, Tex.); G, SAR Goliad site (USGS station 08188500 San Antonio River at Goliad, Tex.); H, SAR McFaddin site (USGS station 

08188570, San Antonio River near McFaddin, Tex.); I, GR Tivoli site (USGS station 08188800, Guadalupe River near Tivoli, Tex.)

2011 2012 2013

Jan. Apr. July Oct. Jan. Apr. July Oct. Jan.*

Daily mean streamflow, in cubic feet per second

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

0.1

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

0.1

1

10

100

1,000

10,000

Calculated daily suspended-sediment load, in tons per day Calculated daily suspended-sediment load, in tons per day

A. SAR Elmendorf

B. SAR Falls City

C. Cibolo Selma

D. Cibolo Falls City

E. Ecleto Runge

Sample collected on May 25, 2013, 

when the daily mean streamflow 

was 2,430 cubic feet per second 

*Except where noted
Methods 7

Figure 2. Daily mean streamflow with sediment sample collection dates and time-series suspended-sediment load estimated from 

daily mean streamflow and daily suspended-sediment concentration (from U.S. Geological Survey, 2013) for selected sites in the San 

Antonio River Basin downstream from San Antonio, Texas, and at a site on the Guadalupe River downstream from the San Antonio 

River Basin, January 2011–May 2013. A, SAR Elmendorf site (U.S. Geological Survey [USGS] station 08181800, San Antonio River near 

Elmendorf, Tex.); B, SAR Falls City site (USGS station 08183500 San Antonio River near Falls City, Tex.); C, Cibolo Selma site (USGS station 

08185000, Cibolo Creek at Selma, Tex.); D, Cibolo Falls City site (USGS station 08186000, Cibolo Creek near Falls City, Tex.); E, Ecleto Runge 

site (USGS station 08186500, Ecleto Creek near Runge, Tex.); F, SAR 72 site (USGS station 08188060, San Antonio River at State Highway 

72 near Runge, Tex.); G, SAR Goliad site (USGS station 08188500 San Antonio River at Goliad, Tex.); H, SAR McFaddin site (USGS station 

08188570, San Antonio River near McFaddin, Tex.); I, GR Tivoli site (USGS station 08188800, Guadalupe River near Tivoli, Tex.)—Continued

2011 2012 2013

Jan. Apr. July Oct. Jan. Apr. July Oct. Jan.

Daily mean streamflow, in cubic feet per second

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

Calculated daily suspended-sediment

load, in tons per day

F. SAR 72

G. SAR Goliad

H. SAR McFaddin

I. GR Tivoli

Note: Short names for sampling sites (table 1) are listed above the upper left corners of figures 2A–2I. For analysis, data collected at 

USGS station numbers 08186500 and 08186550 were combined and reported under short name “Ecleto Runge.”

EXPLANATION

Daily mean streamflow, in cubic feet per second—Dot indicates

 date sediment sample was collected

Calculated daily suspended-sediment load, in tons per day
Figure 3. Samplers used to collect suspended-sediment and bedload samples at sites in the San Antonio River Basin downstream from San AntonioGuadalupe River downstream from the San Antonio River Basin, January 2011–May 2013. A, US DH-81; B, US DH-2; C, US BLH-84; and D, US BL-84; mo

B. US DH–2

C. US BLH–84 D. US BL–84

A. US DH–81
Methods 9

Suspended-sediment samples were analyzed at the USGS 

Kentucky Water Science Center Sediment Laboratory in 

Louisville, Ky., by using methods described in Guy (1969) 

and in accordance with the quality assurance plan documented 

in Shreve and Downs (2005). All 67 suspended-sediment 

samples were analyzed by using filtration or evaporation 

methods and reported in milligrams per liter (mg/L). 

Seventeen suspended-sediment samples were analyzed to 

determine the percentage of particles less than 0.0625 mm, 

which generally corresponds to the particle size separating 

sand-sized (0.0625 to 2.0 mm in diameter) particles from 

finer-size particles of silts and clays (less than 0.0625 mm 

in diameter), commonly referred to as the sand/fine break 

(Guy, 1969). The sand/fine break analysis was done using a 

process known as wet-sieving filtration where the sample was 

washed through a wetted 0.0625-mm mesh sieve. The sand/

fine break data were reported as the percentage of sediment 

less than 0.0625 mm. The remaining 50 suspended-sediment 

samples were analyzed to determine the amount of sediment 

distributed between 11 size classes ranging from less than 

0.002 to 2 mm in diameter (commonly referred to as particlesize distribution). Particle-size distribution analyses were 

done using a combination of sieve, visual-accumulation tube, 

and pipet methods as described by Guy (1969) and by Shreve 

and Downs (2005). Particle-size distributions in suspendedsediment samples were reported as the percentage of sediment 

less than 0.002, 0.004, 0.008, 0.016, 0.031, 0.0625, 0.125, 

0.25, 0.5, 1, and 2 mm.

Bedload Sample Collection and Laboratory 

Analysis

Bedload samples were collected using standard USGS 

methods as described in Edwards and Glysson (1999). 

Samples were collected using pressure-difference samplers 

fitted with mesh bags designed to capture sediment moving 

along the streambed (bedload sediment) while allowing water 

to pass through the bag. As bedload sediment collected in 

the bag, the size of the mesh openings decreased, resulting in 

the collection of particle sizes less than the mesh size of the 

sample bag (0.25 mm) (Hubbell, 1964). When stream depths 

were shallow enough to be waded, a US BLH–84 sampler 

(Davis, 2005) attached to a wading rod was used to collect 

bedload samples (fig. 3). When the stream was too deep to be 

waded, bedload samples were collected using a US BL–84 

sampler (Davis, 2005) attached to a reel and crane system 

(fig. 3). Depending on the site and hydrologic conditions, the 

bedload sampler rested on the streambed at 20–40 equal-width 

increments across the stream for a period of 1–2 minutes each, 

resulting in a composited sample with total collection times of 

32–80 minutes. 

Bedload samples were analyzed at the USGS sediment 

laboratory in Louisville, Ky., for total mass and particlesize distribution. In accordance with methods described by 

Guy (1969), samples were sieved to determine particle sizes 

ranging from less than 0.0625 to less than 16 mm in diameter. 

Quality assurance procedures documented in Shreve and 

Downs (2005) were followed during the analyses. Total 

mass for 22 bedload samples was determined by using the 

evaporation method described in Guy (1969) and reported 

in grams (g) of dry mass. Ten of the 22 bedload samples 

were analyzed for particle-size distribution by dry-sieving 

followed by a combination of sieve, visual-accumulation 

tube-pipet, and sieve-pipet methods (Guy, 1969). Particlesize distributions in bedload samples were reported as the 

percentage of sediment less than 0.0625, 0.125, 0.25, 0.5, 1, 2, 

4, 8, 16, and 32 mm.

Quality Control

Six replicate suspended-sediment samples (table 2) 

were collected from four sites to evaluate potential bias and 

variability introduced during sample processing or laboratory 

analysis. Replicate samples were split from the same churn 

and compared to the associated environmental samples by 

calculating the relative percent difference (RPD) for each 

sample pair of detected constituents (table 2). RPD was 

computed using the equation: 

RPD = |C1 – C2|/[(C1 + C2)/2] × 100, (1)

where

C1 is the concentration or percentage from the 

environmental sample; and 

C2 is the concentration or percentage from the 

replicate sample. 

The median RPD for SSC was 4.0 percent for the six 

sample pairs, and 1.1 percent for the particle-size distribution. 

RPDs exceeded 15 percent for one SSC sample pair and two 

particle-size distribution pairs, which indicates that bias might 

have been introduced to these three results during sample 

collection and processing (for example, while splitting the 

sample with a churn splitter as previously described in this 

section) or during laboratory analysis.
Table 2. Relative percent difference for replicate suspended-sediment data from sites sampled in the San Antonio River Basin downstream from Sasite on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013. [mg/L, milligrams per liter; mm, millimeter; Env, environmental sample; Rep, replicate sample; --, not measured; RPD, relative percent difference; nc, not computed] U.S. Geological Survey station number U.S. Geological Survey station name Short name for sampling site (table 1) Sample date Sample start time Sample type Suspended sediment concentration (mg/L) Suspended sediment (percent less than 0.002 mm) Suspended sediment (percent less than 0.004 mm) Suspended sediment (percent less than 0.008 mm) 

08181800 San Antonio River near Elmendorf, Tex. SAR Elmendorf 6/22/2011 1900 Env 625 60 73 87 

1905 Rep 531 -- -- -- 

RPD 16.3 nc nc nc 

08181800 San Antonio River near Elmendorf, Tex. SAR Elmendorf 2/18/2012 1130 Env 1,430 62 69 74 

1230 Rep 1,420 62 69 75 

RPD 0.7 0 0 1.3 

08186500 Ecleto Ck near Runge, Tex. Ecleto Runge 3/21/2012 1030 Env 789 97 99 100 

1035 Rep 760 96 97 99 

RPD 3.7 1.0 2.0 1.0 

08188500 San Antonio River at Goliad, Tex. SAR Goliad 10/13/2011 945 Env 1,690 58 77 81 

1000 Rep 1,620 73 81 86 

RPD 4.2 23 5.1 6.0 

08188500 San Antonio River at Goliad, Tex. SAR Goliad 3/23/2012 1200 Env 1,450 81 84 88 

1235 Rep 1,330 67 80 89 

RPD 8.6 19 4.9 1.1 

08188800 Guadalupe River near 

Tivoli, Tex.

