U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the U.S. Army Corps of Engineers, Fort Worth District; 

City of Corpus Christi; Guadalupe-Blanco River Authority; San Antonio River Authority; and 

San Antonio Water System

Sources of Suspended-Sediment Loads in the Lower Nueces 

River Watershed, Downstream from Lake Corpus Christi to 

the Nueces Estuary, South Texas, 1958–2010

Sabinal River

98°

99°

100°

30°

29°

28°

Physiographic data from

Wermund, E.G., 1996

River

Nueces Bay 

Corpus 

Christi 

Bay 

Upper

Lake

Creek

Oso

Lake 

Corpus 

Christi

Nueces

Frio River

River

Leona

Frio River

Choke Canyon

Reservoir

San Miguel Creek

Atascosa River

Hondo Creek

GULF OF

 MEXICO

Corpus

 Christi Robstown

Alice Bluntzer

Calallen

San Antonio

Uvalde

Mathis

Three

 Rivers S O U T H T E X A S P L A I N S 

E D W A R D S P L A T E A U

CHOKE

CANYON

 DAM

WESLEY E SEALE DAM 

UPPER DAM

TEXAS

Study area

Nueces

River Basin 

WEBB COUNTY

DUVAL COUNTY

FRIO

COUNTY

BEE

COUNTY

EDWARDS COUNTY

KERR COUNTY

UVALDE

COUNTY

DIMMIT COUNTY

KINNEY COUNTY

ZAVALA

COUNTY

MEDINA

COUNTY

REAL

COUNTY

MAVERICK COUNTY

LIVE OAK COUNTY

MC MULLEN

COUNTY

WILSON

COUNTY

KARNES

COUNTY

BANDERA

COUNTY

SAN PATRICIO

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

NUECES COUNTY

KLEBERG COUNTY

LA SALLE

COUNTY

JIM WELLS COUNTY

Scientific Investigations Report 2013–5059
Cover: Aerial photograph of the Nueces River and Nueces Estuary downstream from Interstate Highway 

37, near Corpus Christi, Texas, May 21, 2004 (photograph by Dr. Carlos Fernandez, Texas AgriLife 

Research and Extension Center at Corpus Christi, Texas). 
Sources of Suspended-Sediment Loads 

in the Lower Nueces River Watershed, 

Downstream from Lake Corpus Christi 

to the Nueces Estuary, South Texas, 

1958–2010

By Darwin J. Ockerman, Franklin T. Heitmuller, and Loren L. Wehmeyer

Prepared in cooperation with the U.S. Army Corps of Engineers, Fort Worth 

District; City of Corpus Christi; Guadalupe-Blanco River Authority; San Antonio 

River Authority; and San Antonio Water System

Scientific Investigations Report 2013–5059

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: 2013

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ockerman, D.J., Heitmuller, F.T., and Wehmeyer, L.L., 2013, Sources of suspended-sediment loads in the lower 

Nueces River watershed, downstream from Lake Corpus Christi to the Nueces Estuary, south Texas, 1958–2010: 

U.S. Geological Survey Scientific Investigations Report 2013–5059, 56 p. 
iii

Contents

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

Introduction ....................................................................................................................................................2

Purpose and Scope .............................................................................................................................5

Description of the Lower Nueces River Watershed ......................................................................5

Suspended-Sediment Concentrations and Loads of the Lower Nueces River ..................................5

Review of Historical Suspended-Sediment Data and Studies in the Lower 

Nueces River ...........................................................................................................................5

Recent (2006–7 and 2010) Suspended-Sediment Sampling...........................................................7

Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the 

Lower Nueces River Watershed .................................................................................................10

Functional Description of Hydrological Simulation Program—FORTRAN ...............................10

Previous Watershed Modeling Study and Report ........................................................................12

Input Data for the Lower Nueces River Watershed Model ........................................................15

Model Development ..........................................................................................................................22

Model Calibration ...............................................................................................................................22

Hydrology ...................................................................................................................................22

Suspended Sediment ...............................................................................................................27

Estimation of Suspended-Sediment Loads from Lake Corpus Christi .....................27

Model Calibration ............................................................................................................30

Parameter Calibration Values .................................................................................................34

Sensitivity Analysis ............................................................................................................................34

Model Limitations ...............................................................................................................................36

Estimated Suspended-Sediment Loads, by Source, to the Nueces Estuary, 1958–2010 .................36

Estimated Suspended-Sediment Yields by Subwatershed for Selected Runoff 

Events during 2009–10 .........................................................................................................41

Summary .......................................................................................................................................................45

References ...................................................................................................................................................46

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) Parameters Used to 

Simulate Hydrologic and Sediment Processes in the Lower Nueces River 

Watershed, South Texas ...................................................................................................51

Figures

1. Map showing location of Nueces River Basin, including lower Nueces River 

study area, south Texas ..............................................................................................................3

2. Map showing location of lower Nueces River watershed, south Texas ............................4

3. Graph showing annual suspended-sediment loads for the Nueces River near 

Mathis, Texas, 1942–57 and 1962–82 ........................................................................................7

4. Diagram showing hydrological Simulation Program—FORTRAN (HSPF) flowchart 

for hydrologic processes on A, impervious and B, pervious land segments ..................13

5. Diagram showing hiagram showing Hydrological Simulation Program—

FORTRAN (HSPF) flowchart for sediment processes on A, impervious land 

segments, B, pervious land segments, and C, stream reaches .........................................14
iv

6. Map showing land-cover categories in the lower Nueces River watershed, 

south Texas .................................................................................................................................16

7. Map showing surficial geology of the lower Nueces River watershed, south 

Texas ............................................................................................................................................17

8. Map showing relative soil infiltration rates in the lower Nueces River watershed, 

south Texas .................................................................................................................................18

9. Map showing subwatershed delineation for the lower Nueces River watershed 

model, south Texas ....................................................................................................................19

10. Map showing locations of U.S. Geological Survey streamflow-gaging stations 

and National Weather Service rainfall stations (with associated Thiessen rainfall 

areas) providing data for the lower Nueces River watershed model, south Texas .......21

11. Graphs showing measured and simulated daily mean streamflow at U.S. 

Geoloical Survey streamflow-gaging station 08211000 Nueces River at Bluntzer, 

Texas, 2005–6 ..............................................................................................................................23

12. Graphs showing measured and simulated 7-day mean streamflow at U.S. 

Geoloical Survey streamflow-gaging station 08211500 Nueces River at Calallen, 

Texas, 2001–10 ............................................................................................................................26

13. Graphs showing relation between streamflow and suspended-sediment load and 

plot of regression error residuals based on 22 streamflow-load data pairs from 

08211000 Nueces River near Mathis, Texas, 1964–71 and 2006–7 .....................................28

14. Graphs showing relation between streamflow and suspended-sediment load 

and plot of regression error residuals based on 10 streamflow-load data pairs 

from 08211500 Nueces River at Calallen, Texas, 2006–7 .....................................................32

15. Graph showing estimated annual streamflow and suspended-sediment loads 

to the Nueces Estuary, south Texas, 1958–2010 ...................................................................37

16. Map showing model-estimated sediment yield by subwatershed for the runoff 

event of November 20–21, 2009, lower Nueces River watershed, south Texas ..............42

17. Map showing model-estimated sediment yield by subwatershed for the runoff 

event of September 7–8, 2010, lower Nueces River watershed, south Texas .................43

18. Map showing model-estimated sediment yield by subwatershed for the runoff 

event of September 20–21, 2010, lower Nueces River watershed, south Texas .............44

Tables

1. Annual suspended-sediment loads for the Nueces River near Mathis, Texas, 

1942–58 and 1961–82 ...................................................................................................................8

2. Streamflow and suspended-sediment loads for selected flow releases from

Wesley E. Seale Dam on the Nueces River near Mathis, Texas, October 1964–

September 1971 ............................................................................................................................9

3. Suspended-sediment sampling results, lower Nueces River watershed, south 

Texas, 2006–7, 2010 ....................................................................................................................11

4. Data-collection sites providing data for the lower Nueces River watershed 

model, south Texas ....................................................................................................................20

5. Streamflow calibration and testing results for the lower Nueces River 

watershed model, south Texas ................................................................................................24

6. Measured and simulated streamflows, measured suspended-sediment particle 

size, and measured and simulated suspended-sediment concentrations and 

loads for selected sampling events at 08211000 Nueces River near Mathis, 

Texas, 2006–7, 2010 ....................................................................................................................29
v

7. Comparison of literature estimates and simulation results for sediment yields 

from selected land-cover and land-use types in the coastal area of south Texas 

and the lower Nueces River watershed ................................................................................30

8. Measured and simulated streamflows, measured suspended-sediment particle 

size, and measured and simulated suspended-sediment concentrations and 

loads for selected suspended-sediment sampling events at 08211200 Nueces 

River at Bluntzer, Texas, 2006–7, 2010 ....................................................................................31

9. Suspended-sediment calibration results for the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, south 

Texas, 2001–8 ..............................................................................................................................33

10. Measured and simulated streamflows, measured suspended-sediment particle 

size, and measured and simulated suspended-sediment concentrations and 

loads, for selected sampling events at 08211500 Nueces River near Calallen, 

Texas, 2006–7, 2010. ...................................................................................................................33

11. Summary of calibrated values for selected hydrologic parameters for the 

Hydrological Simulation Program—FORTRAN model of the lower Nueces River 

watershed, south Texas ............................................................................................................34

12. Summary of calibrated values for selected sediment-related parameters for the 

Hydrological Simulation Program—FORTRAN model of the lower Nueces River 

watershed, south Texas ............................................................................................................35

13. Sensitivity of simulated streamflow volumes and suspended-sediment loads to 

changes in selected process-related parameters for the lower Nueces River 

watershed model, south Texas ................................................................................................35

14. Estimated annual streamflows and suspended-sediment loads to the Nueces 

Estuary, south Texas, 1958–2010 .............................................................................................38

15. Estimated annual suspended-sediment loads at selected stations, lower Nueces 

River watershed, south Texas, 1958–2010 .................................................................................39

16. Estimated annual streamflows and suspended-sediment loads, by sediment 

source, simulated by the Hydrological Simulation Program—FORTRAN model 

of the lower Nueces River watershed, south Texas, 1958–2010 ........................................40
vi

Conversion Factors, Datum, and Water-Quality Abbreviation

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

inch per second 0.0254 meters per second (m/s)

inch per hour 0.0254 meters per hour (m/h)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

acre-foot per years (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

cubic foot per second (ft3

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

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 megagram (Mg) 

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

ton per acre 2,242 kilogram per hectare (kg/ha)

Pressure

pound per square foot (lb/ft2

) 0.04788 kilopascal (kPa) 

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Yield

pound per acre (lb/acre) 1.121 kilogram per hectare kg/ha)

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

°C=(°F-32)/1.8
vii

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

Volume

milliliter (mL) 0.03382 ounce, fluid (fl. oz)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

Density

gram per cubic centimeter (g/cm3

) 62.4220 converted to pound per cubic foot

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

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Sources of Suspended-Sediment Loads in the Lower 

Nueces River Watershed, Downstream from Lake Corpus 

Christi to the Nueces Estuary, South Texas, 1958–2010

By Darwin J. Ockerman, Franklin T. Heitmuller, and Loren L. Wehmeyer

Abstract

The U.S. Geological Survey (USGS), in cooperation 

with the U.S. Army Corps of Engineers, Fort Worth District; 

City of Corpus Christi; Guadalupe-Blanco River Authority; 

San Antonio River Authority; and San Antonio Water System, 

developed, calibrated, and tested a Hydrological Simulation 

Program—FORTRAN (HSPF) watershed model to simulate 

streamflow and suspended-sediment concentrations and loads 

during 1958–2010 in the lower Nueces River watershed, 

downstream from Lake Corpus Christi to the Nueces Estuary 

in south Texas. Data available to simulate suspended-sediment 

concentrations and loads consisted of historical sediment data 

collected during 1942–82 in the study area and suspendedsediment concentration data collected periodically by the 

USGS during 2006–7 and 2010 at three USGS streamflowgaging stations (08211000 Nueces River near Mathis, Tex. 

[the Mathis gage], 08211200 Nueces River at Bluntzer, Tex. 