GR Tivoli 2/23/2012 1020 Env 317 89 93 96 

1118 Rep 322 89 92 96 

RPD 1.6 0 1.1 0 
Table 2. Relative percent difference for replicate suspended-sediment data from sites sampled in the San Antonio River Basin downstream from Sasite on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013.—Continued[mg/L, milligrams per liter; mm, millimeter; Env, environmental sample; Rep, replicate sample; --, not measured; RPD, relative percent difference; nc, not computed]

U.S. Geological 

Survey station 

number

U.S. Geological Survey

station name

Short name for 

sampling site 

(table 1)

Sample 

date

Sample 

start 

time

Sample 

type

Suspended 

sediment

(percent 

less than 

0.0625 mm)

Suspended 

sediment

(percent 

less than 

0.125 mm)

Suspended 

sediment

(percent 

less than 

0.25 mm)

Suspended 

sediment

(percent 

less than 

0.5 mm)

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 6/22/2011 1900 Env 96 98 99 100 

1905 Rep 99 -- -- -- 

RPD 3.1 nc nc nc 

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 2/18/2012 1130 Env 92 98 100 -- 

1230 Rep 93 98 100 -- 

RPD 1.1 0 0 nc 

08186500 Ecleto Ck near Runge, 

Tex.

Ecleto Runge 3/21/2012 1030 Env -- -- -- -- 

1035 Rep 99 100 -- -- 

RPD nc nc nc nc 

08188500 San Antonio River at 

Goliad, Tex.

SAR Goliad 10/13/2011 945 Env 92 95 98 100 

1000 Rep 95 97 99 100 

RPD 3.2 2.1 1.0 0 

08188500 San Antonio River at 

Goliad, Tex.

SAR Goliad 3/23/2012 1200 Env 95 96 98 100 

1235 Rep 97 98 99 100 

RPD 2.1 2.1 1.0 0 

08188800 Guadalupe River near 

Tivoli, Tex.

GR Tivoli 2/23/2012 1020 Env 98 99 100 -- 

1118 Rep 99 99 100 -- 

RPD 1.0 0 0 nc 
12 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Turbidity Measurements

Turbidity, the measure of water clarity, was measured 

from January 6, 2011, through September 30, 2013, at the 

SAR Elmendorf site (map identifier 1; fig. 1; table 1) by using 

a YSI 6920 multi-sensor water-quality monitor equipped with 

a YSI 6136 optical turbidity sensor (Xylem Analytics, 2014a). 

Turbidity was measured at the time of sample collection from 

the centroid of the stream. In addition, a real-time turbidity 

probe was deployed and measured turbidity every 15 minutes 

that was recorded by a data-collection platform (DCP) and 

transmitted to the USGS’s National Water Information System 

(NWIS) database (U.S. Geological Survey, 2013) every hour. 

The monitor was cleaned and calibrated, and the data were 

processed following methods described by Wagner and others 

(2006). The time between visits for cleaning and calibration 

ranged from 6 to 32 days with an average time between visits 

of about 18 days. Turbidity sensor calibration occurred in a 

laboratory environment and consisted of a visual inspection of 

the probe to ensure the wiper used to clean the optical surface 

of the probe was parking in the correct location in relation 

to the optical surface, replacement of the wiper pad, and a 

two-point calibration using deionized water as a 0 formazin 

nephelometric unit (FNU) standard and a commercial 100 

FNU formazin standard. The YSI 6136 optical turbidity sensor 

has a reported range of 0 to 1,000 FNU (Xylem Analytics, 

2014b); field measurements indicate that the sensor reaches 

a maximum at approximately 1,100–1,200 FNU, depending 

on the individual sensor. The accuracy of YSI 6136 optical 

turbidity sensor is plus or minus 2 percent of the reading or 

0.3 FNU, whichever is greater (Xylem Analytics, 2014b). 

Additionally, turbidity measurements made prior to May 23, 

2012, were affected by a programming problem in the DCP 

that caused turbidity measurements greater than 330 FNU 

to be recorded as 330 FNU. Daily mean turbidity values 

were calculated from the 15-minute turbidity measurements. 

Missing daily mean turbidity values represent days in which 

17 or more 15-minute turbidity measurements (about 18 

percent) were missing from the daily record. In this instance, 

daily mean turbidity values were not computed or reported. 

The time-series turbidity data described in this report are 

available from the USGS NWIS (U.S. Geological Survey, 

2013). 

Instantaneous Suspended-Sediment Load, 

Bedload, and Total Sediment Load 

Streams transport sediment by maintaining the finer 

particles in suspension with turbulent currents (SSL) and by 

intermittent entrainment and movement of coarser particles 

along the streambed (bedload) (Ellison and others, 2014). 

Instantaneous streamflow data were obtained from stagedischarge rating curves at the streamflow-gaging station 

(Rantz and others,1982; Turnipseed and Sauer, 2010) 

where suspended-sediment samples were collected or at the 

streamflow-gaging station directly upstream from the sample 

site (as was the case for the SAR Falls City and Ecleto Runge 

sites). The SSL transported by the stream past a site at the time 

of sample collection (hereinafter referred to as instantaneous 

SSL) was estimated using equation 2 (modified from 

Porterfield, 1972): 

SSL = Qi×SSC×k, (2)

where

SSL is the suspended-sediment load, in tons 

per day; 

Qi is the instantaneous streamflow, in cubic feet 

per second; 

SSC is the suspended-sediment concentration, in 

milligrams per liter; and 

k is a unit conversion factor of 0.0027 in 

tons per day per cubic foot per secondmilligrams per liter.

Bedload refers to the sediment transported by rolling, 

saltating, or bouncing along a riverbed or the component of 

sediment in transport from the surface of the riverbed up to the 

height of the top of the sampler nozzle (Galloway and others, 

2013). The bedload transported past a sampled site at the time 

of sampling (hereinafter referred to as instantaneous bedload) 

was calculated by using equation 3 (modified from Edwards 

and Glysson, 1999): 

BL=K×(WT/tT)×MT (3)

where

BL is the instantaneous bedload mass, in tons per 

day; 

K is a unit conversion factor (0.381 for a 3-inch 

wide nozzle); 

WT is the total width of the stream, in feet, and 

is equal to the increment width multiplied 

by the total number of vertical samples 

measured in the stream cross section; 

t

T is the total time the sampler remained on the 

streambed, in seconds, and is equal to the 

sample time multiplied by the total number 

of verticals from which samples were 

collected; and 

MT is the total mass of sample collected from 

all verticals sampled in the stream cross 

section, in grams.

Instantaneous total sediment load for a site, reported in 

tons per day, was calculated by summing the instantaneous 

SSL and instantaneous bedload. Instantaneous total sediment 

load was calculated for 21 sampling events at six sites for 

which both an instantaneous SSL and an instantaneous 

bedload were computed.
Methods 13

Estimating Suspended-Sediment Loads from 

Streamflow

To estimate SSLs for days when suspended-sediment 

samples were not collected, log-linear regressions were 

developed to estimate SSCs based on streamflow for the five 

sites for which sufficient data were available (SAR Elmendorf, 

SAR Falls City, Cibolo Falls City, Ecleto Runge, and SAR 

Goliad). The historical (appendix 1) and the 2011–13 data 

were included in the regression analyses. The measured 

SSC and instantaneous streamflow data were nonnormally 

distributed and, consequently, were log transformed before 

the regression lines were calculated. The regression equation 

for estimating SSC (retransformed into original units 

incorporating the bias correction factor [Helsel and Hirsch, 

2002]) from streamflow is represented as:

SSC = β0 x Qd

β1

 x Φ (4)

where 

SSC is estimated suspended-sediment 

concentration, in milligrams per liter;

β0 and β1 are regression coefficients;

Qd is the daily mean streamflow, in cubic feet per 

second; and

Φ is the bias-correction factor.

Similar to instantaneous loads, the estimated SSCs were used 

with daily mean streamflow in equation 2 to estimate daily 

mean SSLs. 

Summary statistics were computed for each of the 

regression equations and include a coefficient of determination 

(R2), a root mean square error (RMSE), and a mean absolute 

error (MAE). R2 values range from 0 to 1 and estimate the 

proportion of variability explained by a regression model. 