[the Bluntzer gage], and 08211500 Nueces River at Calallen, 

Tex. [the Calallen gage]), and at one ungaged location on a 

Nueces River tributary (USGS station 08211050 Bayou Creek 

at Farm Road 666 near Mathis, Tex.). The Mathis gage is 

downstream from Wesley E. Seale Dam, which was completed 

in 1958 to impound Lake Corpus Christi. Suspended-sediment 

data collected before and after completion of Wesley E. Seale 

Dam provide insights to the effects of the dam and reservoir 

on suspended-sediment loads transported by the lower Nueces 

River downstream from the dam to the Nueces Estuary. 

Annual suspended-sediment loads at the Nueces River near the 

Mathis, Tex., gage were considerably lower for a given annual 

mean discharge after the dam was completed than before the 

dam was completed.

Most of the suspended sediment transported by the 

Nueces River downstream from Wesley E. Seale Dam 

occurred during high-flow releases from the dam or during 

floods. During October 1964–September 1971, about 536,000 

tons of suspended sediment were transported by the Nueces 

River past the Mathis gage. Of this amount, about 473,000 

tons, or about 88 percent, were transported by large runoff 

events (mean streamflow exceeding 1,000 cubic feet per 

second).

To develop the watershed model to simulate suspendedsediment concentrations and loads in the lower Nueces River 

watershed during 1958–2010, streamflow simulations were 

calibrated and tested with available data for 2001–10 from the 

Bluntzer and Calallen gages. Streamflow data for the Nueces 

River obtained from the Mathis gage were used as input to 

the model at the upstream boundary of the model. Simulated 

streamflow volumes for the Bluntzer and Calallen gages 

showed good agreement with measured streamflow volumes. 

For 2001–10, simulated streamflow at the Calallen gage was 

within 3 percent of measured streamflow. 

The HSPF model was calibrated to simulate suspended 

sediment using suspended-sediment data collected at the 

Mathis, Bluntzer, and Calallen gages during 2006–7. Model 

simulated suspended-sediment loads at the Calallen gage 

were within 5 percent of loads that were estimated, by 

regression, from suspended-sediment sample analysis and 

measured streamflow. The calibrated watershed model was 

used to estimate streamflow and suspended-sediment loads 

for 1958–2010, including loads transported to the Nueces 

Estuary. During 1958–2010, on average, an estimated 288 tons 

per day (tons/d) of suspended sediment were delivered to the 

lower Nueces River; an estimated 278 tons/d were delivered 

to the estuary. The annual suspended-sediment load was 

highly variable, depending on the occurrence of runoff events 

and high streamflows. During 1958–2010, the annual total 

sediment loads to the estuary varied from an estimated 3.8 

to 2,490 tons/d. On average, 113 tons/d, or about 39 percent 

of the estimated annual suspended-sediment contribution, 

originated from cropland in the study watershed. Releases 

from Lake Corpus Christi delivered an estimated 94 tons/d 

of suspended sediment or about 33 percent of the 288 tons/d 

estimated to have been delivered to the lower Nueces River. 

Erosion of stream-channel bed and banks accounted for 44 

tons/d or about 15 percent of the estimated total suspendedsediment load. All other land categories, except cropland, 

accounted for an estimated 36 tons/d, or about 12 percent of 

the total. An estimated 10 tons/d of suspended sediment or 

about 3 percent of the suspended-sediment load delivered to 

the lower Nueces River were removed by water withdrawals 

before reaching the Nueces Estuary.
2 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

During 2010, additional suspended-sediment data were 

collected during selected runoff events to provide new data 

for model testing and to help better understand the sources of 

suspended-sediment loads. The model was updated and used 

to estimate and compare sediment yields from each of 64 

subwatersheds comprising the lower Nueces River watershed 

study area for three selected runoff events: November 20–21, 

2009, September 7– 8, 2010, and September 20–21, 2010. 

These three runoff events were characterized by heavy rainfall 

centered near the study area and during which minimal 

streamflow and suspended-sediment load entered the lower 

Nueces River upstream from Wesley E. Seale Dam. During 

all three runoff events, model simulations showed that the 

greatest sediment yields originated from the subwatersheds, 

which were largely cropland. In particular, the Bayou Creek 

subwatersheds were major contributors of suspendedsediment load to the lower Nueces River during the selected 

runoff events. During the November 2009 runoff event, high 

suspended-sediment concentrations in the Nueces River water 

withdrawn for the City of Corpus Christi public-water supply 

caused problems during the water-treatment process, resulting 

in failure to meet State water-treatment standards for turbidity 

in drinking water. Model simulations of the November 2009 

runoff event showed that the Bayou Creek subwatersheds were 

the primary source of suspended-sediment loads during that 

runoff event.

Introduction

The Nueces River extends approximately 315 miles 

(mi) from its headwaters in the southern Edwards Plateau in 

south Texas to Nueces Bay near Corpus Christi, Tex., and has 

a drainage area of approximately 16,700 square miles (mi2) 

(fig. 1). The river exits the Edwards Plateau near Uvalde, Tex., 

and enters the South Texas Plains, also referred to as the South 

Texas Brush Country (Texas Parks and Wildlife Department, 

2013), where the majority of its length and drainage area 

are located. The Frio River, a major tributary, joins the 

Nueces River near Three Rivers, Tex. Major impoundments 

in the Nueces River watershed in the South Texas Plains 

include Upper Lake (also known as Upper Nueces Reservoir), 

formed in 1948 by Upper Dam; Choke Canyon Reservoir, 

formed in 1982 by Choke Canyon Dam (on the Frio River); 

and Lake Corpus Christi, impounded by Mathis Dam in 

1935 (surface area 5,493 acres, conservation storage volume 

43,800 acre-feet [acre-ft]) (Texas Water Development Board, 

2002) and impounded since 1958 by Wesley E. Seale Dam 

(surface area 19,251 acres, conservation storage volume 

257,260 acre-ft) (City of Corpus Christi, 2010). Downstream 

from Wesley E. Seale Dam (fig. 2), the Nueces River flows 

about 50 mi through an alluvial valley to the Nueces Bay. 

The Nueces Estuary (fig. 2) consists of two areas of 

nearly equal size, Nueces Bay and the Nueces River Delta. 

Nueces Bay is a shallow, 27 mi2

 secondary bay of Corpus 

Christi Bay. Nueces Bay has a mean depth of about 2.5 feet 

(ft) and a volume of about 40,000 acre-ft. Bottom sediments 

in Nueces Bay are deposited mostly by the Nueces River 

(Yeager and others, 2006). The Nueces River Delta in southern 

San Patricio County is a 28-mi2 area of vegetated salt and 

brackish marshes, land subject to inundation from river or 

tidal flooding, and open water formed where the Nueces River 

flows into Nueces Bay (fig. 2). Currently (2013), the Nueces 

River channel is located along the southern margins of the 

delta. The distributary network includes a manmade overflow 

channel in the northwestern part of the delta that connects the 

Nueces River to Rincon Bayou (U.S. Bureau of Reclamation, 

2000; Ockerman, 2001).

In November 2005, during a resource agency meeting 

in San Antonio, Tex., the U.S. Army Corps of Engineers 

(USACE) highlighted 12 ecological problems in the Nueces 

River watershed. One of these was a “loss of sediment loading 

and nutrient loads to estuaries” (Marcia Hackett, U.S. Army 

Corps of Engineers, Fort Worth District, written commun., 

2005). A number of the other ecological problems were 

directly related, including a reduction of overbank flows 

downstream from reservoirs and a decrease in freshwater 

inflows to the Nueces Estuary. The reduction in sediment 

loads to the Nueces Estuary is the result of sedimentation in 

large impoundments, notably Lake Corpus Christi (Leibbrand, 

1987). Downstream from the reservoirs, ecological problems 

caused by sedimentation impoundment are expected to 

include river channel incision (Williams and Wolman, 1984; 

Salant and others, 2006), channel-bed armoring (Williams 

and Wolman, 1984; Vericat and others, 2006), and deltaic 

and shoreline erosion (Jaffe and others, 1998; Fan and others, 

2006; Yang and others, 2006). Reductions in the extent of 

marshland and vegetated areas in the Nueces River Delta 

occurred following the initial impoundment of Lake Corpus 

Christi (Morton and Paine 1984; White and Calnan, 1991). 

The decreased sediment loads of the Nueces River, combined 

with relative sea-level rise and subsidence, are responsible for 

deltaic erosion and the conversion of wetland habitat to open 

water and shallow flats (Day and others, 1995; White and 

others, 2002; Yeager and others, 2006).

The USACE, Fort Worth District, began a study in 

2002 to identify opportunities for flood-damage reduction, 

ecosystem restoration, and implementation of multipurpose 

projects in the Nueces River Basin (U.S. Army Corps of 

Engineers, 2009). The purpose of the USACE study was to 

participate with other (Federal and non-Federal) sponsor 

agencies to identify and conduct detailed studies of waterresource problems in the Nueces River Basin (fig. 1 ), 

including documenting existing hydrologic, engineering, 

and environmental conditions of the study area. One of the 

specific feasibility investigations outlined in the USACE study 

related to defining the existing conditions and opportunities 

for ecosystem restoration in the Nueces Estuary, namely, an 

investigation of the current conditions of suspended-sediment 

concentrations and loads delivered by the Nueces River to the 

estuary. As part of this feasibility study, the U.S. Geological 
Figure 1. Location of Nueces River Basin, including lower Nueces River study area, south Texas.

Sabinal River

98°

99°

100°

30°

29°

28°

Base modified from U.S. Geological Survey digital data, 1:24,000Texas Centric Mapping System, Albers Equal Area Projection

North American Datum of 1983 

0 15 30 60 MILES 

0 15 30 60 KILOMETERS 

45

45

River

Upper

Lake

Lake 

Corpus 

Christi

Nueces

Frio River

River

Leona

Frio River

Choke Canyon

Reservoir

San Miguel Creek

Atascosa River

Hondo Creek

R

Alice Bluntzer

San Antonio

Uvalde

M

Three Rivers

S O U T H T E X A S P L A I N S 

E D W A R D S P L A T E A U

EXPLANATION

Study area—Lower Nueces River watershedNueces River BasinMarshLand subject to inundationPhysiographic boundary

CHOKE

CANYON

 DAM

WESLEY E SEALE DAM 

UPPER DAM

Study 

WEBB COUNTY

DUVAL COUNTY

FRIO

COUNTY

B

CO

EDWARDS COUNTY

KERR COUNTY

UVALDE

COUNTY

DIMMIT COUNTY

KINNEY COUNTY

ZAVALA

COUNTY

MEDINA

COUNTY

REAL

COUNTY

MAVERICK COUNTY

LIVE OAK COUNTY

MC MULLEN

COUNTY

WILSON

COUNTY

KARNES

COUNTY

BANDERA

COUNTY

BEXAR COUNTY ATASCOSA COUNTY

NUE

KLEB

LA SALLE

COUNTY

JIM WELLS COUNTY
4 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Survey (USGS), in cooperation with the USACE Fort 

Worth District, City of Corpus Christi, Guadalupe-Blanco 

River Authority, San Antonio River Authority, and San 

Antonio Water System, developed, calibrated, and tested 

a watershed model of the lower Nueces River watershed 

to simulate existing hydrologic conditions and suspendedsediment concentrations and loads to the Nueces Estuary. The 

USGS study simulated streamflow and suspended-sediment 

concentrations and loads during 1958–2008 in the lower 

Nueces River watershed, downstream from Lake Corpus 

Christi to the Nueces Estuary in south Texas. In 2010, a USGS 

Scientific Investigations Report (Ockerman and Heitmuller, 

2010) was published documenting data collection, model 

calibration, and model simulation results of the study.

In November 2009, the City of Corpus Christi failed to 

meet State water-treatment standards for turbidity in drinking 

Figure 2. Location of lower Nueces River watershed, south Texas. 