As the R2 value approaches 1, the models approach perfect 

correlation. RMSE is an estimator that quantifies the 

difference between values estimated by a model and the actual 

values that were measured. MAE is a metric for measuring 

how far estimated values deviate from measured values. As 

RMSE and MAE values approach 0, the models approach 

perfect estimation (Helsel and Hirsch, 2002). 

Limitations in Estimating Suspended-Sediment 

Load from Streamflow 

Suspended-sediment loads estimated from log-linear 

regressions that relate SSC to streamflow provide a general 

characterization of the sources, timing, and quantity of SSLs 

in the study area. Limitations in the available suspendedsediment data and the effects of various assumptions might 

contribute to uncertainty in the development of the regression 

equations and the estimated SSLs produced from them. 

The regression equations used to estimate SSCs 

incorporate historical and 2011–13 suspended-sediment 

sample data. Historical data from as early as 1966 to as late 

as 2004 are available from five sites (31 to 162 suspendedsediment samples per site) (table 1). The combination of these 

historical data with the 2011–13 suspended-sediment samples 

increases the robustness of the regressions by representing a 

wider range of hydrologic conditions for which suspendedsediment data were collected at the sites. The historical 

samples were collected during a wide range of streamflow 

conditions, including sampling during base-flow conditions. 

Most historical samples were collected on a routine schedule 

regardless of streamflow or sediment conditions; some were 

collected during storm runoff and represent different parts of 

storm hydrographs. In contrast, the 2011–13 samples were 

collected primarily during the rising limb to peak of the 

hydrograph during storm events, when SSLs often reach a 

maximum. Because the 2011–13 sampling plan intentionally 

targeted the rising limb to peak of the hydrograph during 

storm events, combination of the historical and 2011–13 

data represents a wider range of hydrologic and antecedent 

conditions compared to those represented by the 2011–13 

data alone. In addition to the historical samples potentially 

representing somewhat different hydrologic conditions 

compared to the 2011–13 samples, there are factors that might 

cause SSCs to change over time. These factors include (but 

are not limited to) (1) changes in the amount of urbanization 

and other changes in land use; (2) placement or removal of 

physical structures such as dams or levees; (3) implementation 

of best management practices within the contributing 

watershed; and (4) differences in sediment yields from land 

surfaces as a result of antecedent moisture conditions, time 

since last rainfall, rainfall intensity, and where the rainfall 

occurred within the basin (Glymph, 1951; Griffiths, 1981). 

At some sites, there are several years to several decades 

between the collection of historical and 2011–13 data. It was 

assumed that relations between streamflow and SSCs did not 

change at any of the sites because quantification of the factors 

that might cause SSCs to change over time was not possible 

when combining the historical and 2011–13 datasets. The 

combined dataset likely has a larger (and unquantified) degree 

of measurement uncertainty compared to the 2011–13 data 

because of the unknown magnitude of the factors previously 

mentioned in this paragraph that can affect SSC over time.

Another factor contributing to the estimated SSL 

uncertainty at two sites was that the sampling locations 

changed; the historical sampling locations were replaced with 

new sampling locations. Changes in the 2011–13 sampling 

locations at the SAR Falls City and Ecleto Runge sedimentsampling sites compared to their historical sampling locations 

at streamflow-gaging stations were made because the 

historical sediment-sampling sites were considered unsafe for 

the collection of sediment samples (the historical SAR Falls 

City and Ecleto Runge sampling sites continue to function 

as streamflow-gaging stations). No sediment samples were 

collected during 2011–13 at the historical SAR Falls City 

site because of safety considerations. Three samples were 

collected from January through March 2012 at the historical 

Ecleto Runge sampling site before this site was deemed unsafe 
14 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

for any additional sediment sampling. Replacement sampling 

sites were selected downstream as near as possible to the 

historical sampling sites and upstream from any tributaries. 

Land uses in the drainage areas upstream from the historical 

and 2011–13 sampling sites were similar, and no other 

notable factors were identified that might result in appreciable 

differences between the historical and 2011–13 suspended 

sediment concentrations. The SSLs in the study area are 

dominated by very fine suspended-sediment particle sizes that 

tend to remain in suspension during transport between the two 

sites, providing additional corroborating evidence that changes 

in SSLs between the historical and 2011–13 sampling sites 

were unlikely. 

It was not practical to make streamflow measurements 

at the 2011–13 SAR Falls City and Ecleto Runge 

sampling sites; consequently, streamflows at these two 

sampling sites were estimated as the gaged streamflow 

measured at their respective upstream streamflow-gaging 

stations, taking into account the streamflow traveltime 

between the locations. To account for streamflow traveltime 

between the SAR Falls City gaging station and the 2011–13 

sampling site (approximately 2 hours for streamflow greater 

than 1,000 ft3/s), streamflow measured at the gaging station 

2 hours before the sample collection was used to represent 

streamflow conditions at the 2011–13 sample sites during 

stormflow events. Streamflow was not adjusted during 

base-flow conditions. Similarly, during sampling events at 

the Ecleto Runge site, gaged streamflow recorded 2 hours 

before the sampling event was used to represent streamflow 

conditions during the sample collection.

The regression of SSCs to streamflow data (eq. 4) 

collected at each of the sampling sites exhibits generally 

reasonable correlations between observed data and the 

regression estimates; however, during increased streamflow 

(typically, greater than the 80th percentile daily streamflow), 

SSCs tend to approach an upper limit (concentrations 

remain constant with increasing streamflow), which is likely 

attributable to watershed limits to the supply of available 

sediment in runoff (Porterfield, 1972; Gao, 2008). Therefore, 

the regression equations tend to oversimulate sediment 

concentrations, compared with observed SSC values, during 

increased streamflow. Also, extrapolation of the regression 

equations to simulate SSCs and SSLs for streamflow values 

that are greater than observed SSCs and SSLs during sample 

collection results in (possibly extreme) overestimates of 

SSCs and SSLs. During major runoff events, particularly for 

large events that were not sampled for suspended sediment, 

prediction of SSCs by regression would potentially result in 

overestimation of daily and event SSLs; therefore, to avoid 

overestimation of SSCs and SSLs, the regression-estimated 

SSCs (eq. 4) used to calculate daily SSLs at each sampling 

location (eq. 2) were limited (truncated) at a maximum value 

when the observed (sampled) SSCs approached the upper 

concentration limit.

Estimating Suspended-Sediment Loads from 

Turbidity 

A correlation between turbidity and SSC has been 

documented in numerous reports for different geographic 

settings in the United States and in other parts if the world. 

Studies in the United States were done in the Pacific 

Northwest (Kunkle and Comer, 1971), Vermont (Beschta, 

1980), Kansas (Christensen and others, 2000; Rasmussen 

and others, 2005, 2009), Oregon (Uhrich and Bragg, 2003; 

Bragg and Uhrich, 2010), the Chesapeake Bay area (Jastram 

and others, 2009), and Kentucky (Williamson and Crawford, 

2011). Additional studies in other parts of the world include 

Indonesia (Brabben, 1981) and Australia (Gippel, 1989). 

By developing a regression equation from the relation of 

turbidity and SSC at a sampling site, a time series of SSCs 

can be estimated from a time series of turbidity data. Once 

the regression equation is developed, the time series of 

estimated SSCs can be used with the time series of streamflow 

to estimate a time series of SSLs (eq. 2). Because sediment 

transport is complex and dependent on many factors, 

regression equations for estimating SSCs are usually site 

specific and subject to change over time. Data collected over 

time can help verify change in sediment load, type, and source 

(Leopold, 1997; Rasmussen and others, 2009). 

Sediment Characteristics

Sediment characteristics in the study area are described 

by comparing the results from various types of historical 

sediment data collected during 1966–94 with recent sediment 

data collected during 2011–13 (table 1). Sediment data were 

historically collected by the U.S. Geological Survey between 

1966 and 2004 at five sites in the study area. Much of this 

historical data was collected during routine sampling efforts 

that typically characterized low-flow conditions, and only 

infrequently were larger flow events characterized. Most of 

the historical data also did not include particle-size 

distributions, and no bedload data were collected. During 

2011–13, SSC and particle-size distribution were measured in 

samples collected at all sites, and bedload mass and particlesize distribution were measured in samples collected at six 

sites. The sediment data, along with streamflow information, 

were used to describe the sediment loads at sites during the 

various flow conditions.
Sediment Characteristics 15

Suspended-Sediment Concentration

Measured SSCs in the 67 samples collected during 

2011–13 ranged from 14 mg/L during base-flow conditions 

at the SAR Falls City site on August 19, 2011, to 4,480 mg/L 

during a stormflow event at the same site on January 26, 

2012 (table 3). It should be noted that not all samples were 

collected during the same storm events, same routine events, 

or even same parts of the storm hydrographs; therefore, 

comparisons of concentrations between sites should be 

considered qualitative. Boxplots created using historical 

and 2011–13 SSC data from the sites are shown in figure 

4. Varying amounts of historical SSC data were available 

for the SAR Elmendorf, SAR Falls City, Cibolo Falls City, 

Ecleto Runge, and SAR Goliad sites. No historical data were 

available for the Cibolo Selma, SAR 72, SAR McFaddin, or 

GR Tivoli sites. 

A comparison of the boxplot for the historical data and 

the 2011–13 data from the SAR Elmendorf site demonstrates 

that the 2011–13 samples represent a wide range of SSCs, 

including some of the largest concentrations measured at 

the site. This is likely, in part, because the samples targeted 

stormflow events that typically elevate SSCs (Wood, 1977; 

VanSickle and Beschta, 1983; Glysson, 1987; Clark and 

others, 2013). The 2011–13 samples from the SAR Falls 

City, Cibolo Falls City, and SAR Goliad sites exhibited SSCs 

that plotted outside of the historical interquartile ranges for 

these sites. These results were not surprising because samples 

collected at these sites during 2011–13 were collected during 

historically low base-flow conditions that produce very low 

SSCs or were collected during stormflow events; whereas, 

the historical data were obtained possibly during different 

hydrologic conditions and potentially during different periods 

of the storm hydrograph. For example, it is not known if 

specific parts of the hydrograph, such as the rising limb or 

peak, were targeted during historical sampling as was the case 

for the 2011–13 samples. The SSCs from 2011–13 samples 

collected at the Ecleto Runge site all plotted at the upper end 

of the interquartile range or higher. Drought conditions were 

common during 2011–13 in the study area; therefore, base 

flow past the Ecleto Runge site was seldom, and the stream 

typically flowed only during storm events. 

Table 3. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing the percentage of sandsized and fine-sized particles in each sample.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mg/L, milligrams per liter; mm, millimeter; *, grab sample]

Date Time

Instantaneous 

streamflow 

(ft3/s)

Suspendedsediment 

concentration 

(mg/L)

Instantaneous 

suspendedsediment load

(tons/day)

Suspended-sediment particle size

Less than 

0.0625 mm

(percent)

Equal to or 

greater than 

0.0625 mm

(percent)

USGS 08181800 San Antonio River near Elmendorf, Tex. (SAR Elmendorf)

1/27/2011 0900 236 21 13 87 13

1

4/7/2011 1100 138 71 26 65 35

5/13/2011 1030 714 131 253 96 4

6/22/2011 1900 1,830 625 3,090 96 4

10/9/2011 1430 4,720 2,630 33,500 98 2

1/25/2012 1230 8,800 2,010 47,800 92 8

1/25/2012 1800 8,570 1,530 35,400 94 6

2/18/2012 1130 3,350 1,430 12,900 92 8

5/11/2012 1000 4,910 1,090 14,500 98 2

9/14/2012 1300 5,360 1,160 16,800 99 1

USGS 08183550 San Antonio River at Highway 181 near Falls City, Tex. (SAR Falls City)2

4/6/2011 1230 171 37 17 95 5

6/24/2011 1000 1,000 930 2,511 98 2

8/19/2011 1100 52 14 2.0 93 7

10/11/2011 0930 2,880 1,990 15,474 94 6

1/26/2012 1400 3,590 2,390 23,200 96 4

Table 3. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing the percentage of sandsized and fine-sized particles in each sample.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mg/L, milligrams per liter; mm, millimeter; *, grab sample]
16 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Table 3. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing the percentage of sandsized and fine-sized particles in each sample.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mg/L, milligrams per liter; mm, millimeter; *, grab sample]

Date Time

Instantaneous 

streamflow 

(ft3/s)

Suspendedsediment 

concentration 

(mg/L)

Instantaneous 

suspendedsediment load

(tons/day)

Suspended-sediment particle size

Less than 

0.0625 mm

(percent)

Equal to or 

greater than 

0.0625 mm

(percent)

USGS 08183550 San Antonio River at Highway 181 near Falls City, Tex. (SAR Falls City)2—Continued

1/26/2012 1630 3,730 4,480 45,118 96 4

3/21/2012 1400 2,130 1,440 8,281 97 3

5/12/2012 1500 2,950 4,030 32,099 98 2

7/12/2012 1230 488 50 66 99 1

USGS 08185000 Cibolo Creek at Selma, Tex. (Cibolo Selma)

9/17/2012 0815 1.9 71 0.36 100 0

9/29/2012 0915 37 262 26 100 0

5/25/2013 1200 10,600 486* 13,909 94 6

USGS 08186000 Cibolo Creek near Falls City, Tex. (Cibolo Falls City)

1

1/26/2011 1350 54 37 5.4 55 45

4/6/2011 1350 30 18 1.5 96 4

6/25/2011 1300 62 53* 9.0 97 3

1/26/2012 0930 1,020 985 2,710 91 9

2/19/2012 1100 2,540 1,480 10,100 94 6

3/21/2012 1200 3,380 1,140 10,400 93 7

9/30/2012 1630 1,820 982 4,830 96 4

USGS 08186500 Ecleto Creek near Runge, Tex. (Ecleto Runge)3

1/26/2012 1335 7.0 399 7.5 99 1

2/19/2012 1430 122 1,180 389 98 2

3/21/2012 1030 140 789 298 100 0

USGS 08186550 Ecleto Creek at County Road 326 near Runge, Tex. (Ecleto Runge)3

3/20/2012 1330 20 347 19 99 1

5/12/2012 1030 397 510 547 93 7

USGS 08188060 San Antonio River at State Highway 72 near Runge, Tex. (SAR 72)

6/25/2011 1500 487 740 973 97 3

8/19/2011 0945 67 129 23 97 3

10/11/2011 1300 2,570 1,660 11,500 89 11

1/27/2012 1230 4,640 2,420 30,300 92 8

3/22/2012 1030 4,840 1,970 25,700 89 11

USGS 08188500 San Antonio River at Goliad, Tex. (SAR Goliad)

1/26/2011 1050 438 69 82 84 16

4/6/2011 0800 311 81 68 98 2

6/25/2011 2000 1,180 811 2,580 95 5

10/13/2011 0945 3,280 1,690 15,000 92 8

1/28/2012 1130 4,650 1,970 24,700 90 10
Sediment Characteristics 17

Table 3. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing the percentage of sandsized and fine-sized particles in each sample.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mg/L, milligrams per liter; mm, millimeter; *, grab sample]

Date Time

Instantaneous 

streamflow 

(ft3/s)

Suspendedsediment 

concentration 

(mg/L)

Instantaneous 

suspendedsediment load

(tons/day)

Suspended-sediment particle size

Less than 

0.0625 mm

(percent)

Equal to or 

greater than 

0.0625 mm

(percent)

USGS 08188500 San Antonio River at Goliad, Tex. (SAR Goliad)—Continued

2/21/2012 1200 4,030 1,350 14,700 96 4

3/22/2012 1300 3,400 1,150 10,600 90 10

3/23/2012 1200 4,520 1,450 17,700 95 5

USGS 08188570 San Antonio River near McFaddin, Tex. (SAR McFaddin)

1/26/2011 0850 544 1,420 2,090 100 0

4/5/2011 1830 329 166 147 98 2

6/27/2011 2000 783 124* 26 100 0

10/13/2011 1300 1,210 1,670 5,460 99 1

2/21/2012 1430 1,650 1,250 5,570 97 3

3/23/2012 1030 1,970 1,490 7,930 98 2

USGS 08188800 Guadalupe River near Tivoli, Tex. (GR Tivoli)

1/25/2011 1440 1,980 67 358 81 19

4/5/2011 1400 941 75 191 95 5

6/28/2011 1200 913 96* 237 99 1

10/16/2011 1530 954 141 363 99 1

1/29/2012 1130 2,880 1,400 10,900 99 1

1/29/2012 1730 3,060 1,130 9,340 99 1

1/30/2012 1100 3,320 788 7,060 98 2

1/31/2012 0930 3,550 469 4,500 93 7

2/22/2012 1200 2,600 457 3,210 99 1

2/23/2012 1020 2,920 317 2,500 98 2

3/26/2012 1230 2,950 187 1,490 97 3

3/27/2012 0800 2,900 147 1,150 96 4

5/15/2012 1230 2,940 366 2,910 96 4

5/16/2012 0830 2,920 472 3,720 98 2

1

Sample not included in analyses because of possible sampling issue but included in table for completeness of data set. 

2

For analysis, data collected at USGS station numbers 08183500 San Antonio River near Falls City, Tex., and 08183550 San Antonio River at Highway 181 

near Falls City, Tex., were combined and reported under short name “SAR Falls City;” adjusted instantaneous streamflow from site 08183500 was reported for 

site 08183550. 