Corpus Christi 

International Airport

 Nueces County 

Airport

LIVE OAK COUNTY

BEE COUNTY

 SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

 REFUGIO COUNTY

37

£¤181

£¤77

£¤281

359

44

£¤77

188

234

359

37

358

357

286

Nueces 

Bay

Oso

Creek

Nueces 

Hondo

Creek

River

Rincon 

Bayou 

Nueces

 Estuary

Bayou Creek

Nueces

 River Delta

 Lake

 Corpus

Christi

WESLEY E 

SEALE DAM

08211000 08211050

08211500

08211200

08211502

Robstown

Corpus Christi Alice

Bluntzer

Calallen

Mathis

Odem

98°00' 97°45' 97°30'

28°15'

28°

27°45'

Base modified from U.S. Geological Survey 1:24,000-scale 

digital data Texas Centric Mapping System, Albers Equal 

Area Projection, North American Datum of 1983

0 2 4 8 MILES

0 4 8 KILOMETERS 2

6

6

EXPLANATION

Study area—Lower Nueces River watershed

Marsh 

Land subject to inundation

U.S. Geological Survey streamflow-gaging 

 station—Table 4 08211502
Suspended-Sediment Concentrations and Loads of the Lower Nueces River 5

water because of high concentrations of suspended sediment 

in raw water withdrawn from the Nueces River near Calallen 

(Corpus Christi Caller Times, 2009). At the time, the Nueces 

River was experiencing runoff caused by heavy rains. During 

this event, releases from Lake Corpus Christi were minimal 

and runoff originated almost entirely from the subwatersheds 

downstream from Lake Corpus Christi and the Wesley E. 

Seale Dam. Because of this problem, the City of Corpus 

Christi expressed interest in gaining a better understanding 

of sources (locations, land uses, land-management practices) 

of sediment and possible methods for reducing Nueces River 

suspended-sediment concentrations during periods of runoff. 

Accordingly, the USGS, in cooperation with the USACE; City 

of Corpus Christi; Guadalupe-Blanco River Authority; San 

Antonio River Authority; and the San Antonio Water System, 

completed a study using an updated version of a previously 

developed watershed model (Ockerman and Heitmuller, 

2010) to characterize sources of suspended sediment during a 

November 2009 runoff event and during selected runoff events 

in 2010 and characterized sediment yields from selected 

subwatersheds in the lower Nueces River watershed.

Purpose and Scope

The purpose of this report is to estimate suspendedsediment concentrations and loads (mass of suspended 

sediment transported by streamflow over a period of time) 

in the lower Nueces River watershed downstream from 

Lake Corpus Christi to the Nueces Estuary during 1958–

2010, characterize the sources of those loads, and estimate 

suspended-sediment yields by subwatershed for selected 

runoff events during 2009–10. To accomplish this, previous 

suspended-sediment data and studies for the study area were 

reviewed, and the simulation period (1958–2008) used by 

Ockerman and Heitmuller (2010) to develop a model of 

the Nueces watershed was extended to simulate the period 

1958–2010 by using additional data collected during 2010. For 

selected runoff events of 2009–10, the model also was used to 

estimate sediment yields, by subwatershed, to help understand 

areas of the watershed that contribute high sediment loads 

during runoff events. Limitations of model-simulated 

estimates of sediment loads are described. The wording and 

presentation of material in this report are based on a previous 

USGS report (Ockerman and Heitmuller, 2010); the contents 

of each section are modified from this previous report. 

Description of the Lower Nueces River 

Watershed

The lower Nueces River study area comprises about 

216 mi2 of the nontidal part of the Nueces River watershed, 

from the outlet of Wesley E. Seale Dam near Mathis (fig. 2) 

to the tidal reach of the river that flows into the Nueces 

Estuary. The study area encompasses parts of Bee, Jim Wells, 

Live Oak, Nueces, and San Patricio Counties in south Texas. 

The largest town in the study area is Mathis, Tex., which had a 

population of 4,942 in 2010 (U.S. Census Bureau, 2013).

The study area has a subtropical, subhumid climate 

characterized by hot summers and mild winters (Larkin and 

Bomar, 1983). The average monthly low temperatures range 

from 46.2 degrees Fahrenheit (oF) in January to 74.5oF in 

August; average daily high temperatures range from 66.0oF 

in January to 93.4oF in August (National Climatic Data 

Center, 2012). The average annual rainfall (1971–2000) in 

the study area is about 32.6 inches (32.2 inches at Mathis 

National Weather Service [NWS] station, cooperative observer 

identification [COOP ID] 415661 and 32.9 inches at Robstown 

NWS station, COOP ID 417677). Daily rainfall greater than 

0.01 inch occurs, on average, 70 days per year (National 

Climatic Data Center, 2012); daily rainfall equal to or 

greater than 1.0 inch occurs, on average, every 39 days 

(Asquith and Roussel, 2003). Although most rainfall occurs 

in spring, early summer, and fall, amounts greater than about 

1.0 inch can occur anytime during the year (Larkin and 

Bomar, 1983). 

Most land use is rangeland and cropland. Elevation in the 

lower Nueces River watershed ranges from about 3 to 230 ft 

above National Geodetic Vertical Datum of 1929 (NGVD 29). 

(U.S. Geological Survey, 2013b). Land slopes are generally 

low, mostly less than 5 percent. Overall, the stream-channel 

slope of the lower Nueces River is about 1 foot per mile 

(ft/mi) over the 38 mi from the outlet of Wesley E. Seale Dam 

to the river crossing at Interstate Highway 37, north of Corpus 

Christi (fig. 2).

Suspended-Sediment Concentrations 

and Loads of the Lower Nueces River

Characterizing suspended-sediment concentrations 

and loads in the lower Nueces River included the following 

major steps: (1) a review of the available suspendedsediment concentration and load data and studies, including 

a compilation of historical estimates of suspended-sediment 

loads; (2) collection of suspended-sediment data during 2010 

(to supplement the 2006–7 data collection in the earlier phase 

of the study); (3) development and calibration of a watershed 

model to simulate suspended-sediment concentrations and 

loads in the lower Nueces River watershed; (4) estimation 

of annual suspended-sediment loads to the Nueces Estuary 

for 1958–2010, including a characterization of sources of 

suspended-sediment loads to the Nueces River during selected 

runoff events of 2009–10.

Review of Historical Suspended-Sediment Data 

and Studies in the Lower Nueces River

Various studies have investigated the problems associated 

with the reduction in sediment loads to the Nueces Estuary 
6 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

and the associated loss of wetland habitats in the Nueces River 

Delta. Morton and Paine (1984) used aerial photographs to 

show that the propagation of deltaic wetlands in the Nueces 

River Delta ceased sometime between 1930 and 1959. These 

findings were substantiated by White and Calnan (1990), who 

also showed an additional decrease in net vegetated areas 

by 1979. As further support to the importance of sediment 

transport in maintaining the Nueces River Delta, Leibbrand 

(1987) showed that almost all sediment entering Lake Corpus 

Christi between 1972 and 1985 was trapped. Recent studies 

showed that loss of wetlands in the Nueces Estuary not 

only was the result of upstream impoundments but also was 

influenced by gradual subsidence of the delta (White and 

others, 2002). As a consequence of these two factors, White 

and others (2002) also predicted that wetland areas would 

continue to decrease in the Nueces River Delta. Finally, 

Yeager and others (2006) showed that sediment in Nueces Bay 

is supplied mostly by the river rather than by marine sources 

(transport from the Gulf of Mexico by wind or tidal currents), 

supporting claims that a reduction of suspended-sediment 

inflow from the Nueces River will contribute to loss of Nueces 

Estuary wetland habitat.

In 1924, the Texas Board of Water Engineers (whose 

name and functions were subsequently changed several times 

by the Texas Legislature and whose functions, unrelated to 

water rights, were transferred to the Texas Water Development 

Board [TWDB] in 1965) initiated a program to evaluate 

the economic life of reservoirs, which included monitoring 

suspended-sediment loads in selected Texas rivers (Stout 

and others, 1961; Mirabal, 1974). During the program, the 

USGS (in 1942) established one site in the lower Nueces 

River study area (fig. 2), USGS streamflow-gaging station 

08211000 Nueces River near Mathis, Tex. (hereinafter the 

Mathis gage). Suspended-sediment concentration samples 

were collected on a daily basis at this site by TWDB and 

its predecessor agencies; monthly and annual suspendedsediment concentration data and sediment loads (using 

streamflow data from the Mathis gage) were computed until 

1982 and published by various authors, including Stout and 

others (1961), Cook (1967, 1970), Mirabal (1974), Dougherty 

(1979), and Quincy (1988). 

Records for June 1958–June 1961 were invalid (Adey 

and Cook, 1964) and were not used for interpretation in 

this report. Suspended-sediment samples were collected 

during 1942–82 using an 8-ounce (236.3-milliliter) narrowneck bottle, which was held in a 10-pound (4.54-kilogram) 

torpedo-shaped sampler frame positioned approximately 1 ft 

(0.3 meter) below the water surface. Samples were collected 

daily or throughout the day if the stage changed considerably. 

Samples were collected at one-sixth, one-half, and five-sixths 

of the water-surface width using a sampling device called the 

“Texas sampler” (Stout and others, 1961; Welborn, 1967). To 

account for increasing suspended-sediment concentrations 

with depth, the measured percentage of suspended sediment 

by weight was multiplied by a correction factor of 1.102 to 

obtain the mean percentage of suspended sediment in the 

vertical profile (Farris, 1933). Suspended-sediment loads were 

computed with the assumptions that 1 acre-ft of streamflow 

weighed 1,361.25 tons and 1 acre-ft of sediment weighed 

1,524.60 tons (Stout and others, 1961). Since 1982, most 

sediment data have been collected as part of relatively shortterm studies for selected river reaches (for example, Phillips 

and others, 2005; Yeager and others, 2005). 

To verify the accuracy of the suspended-sediment 

sampling method described in the previous paragraph, referred 

to hereinafter as the “Texas sampler method,” Welborn (1967) 

compared the results of samples collected from various 

Texas streams by the Texas sampler method with results of 

concurrent samples collected by a depth-integrated method 

(Edwards and Glysson, 1999). Correlations between the two 

methods for sand-bed rivers in east Texas were poor, but were 

very good (differing by 15 percent or less) for mixed-bed 

and gravel-bed rivers. The lower Nueces River is a mixedsand and gravel-bed channel; therefore, suspended-sediment 

data collected using the Texas sampler method were deemed 

reasonably accurate for computation of historical loads.

Suspended-sediment data collected before and after 

completion of Wesley E. Seale Dam provide insights to the 

effects of the dam and reservoir on suspended-sediment loads 

transported by the lower Nueces River downstream from the 

dam to the Nueces Bay. Suspended-sediment loads (fig. 3; 

table 1) measured by TWDB and its predecessor agencies 

at the Mathis gage decreased appreciably in the years after 

completion of Wesley E. Seale Dam in 1958. During 9 of the 

16 years before completion of the dam, annual suspendedsediment loads exceeded 100,000 tons. The maximum annual 

suspended-sediment load for the period of record was 561,739 

tons in 1957; an amount that likely was larger compared with 

loads for all previous years because of the erosion of sediment 

during construction of Wesley E. Seale Dam immediately 

upstream from the suspended-sediment sampling site. In the 

21 years following completion of the dam, for which reliable 

suspended-sediment data were available (1962–82), more than 

100,000 tons of suspended sediment were transported in only 

2 years—1967 and 1971. These 2 years were characterized 

by relatively high annual mean streamflows at the Mathis 

gage—2,167 and 2,140 cubic feet per second (ft3

/s) in 1967 

and 1971, respectively. During 1962–82, annual suspendedsediment loads were considerably lower, for a given annual 

mean discharge, than in the years before the dam was 

completed (fig. 3; table 1). The relation between annual 

suspended-sediment loads and annual mean streamflow before 

and after completion of Wesley E. Seale Dam is shown on 

figure 3. As an indication of the sediment-retention capacity 

of the dam for comparable annual mean discharges, annual 

suspended-sediment loads after completion of the dam have 

been consistently lower compared with annual suspendedsediment loads before completion of the dam (fig. 3).

Most of the suspended sediment transported by the 

Nueces River downstream from Wesley E. Seale Dam 

occurred during high-flow releases from the dam or during 

floods. During October 1964–September 1971, about 
Suspended-Sediment Concentrations and Loads of the Lower Nueces River 7

536,000 tons of suspended sediment were transported by the 

Nueces River near Mathis (table 1). Of this amount, about 

473,000 tons (table 2), or about 88 percent, were transported 

by large runoff events (mean streamflow exceeding 1,000 

ft3

/s). Furthermore, the suspended-sediment transport rate 

increases with higher magnitude flows (fig. 3). The largest 

flow event listed in table 2 (September 21–October 11, 1967) 

transported 186,302 tons of suspended sediment (an average 

of 8,872 tons per day [tons/d] of suspended sediment).

Recent (2006–7 and 2010) Suspended-Sediment 

Sampling

In addition to historical suspended-sediment data 

collected daily by the TWDB and its predecessor agencies 

at the Mathis gage during 1942–82, the USGS periodically 

collected suspended-sediment samples during 2006–7 as part 

of the earlier study (Ockerman and Heitmuller, 2010) at the 

Mathis gage, station 08211200 Nueces River at Bluntzer, Tex. 

(hereinafter the Bluntzer gage), and station 08211500 Nueces 

River at Calallen, Tex. (hereinafter the Calallen gage) (fig. 2). 

Whereas the historical suspended-sediment data-collection 

program typically involved daily sample collection to compute 

long-term sediment loads, 2006–7 suspended-sediment 

samples represent data only for selected streamflow and 

sediment-load conditions. Eight suspended-sediment samples 

were collected at the Mathis gage, 11 at the Bluntzer gage, and 

10 at the Calallen gage. 

The USGS collected and analyzed suspended-sediment 

samples during two runoff events in September 2010 at 

the three streamflow-gaging stations on the Nueces River. 

One suspended-sediment sample was collected at both the 

Mathis and Calallen gages, and three samples were collected 

at the Bluntzer gage. Two samples also were collected at a 

miscellaneous discharge and sediment sampling site on a 

Nueces River tributary—USGS station 08211050 Bayou 

Creek at Farm Road 666 near Mathis, Tex. (hereinafter the 

Bayou Creek site).

Figure 3. Annual suspended-sediment loads for the Nueces River near Mathis, Texas, 1942–57 and 1962–82.

0 500 1,000 1,500 2,000 2,500 3,000

Annual mean streamflow, in cubic feet per second

0

100,000

200,000

300,000

400,000

500,000

Annual suspended-sediment load, in tons

600,000

1942–57

1962–82

EXPLANATION
8 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Table 1. Annual suspended-sediment loads for the Nueces River near Mathis, Texas, 1942–58 and 1961–82 (from Stout and others, 

1961; Adey and Cook, 1964; Cook, 1967, 1970; Mirabal, 1974).

Hydrologic 

year1,2

Number of days measured 

during hydrologic year

Annual suspendedsediment load 

(tons)

Annual mean suspendedsediment load 

(tons per day)

Annual mean streamflow

(cubic feet per second)

1942 241 546,504 2,268 2,517

1943 365 44,790 123 345

1944 366 323,550 884 1,020

1945 365 125,070 343 378

1946 365 350,430 960 1,294

1947 365 244,730 670 1,273

1948 366 15,170 41 148

1949 365 212,770 583 1,226

1950 365 29,160 80 340

1951 365 106,740 292 583

1952 366 25,670 70 244

1953 365 159,200 436 741

1954 365 67,020 184 465

1955 365 7,269 20 135

1956 366 12,165 33 184

1957 365 561,739 1,539 1,962

1958 243 395,791 1,629 2,179

1961 92 2,088 23 373

1962 365 2,845 8 111

1963 365 2,769 8 110

1964 366 3,445 9 104

1965 365 33,642 92 787

1966 365 23,400 64 452

1967 365 177,600 48 2,167

1968 366 73,640 201 1,232

1969 365 4,810 13 136

1970 365 70,290 193 718

1971 365 149,100 408 2,140

1972 366 67,530 185 1,780

1973 365 14,030 38 714

1974 365 31,370 86 1,244

1975 365 11,450 31 552

1976 366 12,460 34 550

1977 365 34,830 95 1,455

1978 365 4,280 12 307

1979 365 8,210 22 499

1980 366 26,560 73 771

1981 365 35,190 96 1,268

1982 365 9,790 27 479

1

A hydrologic year begins October 1 of previous calendar year and ends September 30 of reported year.

2

Data for June 1958–June 1961 are invalid (Adey and Cook, 1964) and are not included.
Suspended-Sediment Concentrations and Loads of the Lower Nueces River 9

The samples collected during 2010 provided additional 

suspended-sediment concentration and load information to 

compare with results of model simulations that were extended 

beyond the original calibration period of the model (2001–8) 

(Ockerman and Heitmuller, 2010). The inclusion of samples 

from the Bayou Creek site provided data for a relatively large 

part of the study area that is primarily cropland.

Using standard USGS protocols and procedures (U.S. 

Geological Survey, 2006), an isokinetic sampler was used to 

collect suspended-sediment samples in 2006–7, and 2010. 

Davis (2005, p. 1) noted that “an isokinetic sampler collects a 

water-sediment sample from the stream at a rate such that the 

velocity in the intake nozzle is equal to the incident stream 

velocity at the nozzle entrance. The water-sediment sample 

collected is therefore representative of the sediment load at 

that point (in the stream).” 

Samples were collected at 5 or more (usually 10) equally 

spaced intervals across the stream channel and were depth 

integrated by lowering and raising the sampler through the 

water column at a constant rate. The samples from each 

equal-width segment were then combined into a single 

composite sample for analysis. A composite water-sediment 

sample is horizontally and vertically averaged throughout the 

stream cross section and is assumed to represent the average 

streamflow-weighted suspended-sediment concentration 

(Edwards and Glysson, 1999; U.S. Geological Survey, 2006). 

Suspended-sediment samples were collected by wading 

when streamflow conditions permitted. During high-flow 

conditions, when wading was not possible, samples were 

collected from a bridge or by boat. Suspended-sediment 

samples collected by the USGS were analyzed by the USGS 

sediment laboratory in Iowa City, Iowa. Samples were 

analyzed for suspended-sediment concentration and sandseparation analysis (Guy, 1969). Sand-separation analysis 

provides the percentage of sediment, by weight, that is either 

smaller or larger than 0.0625 millimeter. Particle sizes smaller 

than 0.0625 millimeter are defined as silt and clay; particle 

sizes 0.0625 millimeter or larger are defined as sand (Guy, 

1969). 

Each suspended-sediment sample collected by the USGS 

was associated with a streamflow value. Streamflow data were 

usually obtained from the stage-discharge rating curve at the 

streamflow-gaging station where suspended-sediment samples 

were collected (Rantz and others, 1982). For some high-flow 

conditions, streamflow measurements were made when the 

suspended-sediment samples were collected. The availability 

of streamflow data and suspended-sediment concentration data 

allowed computation of suspended-sediment discharge, or 

load, according to equation 1:

Ls = Q x Cs x ks (1)

where

Ls is the instantaneous suspended-sediment load, 

in tons per day;

Q is the streamflow, in cubic feet per second;

Cs is the suspended-sediment concentration, in 

milligrams per liter; and

ks is a conversion factor of 0.0027, which results 

in a sediment load in tons per day, given 

streamflow in cubic feet per second and 

suspended-sediment concentration in 

milligrams per liter (Porterfield, 1972, 

p. 46–47).

Quality control (QC) samples, designed to ensure the 

integrity of the suspended-sediment data analyzed in this 

report, represented 10 percent of the field samples collected. 

QC samples consisted of replicate samples that were collected 

to evaluate bias and variability of data that might have resulted 

from sample collection, processing, and laboratory analysis. 

Sequential replicate samples were collected immediately after 

the primary samples were collected, at the same location and 

according to the same protocols (U.S. Geological Survey, 

2006, section 4.3.2.B).

The results of the suspended-sediment sampling 

and analyses (measured concentrations and particle-size 

Table 2. Streamflow and suspended-sediment loads for selected 

flow releases from Wesley E. Seale Dam on the Nueces River near 

Mathis, Texas, October 1964–September 1971 (from Cook, 1970; 

Mirabal, 1974).

Event

Mean 

streamflow 

(cubic 

feet per 

second)

Suspendedsediment 

load 

(tons)

Mean 

suspendedsediment 

load 

(tons per 

day)

October 5–16, 1964 7,425 7,506 682

February 24–March 11, 1965 2,727 3,939 246

May 20–June 11, 1965 4,465 15,541 676

May 2–June 10, 1966 2,980 18,653 466

September 21–October 11, 1967 35,129 186,302 8,872

October 16–30, 1967 1,545 4,318 288

January 21–February 11, 1968 5,492 14,802 673

May 8–June 5, 1968 4,970 15,806 545

July 12–16, 1968 2,544 4,909 982

October 19–November 19, 1969 1,968 5,438 170

May 25–June 14, 1970 5,543 50,447 2,402

July 8–18, 1971 9,894 17,480 1,589

August 8–September 6, 1971 11,006 74,537 2,485

September 10–30, 1971 14,432 53,578 2,551
10 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

distributions) are shown in table 3. A total of 40 samples, 

including 4 QC samples were collected from the sampling 

locations. Samples were collected for a relatively wide range 

of streamflow conditions (low flows, high flows, releases from 

Wesley E. Seale Dam, and runoff events) at all of the sampling 

locations. The resulting measured suspended-sediment 

concentrations and loads also exhibited a relatively wide range 

of values, thus providing a varied range of streamflow and 

sediment data for model calibration. 

By calculating relative percent differences, it was 

determined that sample concentrations of two replicate 

samples collected at the Bluntzer gage (May 17, 2006, 

and July 20, 2006, samples) were within 10 percent of the 

environmental sample concentrations. Sample concentrations 

of two samples collected at the Mathis gage varied from 

the concentrations of the respective environmental samples 

by 19 percent (May 17, 2006, samples) and 35 percent 

(July 20, 2006, samples) (table 3). An explanation for the 

larger difference in concentrations between replicate and 

primary samples at the Mathis gage, compared with the 

replicate results at the Bluntzer gage, is suspended-sediment 

concentrations were lower at the Mathis gage, and relatively 

small difference in concentrations can result in relatively 

large percentage differences. Also, large sampling errors are 

often observed in routine replicate samples for suspended 

sediment that indicate significant temporal variability for 

sediment can exist over relatively short timeframes (Kelly, and 

others, 2001).

Simulation of Streamflow and 

Suspended-Sediment Concentrations 

and Loads of the Lower Nueces River 

Watershed 

Historical suspended-sediment loads during 1942–82 

are available for the Nueces River near Mathis (at the 

Mathis gage), and suspended-sediment concentrations and 

loads downstream from the Mathis gage were modeled by 

Ockerman and Heitmuller (2010). To better understand 

suspended-sediment conditions in the lower Nueces River, 

estimates of the amount of suspended sediment transported to 

the Nueces Estuary were prepared using an updated version of 

the watershed model developed by Ockerman and Heitmuller 

(2010) to simulate streamflow and suspended-sediment loads 

in the lower Nueces River.

Streamflow and suspended-sediment concentrations 

and loads were simulated with the Hydrological Simulation 

Program—FORTRAN (HSPF) (Bicknell and others, 

2001). HSPF was selected for the study watershed because 

it is one of the more comprehensive watershed models 

available, can simulate a variety of stream and watershed 

conditions with reasonable accuracy, and enables flexibility 

in adjusting the model to simulate alternative conditions 

or scenarios (Donigian and others, 1995). To simulate the 

hydrologic and sediment processes that occur in a watershed, 

different data sources are used as input to the HSPF model 

including rainfall data, potential evapotranspiration, and 

other meteorological parameters; land cover and land use; 

and soil characteristics. The outputs of an HSPF model are 

simulated time series of suspended concentrations, sediment 

loads, or both, as well as streamflow; the time series are 

for a user-specified interval, or time step. A 1-hour time 

step was used for this study. HSPF also can simulate other 

water-quality constituents, including nutrients, metals, and 

organic compounds. Simulations for this study were limited to 

streamflow and suspended sediment.

Continuous (hourly) models enable simulation of 

important watershed processes for a full range of streamflows. 

Ockerman and Roussel (2009, p. 4) noted other investigators’ 

findings that “the relative importance of various processes 

and factors influencing water quality can vary considerably 

with the magnitude of streamflow. For example, processes 

that appreciably affect water-quality conditions during low 

flows might have relatively minor effects on water-quality 

conditions during high flows. For assessment of peakstreamflow characteristics, continuous simulation models can 

provide a more realistic evaluation of antecedent soil-moisture 

conditions than is generally possible with event-based models 

(Martin and others, 2001, p. 66).”