3

For analysis, data collected at USGS station numbers 08186500 Ecleto Creek near Runge, Tex., and 08186550 Ecleto Creek at County Road 326 near Runge, 

Tex., were combined and reported under short name “Ecleto Runge;” adjusted instantaneous streamflow from site 08186500 was reported for site 08186550. 
Figure 4. Distribution of suspended-sediment concentrations at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a downstream from the San Antonio River Basin, 1966–2013.

Short name for sampling site (see table 1)

SAR

Elmendorf

SAR

Falls City1

Cibolo

Falls City

Cibolo

Falls City

Ecleto

Runge3

Ecleto

Runge4

SAR

Falls City2

Cibolo 

Selma

SAR

72

SAR

Goliad

GR

Tivoli

SAR

Goliad

SAR

McFaddin

SAR

Elmendorf

Suspended-sediment concentration, in milligrams per liter.

4,0005,0002,0003,0001,000

0

95 9 47 9 3 35 6 31 5 5 155 8 6 14

xx

oooo

x

xx

o

xxxxxx

ooooooooooo

1Data collected at USGS station number 08183500 reported under short name "SAR Falls City."2Data collected at USGS station number 08183550 reported under short name "SAR Falls City."3Data collected at USGS station number 08186500 reported under short name "Ecleto Runge."4Data collected at USGS station number 08186550 reported under short name "Ecleto Runge."

EXP

Nu

201

His

155

ox
Sediment Characteristics 19

The particle-size distribution data from the 2011–13 

suspended-sediment samples were consistent with historical 

observations that the system is dominated by fine sediment 

particles less than 0.0625 mm in diameter. The elevated 

percentages of sand-sized or larger particles in the samples 

collected on April 4, 2011, at the SAR Elmendorf site and 

on January 26, 2011, at the Cibolo Falls City site were 

considered questionable especially at the streamflows at which 

the samples were collected. It is likely there was a problem 

with the sampling technique that allowed a small number 

of sand-sized or larger particles to enter the sampler during 

collection. These larger particles could easily skew the results 

at such low concentrations of suspended sediment, so these 

data were not used in any analyses for this report although 

they are shown in tables for completeness. Similarly, nine 

historical samples also were identified as potential outliers and 

were not used in the analyses from SAR Falls City, Cibolo 

Falls City, and SAR Goliad but are shown in appendix 1 for 

completeness. A summary of the sand/fine break data for 

the 2011–13 suspended-sediment samples indicates that, on 

average, about 94 percent of suspended-sediment particles 

in the samples were less than 0.0625 mm (silt and clay) 

(table 3). Furthermore, the data from the 50 samples for which 

a complete sediment-size analysis was performed indicate that 

an average of about 69 percent of the particles was less than 

0.002 mm, the smallest size range for which the samples were 

analyzed (table 4).

Table 4. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing complete distribution of 

particle size for each sample.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mm, millimeter; --, no data]

Sample 

date

Sample 

start 

time

Instantaneous 

streamflow 

(ft3/s)

Suspended-sediment diameter 

(percentage)

Less 

than 

0.002 

mm

Less 

than 

0.004 

mm

Less 

than 

0.008 

mm

Less 

than 

0.016 

mm

Less 

than 

0.031 

mm

Less 

than 

0.0625 

mm

Less 

than 

0.125 

mm

Less 

than 

0.25 mm

Less 

than 

0.5 mm

Less 

than 

1 mm

Less 

than 

2 mm

USGS 08181800 San Antonio River near Elmendorf, Tex. (SAR Elmendorf)

6/22/2011 1900 1,830 60 73 87 93 94 96 98 99 100 -- --

10/9/2011 1430 4,720 56 72 83 91 96 98 99 100 -- -- --

1/25/2012 1230 8,800 64 71 77 83 88 92 97 99 100 -- --

1/25/2012 1800 8,570 50 65 73 80 89 94 98 99 100 -- --

2/18/2012 1130 3,350 62 69 74 79 88 92 98 100 -- -- --

5/11/2012 1000 4,910 66 78 84 90 95 98 99 100 -- -- --

9/14/2012 1300 5,360 66 79 84 90 97 99 100 -- -- -- --

USGS 08183550 San Antonio River at Highway 181 near Falls City, Tex. (SAR Falls City)1

6/24/2011 1000 1,000 53 70 88 94 97 98 99 100 -- -- --

8/19/2011 1100 52 -- -- -- -- -- 93 99 100 -- -- --

10/11/2011 930 2,880 59 69 76 84 90 94 98 100 -- -- --

1/26/2012 1400 3,590 58 65 72 82 94 96 99 100 -- -- --

1/26/2012 1630 3,730 67 76 83 89 94 96 99 99 100 -- --

3/21/2012 1400 2,130 53 63 75 86 94 97 99 100 -- -- --

5/12/2012 1500 2,950 59 73 86 95 96 98 99 100 -- -- --

Table 4. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing complete distribution of 

particle size for each sample.

[ft3/s, cubic feet per second; mm, millimeter; USGS, U.S. Geological Survey; --, no data]
20 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Table 4. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing complete distribution of 

particle size for each sample.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mm, millimeter; --, no data]

Sample 

date

Sample 

start 

time

Instantaneous 

streamflow 

(ft3/s)

Suspended-sediment diameter 

(percentage)

Less 

than 

0.002 

mm

Less 

than 

0.004 

mm

Less 

than 

0.008 

mm

Less 

than 

0.016 

mm

Less 

than 

0.031 

mm

Less 

than 

0.0625 

mm

Less 

than 

0.125 

mm

Less 

than 

0.25 mm

Less 

than 

0.5 mm

Less 

than 

1 mm

Less 

than 

2 mm

USGS 08185000 Cibolo Creek at Selma, Tex. (Cibolo Selma)

9/29/2012 915 37 93 99 99 100 -- -- -- -- -- -- --

5/25/2013 1200 10,600 41 58 69 82 92 94 96 96 97 97 100

USGS 08186000 Cibolo Creek near Falls City, Tex. (Cibolo Falls City)

6/25/2011 1300 62 -- -- -- -- -- 97 98 99 100 -- --

1/26/2012 930 1,020 60 67 71 79 87 91 97 99 100 -- --

2/19/2012 1100 2,540 73 78 82 86 91 94 98 99 100 -- --

3/21/2012 1200 3,380 76 80 84 86 91 93 98 100 -- -- --

9/30/2012 1630 1,820 71 77 82 87 92 96 99 99 100 -- --

USGS 08186500 Ecleto Creek near Runge, Tex. (Ecleto Runge)2

1/26/2012 1335 7 97 97 98 99 97 99 100 -- -- -- --

2/19/2012 1430 122 88 90 92 95 96 98 100 -- -- -- --

3/21/2012 1030 140 97 99 100 -- -- -- -- -- -- -- --

USGS 08186550 Ecleto Creek at County Road 326 near Runge, Tex. (Ecleto Runge)2

3/20/2012 1330 20 93 95 98 98 99 99 100 -- -- -- --

5/12/2012 1030 397 63 80 84 90 91 93 97 98 99 100 --

 USGS 08188060 San Antonio River at State Highway 72 near Runge, Tex. (SAR 72)

6/25/2011 1500 487 60 73 87 93 96 97 98 99 100 -- --

8/19/2011 945 67 -- -- -- -- -- 97 99 100 -- -- --

10/11/2011 1300 2,570 57 63 69 76 83 89 97 99 100 -- --

1/27/2012 1230 4,640 62 70 80 85 90 92 96 98 100 -- --

3/22/2012 1030 4,840 68 75 73 82 87 89 90 91 100 -- --

USGS 08188500 San Antonio River at Goliad, Tex. (SAR Goliad)

6/25/2011 2000 1,180 42 51 65 78 90 95 98 99 100 -- --

10/13/2011 945 3,280 58 77 81 87 89 92 95 98 100 -- --

1/28/2012 1130 4,650 72 78 82 87 88 90 92 96 100 -- --

2/21/2012 1200 4,030 60 73 87 92 94 96 97 99 100 -- --
Sediment Characteristics 21

Table 4. Suspended-sediment data collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and at a 

site on the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013, showing complete distribution of 

particle size for each sample.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mm, millimeter; --, no data]

Sample 

date

Sample 

start 

time

Instantaneous 

streamflow 

(ft3/s)

Suspended-sediment diameter 

(percentage)

Less 

than 

0.002 

mm

Less 

than 

0.004 

mm

Less 

than 

0.008 

mm

Less 

than 

0.016 

mm

Less 

than 

0.031 

mm

Less 

than 

0.0625 

mm

Less 

than 

0.125 

mm

Less 

than 

0.25 mm

Less 

than 

0.5 mm

Less 

than 

1 mm

Less 

than 

2 mm

USGS 08188500 San Antonio River at Goliad, Tex. (SAR Goliad)—Continued

3/22/2012 1300 3,400 69 75 78 82 87 90 94 98 100 -- --

3/23/2012 1200 4,520 81 84 88 90 92 95 96 98 100 -- --

USGS 08188570 San Antonio River near McFaddin, Tex. (SAR McFaddin)