Functional Description of Hydrological 

Simulation Program—FORTRAN

HSPF, a continuous, semi-lumped parameter model 

(Singh, 1995), provides continuous water and mass balance 

by tracking rainfall and water-quality constituents through 

the conceptual pathways of the hydrologic cycle. In HSPF, 

a watershed is represented by a group of hydrologically 

similar areas referred to as hydrologic response units (HRUs) 

that drain to a stream segment, lake, or reservoir referred 

to as a ReaCH REServoir (RCHRES). HRUs are areas in a 

subwatershed that have similar hydrologic and water-quality 

characteristics that are determined on the basis of land use, 

surficial geology, soil characteristics, and other factors that 

are deemed to produce similar hydrologic responses 

to rainfall and potential evapotranspiration. HRUs are 

categorized as pervious or impervious land segments, 

termed PERvious LaND (PERLND) or IMPervious LaND 

(IMPLND), respectively. A PERLND is represented 

conceptually within HSPF by three interconnected waterstorage zones—an upper zone, a lower zone, and a 

groundwater zone. An IMPLND is represented by surface 

storage, evaporation, and runoff processes. Each RCHRES 

is associated with a particular subwatershed and receives the 

runoff, sediment, and chemical loads from the PERLNDs and 

IMPLNDs in the subwatershed. The hydraulics of a RCHRES 

are simulated by a storage routing method (Donigian and 

others, 1995). 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 11

Table 3. Suspended-sediment sampling results, lower Nueces River watershed, south Texas, 2006–7, 2010.

[ft3/s, cubic feet per second; mm, millimeters; mg/L, milligrams per liter; mm, millimeters; tons/day, tons per day]

Station 08211000 Nueces River near Mathis, Tex.

Sample date

Measured 

streamflow

(ft3

/s)

Measured sediment 

particle diameter 

less than 0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment load 

(tons/day)

Suspended-sediment 

concentration 

relative percent 

difference2

May 17, 2006 (environmental sample) 139 98 24 8.9 --

May 17, 2006 (replicate sample) 139 90 29 11 19

July 20, 2006 (environmental sample) 110 88 26 8.3 --

July 20, 2006 (replicate sample) 110 82 37 11 35

Sept. 5, 2006 138 99 24 8.7 --

Nov. 20, 2006 72 94 22 4.3 --

May 26, 2007 461 99 16 21 --

May 29, 2007 1,190 98 12 39 --

July 1, 2007 4,830 98 16 215 --

July 11, 2007 17,800 86 44 2,200 --

Sept. 7, 2010 115 64 38 12 --

Station 08211050 Bayou Creek at Farm Road 666 near Mathis, Tex.

Sample date

Measured 

streamflow

(ft3

/s)

Measured sediment 

particle diameter

less than 0.0625 mm 

(percent)1

Measured suspendedsediment load

(mg/L)

Measured suspendedsediment load

(tons/day)

Sept. 7, 2010 140 83 467 177

Sept. 19, 2010 1,860 76 474 2,380

Station 08211200 Nueces River at Bluntzer, Tex.

Sample date

Measured

 streamflow

(ft3

/s)

Measured sediment 

particle diameter

less than 0.625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment load 

(tons/day)

Suspended-sediment 

concentration 

relative percent 

difference2

May 17, 2006 121 97 48 16 --

May 17, 2006 (replicate sample) 121 98 45 15 -6.5

July 20, 2006 116 88 51 16 --

July 20, 2006 (replicate sample) 116 94 54 17 5.7

Sept. 5, 2006 136 97 52 19 --

Sept. 19, 2006 1,050 97 1,070 3,040 --

Nov. 20, 2006 82 89 25 5.5 --

Jan. 4, 2007 153 81 293 121 --

Jan. 25, 2007 243 99 760 499 --

May 26, 2007 471 100 94 120 --

May 29, 2007 1,330 97 54 194 --

July 1, 2007 5,980 92 45 727 --

July 11, 2007 15,000 79 40 1,620 --

Sept. 7, 2010 160 78 78 34 --

Sept. 19, 2010 1,500 85 540 2,190 --

Sept. 20, 2010 4,270 94 206 2,370 --
12 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

HSPF is composed of a series of computational routines 

that separately simulate processes of the hydrologic cycle. 

Specifically, HSPF simulates the hydrologic cycle as an 

interconnected series of storage (and processing) segments 

with water fluxes (volume per unit area per unit time) and 

constituent fluxes (mass [weight] per unit area per unit time) 

moving between the various storages. Figure 4 shows a 

flowchart of HSPF hydrologic processes for IMPLNDs and 

PERLNDs. Figure 5 shows a flowchart of HSPF sediment 

processes for IMPLNDs, PERLNDs, and RCHRESs. The 

movement of water and suspended sediment from IMPLNDs 

and PERLNDs and between storage zones is controlled by 

various process-related parameters. Although some parameters 

are directly measurable, most are estimated during model 

calibration (Martin and others, 2001).

The HSPF model of the lower Nueces River watershed 

was developed by (1) compiling and processing required 

input data, (2) configuring the model to represent the 

watershed, (3) calibrating the model to improve simulation 

accuracy, and (4) comparing calibrated model simulations 

with additional streamflow and suspended-sediment data to 

test the model simulation accuracy. The definitions of the 

HSPF model process parameters used in the lower Nueces 

River watershed model are listed in appendix 1. A complete 

description of the computational processes and required input 

model parameters is provided in the HSPF users’ manual 

(Bicknell and others, 2001). 

Previous Watershed Modeling Study and 

Report

As described in the “Introduction” section of this 

report, the USGS, in cooperation with the USACE and 

other cooperators, published a report that documented the 

calibration of a watershed model and use of the model to 

simulate streamflow and suspended-sediment concentrations 

and loads in the lower Nueces River for the period 1958–

2008 (Ockerman and Heitmuller, 2010). Because of interest 

in Nueces River suspended-sediment conditions during a 

runoff event in November 2009, and with the collection of 

additional suspended-sediment data in 2010, the previous 

model was updated to extend the simulation period 

through 2010.

The original model by Ockerman and Heitmuller 

(2010) was configured according to land-cover categories 

and was well suited for simulating sources of sediment 

during runoff events. Updating of the original model for 

this study involved extending the simulation period by 

including the necessary model input data (rainfall, potential 

evapotranspiration, inflow from releases from Lake Corpus 

Christi, and withdrawals) for the period 2009–10. The model 

configuration data and model calibration parameters remained 

unchanged from the original model. 

Table 3. Suspended-sediment sampling results, lower Nueces River watershed, south Texas, 2006–7, 2010.—Continued

[ft3/s, cubic feet per second; mm, millimeters; mg/L, milligrams per liter; mm, millimeters; tons/day, tons per day]

Station 08211500 Nueces River at Calallen, Tex.

Sample date

Measured

streamflow

(ft3

/s)

Measured sediment 

particle diameter 

less than 0.0625 mm 

(percent)1

Measured suspendedsediment concentration 

(mg/L)

Measured suspendedsediment load 

(tons/day)

May 16, 2006 15 82 58 2.3

Sept. 19, 2006 1,150 100 429 1,330

Jan. 5, 2007 66 98 19 3.4

Jan. 25, 2007 163 89 32 14

Mar. 15, 2007 221 32 134 80

July 2, 2007 5,100 94 231 3,180

July 4, 2007 6,530 93 50 882

July 5, 2007 8,960 84 64 1,550

July 11, 2007 19,800 49 45 2,410

Aug. 16, 2007 4,050 94 22 241

Sept. 20, 2010 1,970 89 381 2,030

1

Percent by weight of sediment sample that passes through a 0.0625 millimeter sieve.

2

Relative percent difference is [|sample 1 – sample 2|/(sample 1 + sample 2)/2] × 100. 
Figure 4. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for hydrologic processes on A, impervious and B, pervious land segments. A . Impervious land segments B. Pervious land segments INFILT* INFILD* INFEXP* Rainfall potential ET Interception storage Surface detention storage Interflow storage Active groundwater storage 

Upper-zone storage Delayed Infiltration Direct infiltration Lower- zone storage Actual evaporation Actual ET ETETETETET Inactive groundwater To stream Potential runoff RETSC* CEPSC* LZETP* LZSN* INTFW* AGWETP* BASETP* DEEPFR* UZSN* LSUR* NSUR* SLSUR* IR

KVAR

AGWR

Surface detention storage Surface retention storage Rainfall potential ET LS

NS

SLS

Overland flow Mode

StoraOutfloDecisEvapoDesig

HSto 

* EXPLA

ET
Figure 5. Hiagram showing Hydrological Simulation Program—FORTRAN (HSPF) flowchart for sediment processes on A, impervious land segments

and C, stream reaches.

A . Impervious land segments

B. Pervious land segmentsC. Stream reaches

Accumulation

of solids

Solids storage Lateral input of solids 

To

stream

ACCSDP*

SLSLD* Washoff

of solids 

KEIM*, JEIM*

Washoff

of solids 

REMSDP*

Net vertical

sediment input

Detached sediment

storage

Lateral input

of sediment 

To

stream

NVSI*

SLSED*

Total removal

of sediment

KSER*, JSER*

Soil matrix

Detachment of soil by rainfall

KRER*, JRER*

Washoff of detached

sediment by water

Sediment attachment

AFFIX*

Scour of matrix

soil by water 

KGER*, JGER*

Suspended

sediment

storage

Sediment inflow

to RCHRES

Sediment outflow

from RCHRES

Bed

sediment

storage

Deposition TAUCD*, TAUCS*

Scour

sand 

silt

clay 

clay and silt

sand

KSAND*, EXPSND*

*

RCHRES

ModStorOutfStreDes H to 

EXPL
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 15

Input Data for the Lower Nueces River 

Watershed Model

Input data for the lower Nueces River watershed model 

included spatial data (land cover, geology, soils, topography, 

and drainage characteristics such as subwatershed boundaries 

and stream-reach length and cross-section data) and timesseries data, including meteorological data (rainfall and 

potential evapotranspiration), streamflow data, and suspendedsediment load data. Spatial data were used to create the model 

HRUs and RCHRESs. Streamflow and suspended-sediment 

data were used to calibrate the model.

Land-cover data for the study area were obtained from 

the U.S. Environmental Protection Agency’s Multi-Resolution 

Land Characteristics Consortium (Homer and others, 2004). In 

the lower Nueces River watershed, there were 15 land covers 

and land uses as classified by the 2001 National Land Cover 

Database (NLCD) (Multi-Resolution Land Characteristics 

Consortium, 2008). To simplify the model configuration, 

the 15 classes were reclassified into 8 land-cover categories 

shown on figure 6. Areas classified as open water were 

modeled as part of a RCHRES. Developed land use was 

principally classified as low-intensity development, of which 

about 15 percent was simulated as impervious area. Barren 

land was grouped with developed open space and the resulting 

acreage also was considered to be 15 percent impervious. 

Surficial geology data (fig. 7) were obtained from 

two areal geologic maps published in the “Geologic Atlas 

of Texas” by the University of Texas Bureau of Economic 

Geology for the lower Nueces River study area: Beeville-Bay 

City sheet (Aronow and others, 1975) and Corpus Christi 

sheet (Aronow and Barnes, 1975). The predominant surficial 

geology of the most upstream parts of the study area includes 

the Pleistocene-age Lissie and Deweyville Formations. In 

the rest of the study area, the surficial geology consists of the 

Beaumont Formation (Pleistocene age) or alluvium (Holocene 

age) (Aronow and Barnes, 1975). In the Lissie and Deweyville 

Formations, the sediments, in order of dominance, consist 

mostly of sand, silt, and clay. In the Beaumont Formation and 

alluvium, the sediments, in order of dominance, consist mostly 

of clay, silt, and sand (Minzenmayer, 1979). The data obtained 

from the Beeville-Bay City sheet (northern part of the study 

area) provide a more detailed breakdown of the areas of the 

Beaumont Formation that are dominated by clay or sand, 

compared with the data available from the Corpus Christi 

sheet (southern part of the study area). 

Soils data for the study area were compiled from the U.S. 