10/13/2011 1300 1,210 62 71 78 90 98 99 100 -- -- -- --

2/21/2012 1430 1,650 56 62 69 77 91 97 100 -- -- -- --

3/23/2012 1030 1,970 60 66 72 78 90 98 100 -- -- -- --

USGS 08188800 Guadalupe River near Tivoli, Tex. (GR Tivoli)

6/28/2011 1200 913 -- -- -- -- -- 99 100 -- -- -- --

10/16/2011 1530 954 72 80 84 91 95 99 100 -- -- -- --

1/30/2012 1100 3,320 82 88 91 95 95 98 99 100 -- -- --

1/31/2012 930 3,550 74 79 82 85 90 93 96 99 100 -- --

2/22/2012 1200 2,600 82 88 91 96 98 99 99 100 -- -- --

2/23/2012 1020 2,920 89 93 96 97 98 98 99 100 -- -- --

3/26/2012 1230 2,950 82 88 89 94 96 97 98 100 -- -- --

3/27/2012 800 2,900 86 90 91 95 96 96 98 99 100 -- --

5/15/2012 1230 2,940 91 95 96 96 96 96 98 99 100 -- --

5/16/2012 830 2,920 81 95 98 98 98 98 99 100 -- -- --

1

For analysis, data collected at USGS station numbers 08183500 San Antonio River near Falls City, Tex., and 08183550 San Antonio River at Highway 181 

near Falls City, Tex., were combined and reported under short name “SAR Falls City;” adjusted instantaneous streamflow from site 08183500 was reported for 

site 08183550. 

2

For analysis, data collected at USGS station numbers 08186500 Ecleto Creek near Runge, Tex., and 08186550 Ecleto Creek at County Road 326 near 

Runge, Tex., were combined and reported under short name “Ecleto Runge;” adjusted instantaneous streamflow from site 08186500 was reported for 

site 08186550. 
22 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Bedload

The mass from the 22 bedload samples collected during 

2011–13 ranged from 12 g at the site furthest downstream 

(GR Tivoli) during base-flow conditions on April 5, 2011, to 

8,900 g at the SAR Goliad site during a stormflow event on 

January 28, 2012 (table 5). The data from the particle-size 

analysis for 10 bedload samples collected during 2011–13 

(table 6) indicated that, on average, about 91 percent of 

bedload was composed of sand-sized particles or smaller (less 

than 2 mm in size) with sediment in the 0.25–0.5-mm size 

range accounting for a majority (51 percent, on average) of 

bedload. The bedload samples collected at upstream sample 

sites exhibited a wider range of sediment sizes and included 

greater percentages of larger particle sizes than those collected 

at downstream sites, which showed a very narrow and uniform 

particle-size distribution despite streamflow variation. This 

distribution is common in most river systems as sediment 

matures (becomes finer) as it is exposed to ongoing erosion 

as it moves through a basin. In addition, energy available 

for transporting sediment generally decreases as the stream 

approaches a delta and slope decreases.

Sediment Loads 

Instantaneous Suspended-Sediment Loads, 

Bedloads, and Total Loads 

Instantaneous SSLs computed from the 67 SSC samples 

collected during 2011–12 ranged from 0.36 tons per day at the 

Cibolo Selma site during base flow conditions on September 

17, 2012, to 47,800 tons per day at the SAR Elmendorf site 

during a stormflow event on January 25, 2012 (table 3). 

Instantaneous bedload transport computed for the 22 samples 

collected during 2011–12 ranged from 0.15 tons per day at 

SAR Elmendorf during base-flow conditions on January 

27, 2011, to 155 tons per day at site SAR Goliad during a 

stormflow event on January 28, 2012 (table 5). Instantaneous 

total sediment loads computed for the 21 samples for 

which both instantaneous SSL and instantaneous bedload 

were computed ranged from 13.1 tons per day at the SAR 

Elmendorf site on January 27, 2011, to 47,828 tons per day at 

the same site on January 25, 2012 (table 5). The percentage 

of instantaneous total sediment load that was bedload ranged 

from 0 to 7.2 percent, with bedload accounting for less than 

1 percent of total sediment load in 13 of the 21 samples 

(table 5). The small percentage of bedload is not unexpected 

in the basin because the SSL is dominated by the fine sediment 

fraction.

Guadalupe River near Tivoli, Texas.
Table 5. Bedload mass and instantaneous bedload transport for samples collected at sites in the San Antonio River Basin downstream from San Anon the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013.—Continued[SSL, suspended sediment load; CR, County Road; --, no data]

U.S. Geological 

Survey 

station 

number

U.S. Geological Survey 

station name

Short name for 

sampling site

Sample

date

Bedload 

sample 

mass

(grams)

Composited 

samples 

in crosssectional 

bedload 

measurement

(number)

Stream 

width

(feet)

Rest 

time on 

bed for 

bedload 

sample 

(seconds)

Instantaneous 

bedload 

transport

(tons 

per day)

Instantaneous 

SSL 

(tons 

per day)

Insta

neo

tot

sedim

loa

(to

per d

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 1/27/2011 32 40 44 90 0.15 13 

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 4/7/2011 112 40 36 120 0.32 26 

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 5/13/2011 5,450 40 45 120 20 253 2

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 6/22/2011 3,060 40 62 60 30 3,090 3,1

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 10/9/2011 1,050 40 117 60 20 33,500 33,5

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 1/25/2012 1,360 40 130 60 28 47,800 47,8

08186000 Cibolo Creek near Falls 

City, Tex.

Cibolo Falls City 2/19/2012 313 40 68 60 3.4 10,100 10,1

08186500 Ecleto Creek near 

Runge, Tex.

Ecleto Runge2 2/19/2012 277 32 31 60 1.7 389 3

08186550 Ecleto Creek at CR 326 

near Runge, Tex.

Ecleto Runge2 3/20/2012 42 38 34 60 0.24 19 

08186550 Ecleto Creek at CR 326 

near Runge, Tex.

Ecleto Runge2 5/12/2012 3,280 40 52 120 14 547 5

08188500 San Antonio River at 

Goliad, Tex.

SAR Goliad 6/25/2011 1,440 38 65 60 16 2,580 2,5

08188500 San Antonio River at 

Goliad, Tex.

SAR Goliad 8/18/2011 105 40 60 120 0.50 -- -

Table 5. Bedload mass and instantaneous bedload transport for samples collected at sites in the San Antonio River Basin downstream from San Antthe Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013.[SSL, suspended sediment load; CR, County Road; --, no data]
Table 5. Bedload mass and instantaneous bedload transport for samples collected at sites in the San Antonio River Basin downstream from San Anon the Guadalupe River downstream from the San Antonio River Basin, January 2011–May 2013.—Continued [SSL, suspended sediment load; CR, County Road; --, no data] U.S. Geo- logical Survey station number U.S. Geological Survey station name Short name for sampling site Sample date Bedload sample mass (grams) Composited samples in cross- sectional bedload measurement (number) Stream width (feet) Rest time on bed for bedload sample (seconds) Instan- taneous bedload transport (tons per day) Instan- taneous SSL (tons per day) Insta

neo

tot

sedim

loa

(to

per d

08188500 San Antonio River at Goliad, Tex. SAR Goliad 1/28/2012 8,900 40 110 60 155 24,700 24,808188800 Guadalupe River near Tivoli, Tex. GR Tivoli 1/25/2011 485 40 127 60 10 358 308188800 Guadalupe River near Tivoli, Tex. GR Tivoli 4/5/2011 12 20 138 120 0.27 191 108188800 Guadalupe River near Tivoli, Tex. GR Tivoli 6/28/2011 66 40 126 60 1.3 237 208188800 Guadalupe River near Tivoli, Tex. GR Tivoli 10/16/2011 25 40 125 120 0.24 363 308188800 Guadalupe River near Tivoli, Tex. GR Tivoli 1/30/2012 1,270 40 130 60 26 7,060 7,008188800 Guadalupe River near Tivoli, Tex. GR Tivoli 1/31/2012 518 40 130 60 11 4,500 4,508188800 Guadalupe River near Tivoli, Tex. GR Tivoli 2/22/2012 425 40 132 60 8.9 3,210 3,208188800 Guadalupe River near Tivoli, Tex. GR Tivoli 3/26/2012 735 40 130 60 15 1,490 1,508188800 Guadalupe River near 

Tivoli, Tex.

GR Tivoli 5/15/2012 641 40 130 120 6.6 2,910 2,9

1Instantaneous total sediment load is the sum of instantaneous bedload transport plus instantaneous SSL. Unrounded values are reported for the sum because rounding to three 

in 100 percent suspended-sediment load and 0 percent bedload at most of the sampling sites.

2Data collected at sites 08186500 and 08186550 were reported under sampling site short name “Ecleto Runge.”
Table 6. Bedload particle-size distribution for samples collected at sites in the San Antonio River Basin downstream from San Antonio, Texas, and aRiver downstream from the San Antonio River Basin, January 2011–May 2013.[USGS, U.S. Geological Survey; mm, millimeter; --, no data; CR, County Road]

USGS 

station 

number

USGS

station name

Short name for 

sampling site 

(table 1) 

Sample 

date

Sample 

start 

time

Bedload sediment diameter 

(percentage)

Less 

than

0.0625 

mm

Less 

than 

0.125 

mm

Less 

than 

0.25 

mm

Less 

than 

0.5 mm

Less 

than 

1 mm

Less 

than 

2 mm

Less 

than 

4 mm

08181800 San Antonio River near 

Elmendorf, Tex.

SAR Elmendorf 1/25/2012 1330 0 1 2 18 56 86 96 

08186000 Cibolo Creek near Falls 

City, Tex.

Cibolo Falls City 2/19/2012 1200 2 4 5 40 76 83 87 

08186500 Ecleto Creek near 

Runge, Tex.

Ecleto Runge1 2/19/2012 1530 0 1 2 33 76 91 95 

08186550 Ecleto Creek at CR 326 

near Runge, Tex.

Ecleto Runge1 3/20/2012 1300 0 1 3 40 76 88 95 

08186550 Ecleto Creek at CR 326 

near Runge, Tex.

Ecleto Runge1 5/12/2012 1200 1 1 2 53 89 96 98 

08188500 San Antonio River at 

Goliad, Tex.

SAR Goliad 1/28/2012 1500 0 0 3 54 89 95 97 

08188800 Guadalupe River near 

Tivoli, Tex.

GR Tivoli 1/31/2012 1130 2 2 7 81 89 92 95 

08188800 Guadalupe River near 

Tivoli, Tex.

GR Tivoli 2/22/2012 1400 1 2 5 80 92 95 98 

08188800 Guadalupe River near 

Tivoli, 

GR Tivoli 3/26/2012 1430 2 2 10 83 92 95 97 

08188800 Guadalupe River near 

Tivoli, Tex.

GR Tivoli 5/15/2012 1400 1 2 6 73 81 85 91 

1Data collected at USGS station numbers 08186500 and 08186550 were reported under sampling site short name “Ecleto Runge.”
26 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Estimated Daily Suspended-Sediment Loads 

from Streamflow

Daily SSLs were estimated using the SSC to streamflow 

regression equations for each site (fig. 5; table 7; appendix 2). 

The regression equations and summary statistics are listed 

in table 7, and the residuals were approximately uniformly 

distributed. The estimated daily SSLs for each of the sites 

indicated that during 2011–12, the majority of the SSL in 

the basin originated upstream from the SAR Elmendorf site. 

The estimated average daily SSL was 237 tons per day at 

the SAR Elmendorf site and 454 tons per day at the SAR 

Goliad site during 2011–12. Downstream from the SAR 

Elmendorf site is the SAR Falls City site, where the estimated 

average daily SSL was 332 tons per day (table 7). This might 

be expected given that the majority of the streamflow at 

the SAR Falls City site originates upstream from the SAR 

Elmendorf site. Note that the estimated SSLs during 2011–12 

at the SAR Elmendorf and SAR Falls City sites were based 

on independent streamflow measurements and independent 

SSCs measurements at the respective sites, indicating that 

the regression method used for estimating loads produces 

consistent results. Because the Cibolo Selma site often did 

not flow, the estimated average daily SSL at Cibolo Falls 

City (57 tons per day) likely originates from within the study 

watershed, except during large stormflow events when there is 

streamflow at the Cibolo Selma site. During 2011–12, Ecleto 

Runge had the lowest SSL of 9 tons per day, which is expected 

because it drains a smaller watershed and streamflow generally 

only occurs during storm events. Regression analyses were 

not performed and SSLs were not estimated for the SAR 72, 

SAR McFaddin, and GR Tivoli sites because the relatively 

small number of data available for these sites represents only a 

limited range of hydrologic conditions. 

Cibolo Creek at Selma, Texas. 
Sediment Loads 27

Figure 5. Relations between suspended-sediment concentration and streamflow at sites A, SAR Elmendorf site (U.S. Geological 

Survey [USGS] station 08181800, San Antonio River near Elmendorf, Texas); B, SAR Falls City site (USGS station 08183500 San Antonio 

River near Falls City, Tex.); C, Cibolo Falls City site (USGS station 08186000, Cibolo Creek near Falls City, Tex.); D, Ecleto Runge site (USGS 

station 08186500, Ecleto Creek near Runge, Tex.); and E, SAR Goliad site (USGS station 08188500 San Antonio River at Goliad, Tex.).

Streamflow, in cubic feet per second

Streamflow, in cubic feet per second

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

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

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

0.01 0.1 1 10 100 1,000 10,000 100,000

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

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

Suspended-sediment concentration, in milligrams per liter

R2 = 0.66

SSC = 0.151 x Q1.026

R2 = 0.83

SSC = 4.32 x Q0.72

R2 = 0.42

SSC = 0.46 x Q0.949

R2 = 0.66

SSC = 0.0483 x Q1.336

R2 = 0.48

SSC = 118.5 x Q0.308

Note: Short names for sampling sites (table 1) are listed above the upper left corners of figures 5A–5E. 

For analysis, data collected at USGS station numbers 08183500 and 08183550 were combined and reported under short name “SAR Falls City.” 

For analysis, data collected at USGS station numbers 08186500 and 08186550 were combined and reported under short name “Ecleto Runge.”

A. SAR Elmendorf B. SAR Falls City

C. Cibolo Falls City D. Ecleto Runge

E. SAR Goliad

EXPLANATION

Relation between suspended-sediment concentration

 (SSC) and streamflow (Q)—Dotted where suspended

 sediment no longer increases with increasing streamflow

Historical sample data

2011–12 sample data

Coefficient of determination for the log-transformed

 regression equation, where suspended sediment 

 increases with increasing streamflow

R2
Table 7. Regression equations, average estimated suspended-sediment load, and summary statistics for suspended-sediment concentration regres[Tex., Texas; SSC, suspended-sediment concentration, in milligrams per liter; Q, streamflow, in cubic feet per second; ft3/s, cubic feet per second] U.S. Geologi- cal Survey station number U.S. Geological Survey station name Short name of sampling site (table 1) Suspended- sediment concentration to streamflow regression equation1 Range of streamflow when linear regression is appli- cable2 (ft3/s) Number of mea- sure- ments (n) Coeffi- cient of determi- nation (R2) Mean absolute error (MAE) Root mean square error (RMSE) Bia

cor

recti

facto

(Φ) 

8181800 San Antonio River near Elmendorf, Tex. SAR Elmendorf SSC = 0.151 x Q1.026 0–1,000 104 0.66 0.25 0.31 1.27 8183500 and 8183550 San Antonio River near Falls City, Tex., and San Antonio River at Highway 181 near Falls City, Tex. SAR Falls City3 SSC = 0.0483 x Q1.336 0–1,910 38 0.66 0.27 0.33 1.32 8186000 Cibolo Creek near Falls City, Tex. Cibolo Falls City SSC = 4.32 x Q0.72 0–2,000 30 0.83 0.16 0.20 1.18186500 and 08186550 Ecleto Creek near Runge, Tex., and Ecleto Creek at County Road 326 near Runge, Tex. Ecleto Runge4 SSC = 118.5 x Q0.308 0–200 27 0.48 0.28 0.33 1.32 8188500 San Antonio River at Goliad, Tex. SAR Goliad SSC = 0.46 x Q0.949 0–3,500 149 0.42 0.22 0.28 1.20 

1Regression equations incorporate the bias correction factor. 2When streamflow exceeded the upper range limit, SSC was set to a constant value equal to the estimated SSC based on streamflow at the upper value of the range. 3For analysis, data collected at USGS station numbers 08183500 and 08183550 were combined and reported under short name “SAR Falls City.” 4For analysis, data collected at USGS station numbers 08186500 and 08186550 were combined and reported under short name “Ecleto Runge.”
Sediment Loads 29

Estimated Daily Suspended-Sediment Loads 

from Turbidity 

Turbidity was measured every 15 minutes at the SAR 

Elmendorf site from January 6, 2011, through September 30, 

2013. With the exception of two increases in turbidity (July 

11, 2012, and January 9, 2013), the turbidity measurements 

truncated near the upper limit of the turbidity probe at 

approximately 1,100–1,200 FNU. The daily mean turbidity 

values (computed from the 15-minute turbidity values) at the 

SAR Elmendorf site ranged from 4.6 FNU on September 3, 

2011, to 710 FNU on August 19, 2012 (fig. 6; appendix 3). 

The data truncated by the limitations of the turbidity probe 

and issues with the DCP program likely caused daily mean 

turbidity values to be underestimated, especially during storm 

events. 