Department of Agriculture, Natural Resources Conservation 

Service (2009) “Soil Survey Geographic (SSURGO) 

Database.” These data were used to provide initial estimates 

for selected HSPF process-related parameter values, primarily 

soil infiltration rate (INFILT). Figure 8 shows the relative soil 

infiltration rates for the study area. 

Topography (slope) data for the study area were obtained 

from USGS 7.5-minute digital elevation models (U.S. 

Geological Survey, 2013b). The digital elevation models also 

were used to delineate subwatersheds as part of the HSPF 

model development. The study area was subdivided into 64 

subwatersheds (fig. 9). The average size of each subwatershed 

is 2,000 acres. The model stream segment (RCHRES) that 

is associated with each subwatershed is identified with the 

same identification number as the subwatershed identification 

number (fig. 9). Spatial data for streams (location and reach 

length) were obtained from the “National Hydrography 

Dataset” (U.S. Geological Survey, 2009). Stream-channel 

cross-section data were obtained from streamflow-discharge 

measurements made at USGS streamflow-gaging stations in 

the study area (U.S. Geological Survey, 2010).

Much of the streamflow in the lower Nueces River 

originates as releases from Wesley E. Seale Dam. These 

releases are measured at the USGS Mathis gage. The Mathis 

gage streamflow data were used as input to the upstream 

boundary of the model, providing a relatively accurate 

accounting for inflow to the model (U.S. Geological Survey, 

2013a). Other USGS streamflow-gaging stations in the study 

area provided data that were used for calibration and testing 

of the model simulations. Location information, type of 

data collected, and period of record for sites that provided 

streamflow data for the lower Nueces River watershed are 

listed in table 4. Locations of the streamflow-gaging stations 

are shown on figure 10.

Water is withdrawn from the Nueces River for municipal 

and industrial uses in RCHRES 82 and 84 (fig. 9). During 

2001-10, these withdrawals averaged 64,500 acre-ft/yr or 

the flow volume equivalent to a continuous flow of about 

89 ft3

/s (Nueces River Authority, 2013); withdrawal data are 

available as monthly totals. Within HSPF, the monthly total 

withdrawals were disaggregated to hourly values and input to 

the model.
16 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 6. Land-cover categories in the lower Nueces River watershed, south Texas.

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

LIVE OAK COUNTY

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

 Modified from Multi-Resolution Land

Characteristics Consortium, 2008

National land cover database 2001

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Land-cover category 

 Water

 Developed

 Open space, barren land

 Forest

 Shrub/scrub

 Grassland/pasture

 Cultivated crops

 Wetlands

Lower Nueces River watershed boundary
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 17

Figure 7. Surficial geology of the lower Nueces River watershed, south Texas. 

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

LIVE OAK COUNTY

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

Qd

Qal

Ql

Qb

Qb

Ql

Qd

Qd

Qal

Mg

Ql

Mg

Mg

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

Surficial geology modified from Aronow 

and Barnes, 1975; Aronow and others, 1975

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Surface geology

 Alluvium

 Deweyville Formation

 Beaumont Formation

 Lissie Formation

 Goliad Formation

Lower Nueces River watershed boundary

Qal

Qd

Qb

Ql

Mg
18 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 8. Relative soil infiltration rates in the lower Nueces River watershed, south Texas.

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

LIVE OAK COUNTY

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

Soil data modified from U.S. Department

of Agriculture, Natural Resources

Conservation Service, 2009

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Relative soil infiltration rate

 Moderately rapid

 Moderate

 Moderately slow

 Slow

 Very slow

Lower Nueces River watershed boundary
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 19

Figure 9. Subwatershed delineation for the lower Nueces River watershed model, south Texas.

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

LIVE OAK COUNTY

9

12

16 14

18

6

8

2

5

3

20

24 22

32 36

30 34 38

90

57 42

40

39

92

93

48

50

63

54

73

65

95

74

94

53

64

58

59

61

67

66

70

71

72 76

80

78 97

26

44

46

62

56

55 60

10

91

82

4

68

84

96

98

52

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Subwatershed—Subwatershed identified by number

Lower Nueces River watershed boundary

22
20 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Rainfall and air temperature data were obtained from 

NWS stations Mathis, Mathis 4 SSW, Robstown, Robstown 

Airport (rainfall only), Corpus Christi International Airport, 

and Corpus Christi Maus Field (sites 1–6, respectively, 

fig. 10; table 4). The NWS has operated rainfall stations in 

Mathis and Robstown since 1957 and 1947, respectively. The 

Mathis 4 SSW station replaced the Mathis station in 1977. 

The Robstown station has been in operation since 1947 and 

the Robstown Airport station since 2003. The Mathis and 

Mathis 4 SSW stations (sites 1 and 2, respectively, fig. 10; 

table 4) and the Robstown and Robstown Airport stations 

(sites 3 and 4, respectively, fig. 10; table 4) were the primary 

sources of rainfall data. For modeling purposes, the data from 

the two Mathis stations were combined, and the location of 

the rainfall data time series was considered to be the same as 

the newer Mathis 4 SSW station. Similarly, data from the 

two Robstown stations were combined into a single time 

series that was considered to be located at the newer Robstown 

Airport station. These time series and associated locations 

were then used for the entire simulation period. The spatial 

application of rainfall time-series data to the model is based 

on a Theissen polygon distribution (Linsley and others, 1982). 

Two rainfall areas were defined by the Theissen boundary 

as shown on figure 10. Rainfall to the upstream area of 

the watershed was simulated using the Mathis station data, 

and rainfall to the downstream area of the watershed was 

simulated using the Robstown station data. Rainfall data 

from the Corpus Christi International Airport, Corpus 

Christi Maus Field, and Alice stations (sites 5, 6, and 7, 

respectively, fig. 10; table 4) were used to fill periods of 

missing data when data from the Mathis or Robstown stations 

were not available. 

Time series of potential evapotranspiration (PEVT) 

data are used in HSPF to set the upper limit of actual 

evapotranspiration (ET) that can be simulated for any of 

the HRUs. PEVT is the observed ET if there is an unlimited 

supply of water to satisfy the potential ET rate (Wanielista, 

1990). PEVT was computed from maximum and minimum 

daily air temperature (from NWS station data) using the 

Hamon method (Bidlake, 2002). Similar to the sources of 

rainfall data, the primary sources of air temperature data were 

the Mathis and Robstown NWS stations. PEVT time-series 

data were applied to delineated subwatersheds according to the 

Theissen boundary shown on figure 10.

Table 4. Data-collection sites providing data for the lower Nueces River watershed model, south Texas.

[ddmmss, degrees minutes seconds; NWS, National Weather Service; ID, identifier; --, not available; max, maximum; min, minimum; temp, temperature; USGS, 

U.S. Geological Survey]

Site 

number

(figs. 2, 10)

Station number and name Latitude 

(ddmmss)

Longitude 

(ddmmss)

Type of data 

(period of record available)

1 NWS station Coop ID 415661, Mathis, Jim Wells 

County, Tex.

28° 06′ --′′ 97° 49′--′′ Daily rainfall and daily max and min air 

temp (1957–77)

2 NWS station Coop ID 415661, Mathis 4 SSW, Jim 

Wells County, Tex.

28° 02′14′′ 97° 52′ 21′′ Daily rainfall and daily max and min air 

temp (1977–2010)

3 NWS station Coop ID 417677, Robstown, Nueces 

County, Tex.

27° 47′ 22′′ 97° 39′ 43′ Daily rainfall and daily max and min air 

temp (1947–2010)

4 NWS station WBAN ID 12984, Robstown Airport, 

Nueces County, Tex.

27° 46′ 43′′ 97° 41′ 26′′ Hourly rainfall (2003–10)

5 NWS station COOP ID 412015, Corpus Christi 

International Airport, Nueces County, Tex.

27° 46′ 27′′ 97° 30′ 44′′ Daily rainfall and daily max and min air 

temp (1960–2010)

6 NWS station COOP ID 412015, Corpus Christi Maus 

Field, Nueces County, Tex.

27° 46′ --′′ 97° 27′ --′′ Daily rainfall and daily max and min air 

temp (1934–60)

7 NWS station COOP ID 410144, Alice, Jim Wells 

County, Tex.

27° 43′ 42′′ 98° 04′ 04′′ Daily max and min air temp (1942–2008)

8 USGS streamgaging station 08211000 Nueces River 

near Mathis, Tex.

28° 02′ 17′′ 97° 51′ 36′′ Streamflow (1952–2010); suspended 

sediment (1942–82, 2006–7, 2010)

9 USGS streamgaging station 08211200 Nueces River at 

Bluntzer, Tex. 

(partial-record station)

27° 56′ 15′′ 97° 46′ 32′′ Streamflow (2005–10); suspended sediment 

(2006–7, 2010)

10 USGS streamgaging station 08211500 Nueces River at 

Calallen, Tex.

27° 52′ 58′′ 97° 37′ 30′′ Streamflow (2001–10); suspended sediment 

(2006–8, 2010)

11 USGS streamgaging station 08211502 Nueces River 

near Odem, Tex. 

(partial-record station)

27° 53′ 42′′ 97° 37′ 43′′ Streamflow (flood discharge measurements, 

2001–8)

12 USGS streamgaging station 08211050 Bayou Creek at 

Farm Road 666 near Mathis, Tex. 

(partial-record station)

28° 02′ 36′′ 97° 47′ 20′′ Streamflow (flood discharge measurements, 

2010); suspended sediment (2010)
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 21

Figure 10. Locations of U.S. Geological Survey streamflow-gaging stations and National Weather Service rainfall stations (with 

associated Thiessen rainfall areas) providing data for the lower Nueces River watershed model, south Texas.

Corpus Christi 

International Airport

 Nueces County 

Airport

LIVE OAK COUNTY

BEE COUNTY

 SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

 REFUGIO COUNTY

37

£¤181

£¤77

£¤281

359

44

£¤77

188

234

359

37

358

357

286

Nueces 

Bay

Oso

Creek

Nueces 

Hondo

Creek

River

Nueces

 Estuary

Bayou Creek

Nueces

 River Delta

 Lake

 Corpus

Christi

Rincon 

Bayou 

WESLEY E 

SEALE DAM

8 12

10

9

11

3

2

4 5

6

7

1

Robstown

Corpus Christi Alice

Bluntzer

Calallen

Mathis

Odem

98°00' 97°45' 97°30'

28°15'

28°

27°45'

Base modified from U.S. Geological Survey 1:24,000-scale 

digital data Texas Centric Mapping System, Albers Equal 

Area Projection, North American Datum of 1983

0 2 4 8 MILES

0 4 8 KILOMETERS 2

6

6

EXPLANATION

Subwatershed

Marsh 

Land subject to inundation

Lower Nueces River watershed boundary

Thiessen boundary

U.S. Geological Survey streamflow-gaging 

 map identifier—Table 4

National Weather Service station 

 with map identifier—Table 4

11

7
22 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Model Development 

To develop the model, the stream network of the lower 

Nueces River watershed was segmented into subwatersheds 

(fig. 9), generally on the basis of (1) similar streamflow 

traveltimes that approximated the model time step (1 hour), 

(2) homogeneous channel properties such as slope and 

conveyance, and (3) outlets of subwatersheds at points of 

interest such as streamflow-gaging stations, major tributary 

confluences, and points of water withdrawals.

In each subwatershed, unique pervious and impervious 

HRUs were defined according to three factors: (1) land cover 

and land use, (2) surficial geology and soil characteristics, 

and (3) the location of the nearest rainfall station (to spatially 

distribute meteorological input data [rainfall and PEVT]). 

Spatial information describing these three factors was 

compiled and analyzed using the Geographical Information 

System software ArcGIS (Esri, 2009) to compute the acreage 

of each HRU within a given subwatershed. 

Model Calibration

A primary goal of model calibration was to adjust the 

process-related parameter values to minimize the differences 

between measured and simulated flows at a streamflowgaging station. The model was calibrated in accordance with 

guidelines by Donigian and others (1984) and Lumb and 

others (1994). The calibration of the model proceeded in two 

steps: (1) parameters related to hydrologic processes were 

calibrated, and (2) parameters related to suspended-sediment 

processes were calibrated. Calibration of hydrologic processes 

included adjusting appropriate model parameters to minimize 

differences between measured and simulated streamflow at 

streamflow-gaging stations during 2001–8 over a wide range 

of hydrologic conditions. Model parameters that control landsurface erosion and washoff processes and instream sediment 

transport processes were adjusted to minimize differences 

between measured and simulated suspended-sediment 

concentrations and loads.