Measured SSCs and turbidity data collected at the time of 

sampling were used to develop linear regressions at three sites 

(SAR Elmendorf, SAR Goliad, and GR Tivoli) to be used for 

estimating SSCs based on turbidity values (fig. 7). Regressions 

were not developed at the other sites because too few samples 

were collected at those sites. The strong relation between 

SSC and turbidity established for the three sites (R2 0.93 to 

0.96) indicates the estimated SSC, used with a time-series of 

streamflow, could produce more accurate SSLs than using the 

SSC to streamflow regression relation (R2 0.42-0.83).

The regression developed for the SAR Elmendorf site 

could be used to estimate a time series of SSCs from timeseries turbidity data collected at the SAR Elmendorf site 

(fig. 6). The time series of estimated SSCs could subsequently 

be paired with the time series of streamflow at the site to 

produce a time series of estimated SSLs. Although the 

regression could have been applied to the time series of 

turbidity data collected at the SAR Elmendorf site to produce 

a time series of estimated daily SSLs for this study, truncation 

problems within the turbidity data likely would have caused 

the SSLs to be substantially underestimated. Time-series 

turbidity data currently are not collected at the SAR Goliad 

and GR Tivoli sites, so time series of SSLs could not be 

estimated for those sites. Future suspended-sediment samples 

and corresponding turbidity values incorporated into these 

regressions will increase the accuracy of the regressions.

Figure 6. Daily mean turbidity from January 2011 through September 2013 at SAR Elmendorf site (U.S. Geological Survey station 

08181800, San Antonio River near Elmendorf, Texas), in formazin nephelometric units (FNU).

2011 2012 2013

Jan. Apr. July Oct. Jan. Apr. July Oct. Jan. Apr. July Oct.

1

10

100

1,000

Turbidity, in

formazin nephelometric units

SAR Elmendorf
30 Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas

Summary

To better understand sediment characteristics in the San 

Antonio River Basin downstream from San Antonio, the U.S. 

Geological Survey, in cooperation with the San Antonio River 

Authority and the Texas Water Development Board, collected 

and analyzed suspended-sediment, bedload, particle-size 

distribution, and turbidity data over a wide range of hydrologic 

conditions at sampling sites in the San Antonio River Basin 

downstream from San Antonio, Tex., and at one site on the 

Guadalupe River downstream from the San Antonio River 

Basin from January 2011 through May 2013 and combined 

that data with the available historical sediment data collected 

at the same sampling sites. Turbidity data collected in 

conjunction with suspended-sediment samples at a site were 

used to develop a regression that could be applied to timeseries turbidity data collected at the site to develop an estimate 

of time-series suspended-sediment concentrations (SSCs) at 

the site.

During 2011–13, suspended-sediment samples were 

collected at 10 sites for the analysis of SSC and particle 

size distribution. In addition, samples of bedload material 

were collected at six sites for the analysis of bedload mass 

and particle-size distribution. Samples were collected over 

a variety of hydrologic conditions ranging from a minimum 

streamflow of 1.9 cubic feet per second (ft3/s) at the Cibolo 

Selma site (Cibolo Creek at Selma, Tex.) on September 17, 

2012, to a maximum streamflow of 10,600 ft3/s at the same 

site on May 25, 2013.

SSCs in 67 samples collected during 2011–13 ranged 

from 14 milligrams per liter (mg/L) during base-flow 

conditions at the SAR Falls City site (San Antonio River near 

Falls City, Tex.) on August 19, 2011, to 4,480 mg/L during 

a stormflow event at the same site on January 26, 2012. The 

samples collected during 2011–13 represent a wide range 

of SSCs, including some of the largest concentrations ever 

collected at some of the sites. Conversely, the majority of the 

historical samples collected at this site were routine samples 

that produced a relatively narrow range of concentrations. The 

data from the suspended-sediment samples collected during 

2011–13 support historical observations that the system is 

dominated by the transport of fine sediment particles. On 

average, about 94 percent of suspended-sediment particles in 

all of the samples was less than 0.0625 millimeter (mm) (silt 

and clay). Furthermore, the data from 50 samples for which a 

complete sediment-size analysis was performed indicate that 

an average of about 69 percent of the particles was less than 

0.002 mm, the smallest size range for which the samples were 

analyzed.

The mass from 22 bedload samples collected during 

2011–13 ranged from 12 grams (g) at the site furthest 

downstream (GR Tivoli [Guadalupe River near Tivoli, Tex.]) 

during base-flow conditions on April 5, 2011, to 8,900 g at the 

SAR Goliad (San Antonio River near Goliad, Tex.) site during 

a stormflow event on January 28, 2012. The data from the 

particle-size analysis for 10 bedload samples collected during 

Figure 7. Relation between suspended-sediment concentrations 

and turbidity at A, SAR Elemendorf site (U.S. Geological Survey 

[USGS] station 08181800 San Antonio River near Elmendorf); 

B, SAR Goliad site (USGS station 08188500 San Antonio River at 

Goliad); and C, GR Tivoli site (USGS station 08188800 Guadalupe 

River near Tivoli, Texas).

0 200 400 600 800 1,000 1,200

Turbidity, in formazin nephelometric units

Suspended-sediment concentration, in milligrams per liter

0

500

1,000

1,500

2,000

2,500

0

500

1,000

1,500

2,000

2,500

0

500

1,000

1,500

2,000

2,500

y = 1.78x - 9.98

R2 = 0.96

y = 1.76x + 61.97

R2 = 0.97

y = 1.51x + 59.19

R2 = 0.93

A. SAR Elmendorf

B. SAR Goliad

C. GR Tivoli

Note: Short names for sampling sites (table 1) are listed above the upper left

corners of figures 7A–7C. 

EXPLANATION

Relation between suspended-sediment concentration

 and turbidity

Suspended-sediment sample collected

Suspended-sediment concentration

Turbidity

R Coefficient of determination 2

y

x
References 31

2011–13 indicate, on average, about 91 percent of sediment 

transported as bedload in the streams is sand-sized particles or 

smaller (less than 2 mm in size) with the greatest proportion 

(about 51 percent) being sediment in the 0.25–0.5-mm size 

range.

Instantaneous suspended-sediment loads (SSLs) 

computed from 67 SSC samples collected during 2011–12 

ranged from 0.36 tons per day at the Cibolo Selma site during 

base-flow conditions on September 17, 2012, to 47,800 tons 

per day at the SAR Elmendorf site (San Antonio River near 

Elmendorf, Tex.) during a stormflow event on January 25, 

2012. Instantaneous bedload transport computed for the 

22 samples collected during 2011–12 ranged from 0.15 tons 

per day at SAR Elmendorf during base-flow conditions on 

January 27, 2011, to 155 tons per day at site SAR Goliad 

during a stormflow event on January 28, 2012. Instantaneous 

total sediment loads computed for the 21 samples for which 

both SSL and bedload transport were computed ranged from 

13.1 tons per day at the SAR Elmendorf site on January 27, 

2011, to 47,828 tons per day at the same site on January 25, 

2012. 

Daily SSLs were estimated using the SSC to streamflow 

regression equations for each site. The estimated daily SSLs 

for each of the sites indicated that during 2011–12, the 

majority of the SSL in the basin originated upstream from the 

SAR Elmendorf site. The estimated average daily SSL was 

237 tons per day at the SAR Elmendorf site and 454 tons per 

day at the SAR Goliad site during 2011–12. Downstream from 

the SAR Elmendorf site is the SAR Falls City site, where the 

estimated average daily SSL was 332 tons per day. Because 

the Cibolo Selma site often did not flow, the estimated average 

daily SSL at Cibolo Falls City (Cibolo Creek near Falls City, 

Tex.) (57 tons per day) likely originates from within the study 

watershed, except during large stormflow events when there is 

streamflow at the Cibolo Selma site. During 2011–12, Ecleto 

Runge had the lowest SSL of 9 tons per day, which is expected 

because it drains a smaller watershed and streamflow generally 

only occurs during storm events.

Turbidity and SSC data collected at the SAR 

Elmendorf site were used in the development of a regression 

model for computing SSCs from instantaneous turbidity data. 

The regression could be used to estimate a time series of 

SSCs from time-series turbidity data collected at the SAR 

Elmendorf site. The time series of estimated SSCs could 

subsequently be paired with the time series of streamflow at 

the site to produce a time series of estimated SSLs. Although 

the regression could have been applied to the time series of 

turbidity data collected at the SAR Elmendorf site to produce 

a time series of estimated daily SSLs, truncation problems 

within the turbidity data likely would have caused the SSLs 

to be substantially underestimated. Future suspendedsediment samples and corresponding turbidity values 

incorporated into these regressions will increase the accuracy 

of the regressions.

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Crow and others—Sediment Characteristics in the San Antonio River Basin Downstream From San Antonio, Texas—SIR 2014–5048

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

Printed on recycled paper http://dx.doi.org/10.3133/sir20145048

I SBN 978- 1- 4113-3779- 4

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