Hydrology

To evaluate the goodness of fit between measured and 

simulated streamflows, simulation errors were evaluated by 

comparing total streamflow volume, 50 percent lowest daily 

flows, and 10 percent highest daily flows. Donigian and 

others (1984) presented general guidelines for characterizing 

the goodness of fit of HSPF calibrations. For annual and 

monthly streamflow volumes, model calibration is considered 

very good when the error is less than 10 percent, good when 

the error is 10–15 percent, and fair when the error is 15–25 

percent. 

Additionally, model-fit statistics generated by the 

software program GENeration and analysis of model 

simulation SCeNarios (GenScn) for watersheds (Kittle and 

others, 1998) were used to examine the quality of the model 

fit on an annual, monthly, daily, and hourly basis for the 

(1) coefficient of determination (R-squared [R2]) of the linear 

regression between measured and simulated streamflow 

(Ott and Longnecker, 2001), (2) Nash-Sutcliffe coefficient 

of model-fit efficiency (NSE) (Nash and Sutcliffe, 1970), 

(3) mean absolute error (MAE) (StatSoft, Inc., 2010), and 

(4) root mean square error (RMSE) (StatSoft, Inc., 2010). The 

R2 and NSE are similar because each provides a measure of the 

variation in the measured values explained by the simulated 

values. The NSE, however, provides a generally preferable 

evaluation of the fit quality compared with the R2 because 

the NSE measures the magnitude of the differences between 

measured and simulated values, whereas, the R2 measures the 

difference between mean values (Zarriello and Ries, 2000, 

p. 44). MAE and RMSE express the difference between 

measured and simulated streamflow in original units (cubic 

foot per second) (StatSoft, Inc., 2010).

At selected calibration sites, depending on the availability 

of streamflow data, the calibration process included a separate, 

post-calibration test of the model fit. For sites where the 

testing process was performed, a subset of the observed 

streamflow-gaging data was randomly selected and not used 

during the calibration. After calibration, these withheld data 

were then used for testing the model fit between measured 

and simulated streamflow. Model simulations from 2009–10 

were not part of the original model calibration and testing by 

Ockerman and Heitmuller (2010). When the original model 

simulation period was extended through 2010, 2009–10 

simulated streamflow at the Calallen gage was compared with 

gaged steamflow as an additional testing process. 

Most of the streamflow in the study area originated as 

releases from Lake Corpus Christi (fig. 1). The Mathis gage is 

located immediately downstream from the lake and monitors 

stream discharge from the lake. Daily discharges for this 

station during 1939–2010 were input to the model at the inlet 

of RCHRES 4 (fig. 9) as a boundary condition. Because actual 

gaged streamflow data were used as input to the model at this 

site, no model calibration was necessary at this station.

The Bluntzer gage (outlet of RCHRES 60, fig. 9) is 

operated as a partial-record station. Streamflows of more 

than 2,750 ft3

/s are not measured because streamflowgaging conditions are not favorable at higher flows. Because 

streamflow data from the Bluntzer gage were not suitable 

for calibration of the entire range of streamflow conditions, 

calibration of hydrologic parameters upstream from the 

Bluntzer gage was not based on data from this gage. During 

2005–6, streamflow at the Bluntzer gage did not exceed 

2,750 ft3/s, and the 2005–6 data were used as a test of the 

calibrated model. Streamflow testing results for the Bluntzer 

gage during 2005–6 (fig. 11; table 5) indicate very good 

agreement between measured and simulated streamflow 

volumes; the error in total simulated streamflow volume 

compared with measured streamflow volume during 2005–6 at 

this gage was less than 2 percent. 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 23

Figure 11. Measured and simulated daily mean streamflow at U.S. Geoloical Survey streamflow-gaging station 08211000 Nueces River 

at Bluntzer, Texas, 2005–6.

Line of equal value

10 100 10,000

Measured daily mean streamflow, in cubic feet per second

1,000

1 10

2005

0.2 0.5 2 5 20 30 50 70 80 90 95 98 99 99.5 99.8

Exceedance probability, in percent

Year

2006

10

100

1,000

Simulated daily mean streamflow, in cubic feet per second

10,000

10

1,000

10,000

Daily mean streamflow, in cubic feet per second Daily mean streamflow, in cubic feet per second

100

10

1,000

10,000

100

Measured

Simulated

EXPLANATION
24 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Table 5. Streamflow calibration and testing results for the lower Nueces River watershed model, south Texas.

[acre-ft, acre-feet; ft3s, cubic feet per second]

08211200 Nueces River at Bluntzer, Texas

Testing period 2005–6

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criteria2

(percent)

Total flow volume (acre-ft) 353,000 357,000 1.1 10

Mean flow rate (ft3

/s) 244 247 1.1 10

Highest 10-percent daily flows (acre-ft) 151,000 166,000 9.9 10

Lowest 50-percent daily flows (acre-ft) 72,800 71,600 -1.6 10

Model-fit statistics 2005–6 Annual Monthly Daily Hourly

Number of years, months, days, or hours 2 24 730 17,520

Coefficient of determination (R2

) 1.00 0.98 0.92 0.91

Nash-Sutcliffe coefficient of model-fit efficiency (NSE) 1.00 0.97 0.91 0.90

Mean absolute error (ft3

/s) 4.5 30.7 38.4 38.4

Root mean square error (ft3

/s) 5.3 57.0 117 124

08211500 Nueces River at Calallen, Texas

Calibration period 2005–8

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criteria2

(percent)

Total flow volume (acre-ft) 1,400,000 1,440,000 2.9 10

Mean flow rate (ft3

/s) 483 497 2.9 10

Highest 10-percent daily flows (acre-ft) 1,220,000 1,215,000 -0.4 10

Lowest 50-percent daily flows (acre-ft) 25,400 25,100 -1.2 10

Model-fit statistics 2005–8 Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R2

) 1.00 1.00 0.95 0.95

Nash-Sutcliffe coefficient of model-fit efficiency (NSE) 1.00 0.99 0.93 0.92

Mean absolute error (ft3

/s) 37 66 127 130

Root mean square error (ft3

/s) 39 152 510 523

08211500 Nueces River at Calallen, Texas

Testing period 2001–4

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criteria2

(percent)

Total flow volume, acre-ft 4,220,000 4,010,000 -5.0 10

Mean flow rate (ft3

/s) 1,456 1,384 -5.0 10

Highest 10-percent daily flows, acre-ft 3,170,000 3,070,000 -3.2 10

Lowest 50-percent daily flows, acre-ft 21,500 23,500 9.3 10

Model-fit statistics 2001–4 Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R2

) 1.00 1.00 0.96 0.96

Nash-Sutcliff coefficient of model-fit efficiency (NSE) 0.98 0.99 0.96 0.96

Mean absolute error (ft3

/s) 83 130 307 314

Root mean square error (ft3

/s) 133 265 838 862
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 25

The Calallen gage was operated as a partial-record station 

during 1989–2000; daily streamflows were not reported 

when instantaneous streamflow exceeded 2,750 ft3/s because 

of difficulty measuring higher streamflows at this gage. 

A complete record of streamflow for the Calallen gage is 

available for 2001–10. In June 2000, the Calallen gage was 

moved about 0.4 mile downstream to its present location and 

was converted to a continuous streamflow-gaging station 

capable of measuring the full range of streamflow. Streamflow 

data measured during 2005–8 at the Calallen gage were used 

to calibrate the model, and streamflow data from 2001–4 and 

2009–10 were used to test the calibrated model. Streamflow 

calibration and testing results for the Calallen gage (fig. 12; 

table 5) indicate very good agreement between measured and 

simulated streamflow volumes. The error in total simulated 

streamflow volume at the Calallen gage compared with 

measured streamflow volume during the calibration period 

(2005–8) is less than 3 percent; for the 2001–4 testing period, 

the error in total simulated streamflow volume compared with 

measured streamflow volume is -5 percent; for the 2009–10 

Table 5. Streamflow calibration and testing results for the lower Nueces River watershed model, south Texas—Continued.

[acre-ft, acre-feet; ft3s, cubic feet per second]

08211500 Nueces River at Calallen, Texas

Simulation period 2009–10

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criteria2

(percent)

Total flow volume, acre-ft 109,000 119,000 9.2 10

Mean flow rate (ft3

/s) 75.3 82.2 9.2 10

Highest 10-percent daily flows, acre-ft 87,100 86,600 -0.6 10

Lowest 50-percent daily flows, acre-ft 606 709 17 10

Model-fit statistics 2009–10 Annual Monthly Daily Hourly

Number of years, months, days, or hours 2 24 730 17,520

Coefficient of determination (R2

) 1.00 0.98 0.85 0.83

Nash-Sutcliff coefficient of model-fit efficiency (NSE) 0.99 0.98 0.84 0.82

Mean absolute error (ft3

/s) 5.51 10.2 32.3 34.1

Root mean square error (ft3

/s) 5.93 16.5 88.6 95.4

08211500 Nueces River at Calallen, Texas

Entire calibration and testing period 2001–10

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criteria2

(percent)

Total flow volume, acre-ft 5,730,000 5,570,000 -2.8 10

Mean flow rate (ft3

/s) 791 769 -2.8 10

Highest 10-percent daily flows, acre-ft 4,980,000 4,770,000 -4.2 10

Lowest 50-percent daily flows, acre-ft 38,000 40,300 6.1 10

Model-fit statistics 2001–8 Annual Monthly Daily Hourly

Number of years, months, days, or hours 10 120 3,652 87,648

Coefficient of determination (R2

) 1.00 1.00 0.96 0.96

Nash-Sutcliff coefficient of model-fit efficiency (NSE) 0.99 0.99 0.96 0.96

Mean absolute error (ft3

/s) 53 84 182 187

Root mean square error (ft3

/s) 89 196 628 647

1

Error is [(simulated-measured)/measured] × 100. 

2

Default error criteria from HSPEXP (Lumb and others, 1994).
26 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 12. Measured and simulated 7-day mean streamflow at U.S. Geoloical Survey streamflow-gaging station 08211500 Nueces 

River at Calallen, Texas, 2001–10.

Line of equal value

1 10

1 100 1,000 100,000

0.2 0.5 2 5 20 30 50 70 80 90 95 98 99 99.5 99.8

Year

10,000

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

10

Exceedance probability, in percent

Measured 7-day mean streamflow, in cubic feet per second

1

10

1

100

10,000

100,000

1,000

10

1

100

10,000

100,000

1,000

7-day mean streamflow, in cubic feet per second

100,000

10,000

1,000

100

10

7-day mean streamflow, in cubic feet per second 7-day mean streamflow, in cubic feet per second

Measured

Simulated

EXPLANATION
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 27

testing period, the error in total simulated streamflow volume 

compared with measured streamflow volume was 9.2 percent. 

For the entire 2001–10 calibration and testing period, the error 

in total simulated streamflow volume was -2.8 percent.

Using evaluation criteria by Donigian and others (1984), 

calibration and testing results for streamflow volumes at both 

the Bluntzer (testing only) and Calallen gages (table 5) were 

considered very good. The R2 and NSE values were considered 

acceptable for annual, monthly, daily, and hourly statistics. 

The NSE for hourly streamflows ranged from 0.82 to 0.96 for 

the calibration and testing periods at the Bluntzer and Calallen 

gages. The minimum NSE values for both stations for daily, 

monthly, and annual simulations were 0.84, 0.97, and 0.98, 

respectively (table 5).

Suspended Sediment

Suspended-sediment concentrations and loads were 

simulated using the appropriate HSPF modules: SEDMNT 

for simulation of production and removal of sediment from 

PERLNDs, SOLIDS for simulation of accumulation and 

removal of solids from IMPLNDs, and SEDiment TRaNsport 

(SEDTRN) for simulation of transport of sediment in 

RCHRES. For each PERLND, the processes of detachment 

of sediment from the soil matrix and washoff of this sediment 

were simulated on the basis of rainfall intensity, surface 

runoff, and model parameters that control the accumulation, 

detachment, and transport of soils. For each IMPLND, the 

processes of accumulation and washoff of sediment are 

based on the amount of sediment available and the transport 

capacity of the overland flow. Sediment transport processes in 

RCHRES included deposition and scour, which are functions 

of sediment size, settling velocity, density, erodibility, bed 

depth, and critical shear stress. RCHRES sediment transport 

is computed separately for each sand, silt, and clay fraction of 

sediment size.

Selection of initial values and calibration of sedimentrelated process parameters of the SEDMNT, SOLIDS, and 

SEDTRN modules (appendix 1) were based on published 

guidelines (Donigian and Love, 2003; U.S. Environmental 

Protection Agency, 2006). Calibration of sediment-related 

parameters involved the following steps: (1) estimation 

of suspended-sediment loads carried in flows released 

from Lake Corpus Christi; (2) identification of reasonable 

sediment yields from the various land types in the watershed; 

(3) estimation of the soil-erosion and sediment-washoff 

parameters used to generate sediment washoff from PERLNDs 

and IMPLNDs, respectively; and (4) calibration of sedimenttransport (RCHRES) parameters by comparison of measured 

and simulated suspended-sediment concentrations and loads at 

the streamflow-gaging stations.

Estimation of Suspended-Sediment Loads from Lake 

Corpus Christi

Estimation of suspended-sediment loads in releases 

from Wesley E. Seale Dam (Lake Corpus Christi) were based 

on 14 reported suspended-sediment loads during 1964–71 

(table 2) and 9 suspended-sediment samples collected by 

the USGS during 2006–7, 2010 (table 3). All suspendedsediment data (historical and recent) were collected at the 

Mathis gage downstream from Wesley E. Seale Dam. These 

data represented streamflow ranging from 72 to 35,129 ft3/s 

and corresponding suspended-sediment loading rates ranging 

from 4.3 to 8,872 tons/d. Data collected during 1964–71 

represented mean streamflow and suspended-sediment loads 

during relatively large releases that lasted days or weeks. Data 

collected during 2006–7 represented instantaneous conditions 

(a typical suspended-sediment sample was collected during a 

period of about 1 hour). A least-squares regression equation 

(fig. 13) was developed by relating daily mean discharge at the 

Mathis gage and daily sediment load. Discharge and sediment 

loads were log-transformed before performing the regression 

to improve the quality of the regression fit, then retransformed 

to original units (Helsel and Hirsch, 2002). The resulting 

equation was 

L = 0.026 x Q1.195 (2)

where

L is suspended-sediment load, in tons per day; 

and 

Q is daily mean discharge, in cubic feet per 

second.

The R2 for the regression equation was 0.94; the standard 

error of the residuals (RSE), a measure of the dispersion of 

the data around the regression line (Helsel and Hirsch, 2002, 

p. 244), was 625 tons/d. The residual plot (fig. 13) shows 

the regression residuals (as a percentage of the measured 

suspended-sediment load) plotted as a function of the 

measured suspended-sediment load. The distribution of the 

error residuals was relatively uniform, indicating a reasonable 

regression model. 

A comparison of model-simulated input of suspendedsediment loads with suspended-sediment loads computed 

from nine samples collected at the Mathis gage during 2006–7 

and 2010 is listed in table 6. These results are not considered 

a calibration but rather a test of the ability of the regression 

equation (equation 2) to reasonably simulate instantaneous 

suspended-sediment concentrations and loads entering the 

study area through releases from Wesley E. Seale Dam. In 

general, measured and simulated suspended-sediment loads 

compare favorably. Regression-equation derived sediment 

loads were within 37 percent of measured loads for all 

samples except for the sample collected on November 20, 

2006, for which the simulated load was 116 percent of the 

measured load (table 6). The November 20, 2006, sample 

was collected during a period of low flow and represented a 

relatively small sediment load compared with the relatively 

high flows that were mostly sampled at this site. Therefore, the 

loads computed by the regression equation were considered 

reasonable estimates of the total suspended-sediment loads 

that were discharged from Wesley E. Seale Dam.
28 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 13. Relation between streamflow and suspended-sediment load and plot of regression error residuals based on 22 streamflowload data pairs from 08211000 Nueces River near Mathis, Texas, 1964–71 and 2006–7.

1 10 100 1,000 10,000

10 100 1,000 10,000 100,000

Streamflow, in cubic feet per second

Measured daily suspended-sediment load, in tons per day

Suspended-sediment load, in tons per day

Error residuals of suspended-sediment load, in percent

 (estimated daily load minus measured daily load

divided by measured daily load)

1

10,000

1,000

100

10

250

200

100

0

-100

-200

-250

150

-150

 50

-50

L = 0.026 x Q1.195 

Load (L) estimated by least-squares regression

Load computed from measured streamflow (Q) and

 sample suspended-sediment concentration

EXPLANATION
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 29

The time series of sediment loads from Lake Corpus 

Christi were apportioned between clay and silt particle sizes 

and input to the model in RCHRES 4 (as was done with flows 

from Wesley E. Seale Dam). An apportionment of 40 percent 

silt and 60 percent clay was done on the basis of literature 

synthesis work by White and Calnan (1990).

Several studies of sediment yields (pounds per acre per 

year [lbs/acre/yr] or tons per acre per year [tons/acre/yr]) 

from specific land-use types and land covers have been made 

for the south Texas area (Baird and others, 1996; Ockerman 

and Petri, 2001; Ockerman, 2002; Ockerman and Fernandez, 

2010). These studies provided data for sediment yields for 

cropland, rangeland (grassland/pasture and shrub/scrub), and 

developed land. Data and analyses from these studies were 

compared with simulated sediment yields from similar land 

type PERLNDs. Sediment-related parameters for PERLNDs 

were adjusted so that simulated sediment yields compared 

reasonably with field data results. 

Ockerman and Petri (2001, p. 19), in a study of five 

cropland watersheds in Nueces and Kleberg Counties, reported 

an average sediment yield of 610 lbs/acre/yr during 1996–98. 

Ockerman and Fernandez (2010, p. 20), in a study of two 

primarily cropland watersheds in the Oso Creek watershed in 

Nueces County, reported average sediment runoff yields of 

139 and 522 lbs/acre/yr during 2005–8. In a study of sediment 

runoff from two rangeland study sites in San Patricio County, 

Ockerman (2002) reported an average sediment yield of 

28 lbs/acre/yr during 2000–1. 

Estimates of suspended-solids concentrations in runoff 

from developed land uses and land covers were compiled 

for the south Texas area in a study by Baird and others 

(1996). Median event-mean concentrations for residential, 

commercial, industrial, and transportation land uses were 

compiled from a literature review. These event-mean 

concentrations were used to develop estimates of sediment 

loads in the Oso Creek watershed for 1989–93. The average 

sediment yield for developed land (including residential, 

commercial, industrial, and transportation land uses) was 

52 lbs/acre/yr (Baird and others, 1996, p. 199–218). A 

comparison of sediment yields for selected land-use/landcover types is listed in table 7. The yields from the literature 

review were used as calibration target values for model 

simulations. For comparison, table 7 also lists HSPF-simulated 

sediment yields for selected land uses and land covers for 

1989–2008 in the lower Nueces River watershed (Ockerman 

and Heitmuller, 2010). HSPF parameter values related to 

sediment production (sediment detachment and washoff 

process) were iteratively adjusted until simulated yields 

approximated the target yields determined from the literature 

review.

Annual sediment yields simulated for cropland varied 

from 200 to 530 lbs/acre/yr depending on crop, soil type, 

and rainfall. Annual sediment yields simulated for rangeland 

were 39 to 55 lbs/acre/yr, developed land were 38 to 75 lbs/

acre/yr, and open/undeveloped land were 120 to 340 lbs/acre/

yr. Sediment yields simulated using NWS rainfall data from 

Table 6. Measured and simulated streamflows, measured suspended-sediment particle size, and measured and simulated suspendedsediment concentrations and loads for selected sampling events at 08211000 Nueces River near Mathis, Texas, 2006–7, 2010. 

[ft3/s, cubic feet per second; mm, millimeters; mg/L, milligrams per liter; mm, millimeters; tons/day, tons per day]

Sample 

date

Measured 

streamflow

(ft3

/s)

Simulated 

streamflow

(ft3/s)

Measured 

sediment particle 

diameter less 

than 0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Simulated 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment load 

(tons/day)

Simulated 

suspendedsediment load 

(tons/day)

Error in 

suspendedsediment 

load

(percent)2

May 17, 2006 139 137 98 24 28 8.9 10 12

July 20, 2006 110 117 88 26 31 7.7 10 30

Sept. 5, 2006 138 136 99 24 28 8.7 10 15

Nov. 20, 2006 72 71 94 22 48 4.3 9.3 116

May 26, 2007 461 511 99 16 12 21 17 -19

May 29, 2007 1,190 1,270 98 12 11 39 35 -10

July 1, 2007 4,830 5,120 98 16 18 215 235 9.3

July 11, 2007 17,800 17,600 86 44 45 2,200 2,100 -4.5

Sept. 7, 2010 115 116 64 38 25 12 7.6 -37

1

Percent by weight of sediment sample that passes through a 0.0625 millimeter sieve.

2

Error is [(simulated load-measured load)/measured load] × 100.
30 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Robstown were larger for all land types than sediment yields 

simulated using NWS rainfall data from Mathis. The main 

reason for differences in sediment yields were differences in 

the amount of rainfall measured during 2000–8 at the NWS 

stations for the two parts of the study area; average annual 

rainfall amounts recorded by the NWS during 2000–8 were 

34.1 in. at Robstown and 26.8 in. at Mathis.

Model Calibration

For parameters related to sediment erosion from various 

land types, model calibration and testing were based on 

available studies and data collected during 1989–2010. 

Therefore, calibration of PERLND parameters was based 

on simulations from the same period, 1989–2010. Model 

calibration of RCHRES suspended-sediment processes was 

based primarily on data collected at the Bluntzer and Calallen 

gages during 2006–7. These data were used to develop 

estimates of suspended-sediment loads for 2001–8, which 

were then compared with model simulation results.

Downstream from Lake Corpus Christi, at the Bluntzer 

and Calallen gages, suspended-sediment concentrations and 

loads in streams were calibrated to available suspendedsediment data collected by the USGS during 2006–7. 

RCHRES parameters related to sediment transport were 

adjusted (calibrated) to minimize the differences between 

measured and simulated sediment concentrations and 

computed (from measured concentrations and measured 

streamflows) and simulated loads. Instream transport and 

concentrations of sand are simulated by power functions 

that relate sand concentration to average RCHRES stream 

velocity. The power functions are calibrated by user-selected 

coefficients and exponents for each RCHRES. Proceeding 

in downstream order, the calibration process to determine 

suspended-sediment concentrations and loads was repeated 

for each RCHRES in the intervening areas between sedimentsampling stations. Suspended-sediment calibration results 

at the Bluntzer gage are listed in table 8. Table 8 also lists 

measured and simulated suspended-sediment concentrations 

and measured (computed) and simulated loads from 14 

samples collected during 2006–7, 2010. Errors in simulated 

suspended-sediment loads at the Bluntzer gage (table 8) 

generally were larger compared with those at the Mathis gage 

(table 6), mostly because of differences in the amount of error 

associated with the simulation of streamflow. Overall, the 

amount of error associated with the simulation of streamflow 

at the Bluntzer gage was larger than the amount of error 

associated with the simulation of streamflow at the Mathis 

gage.

Although simulation errors for instantaneous or daily 

suspended-sediment loads were at times relatively large (197 

percent for the September 7, 2010, sample; table 8), simulation 

errors for longer periods (for example, monthly or yearly) 

were deemed acceptable. The percentage error of the sum of 

simulated sediment loads for all 14 samples (12,620 tons/d), 

compared with the sum of the measured (computed) sediment 

loads for all 14 samples (10,960 tons/d) was 15 percent. 

Suspended-sediment data were available for 10 

suspended-sediment samples collected at the Calallen gage 

during 2006–7. By using these data, suspended-sediment loads 

were computed from daily mean streamflows and measured 

suspended-sediment concentrations. Similar to the approach 

used to estimate sediment loads from Lake Corpus Christi 

(fig. 13), the computed loads at Calallen were used to develop 

a regression equation to relate daily suspended-sediment loads 

to daily mean streamflow (fig. 14). The resulting equation was

L = 0.119 × Q1.061 (3)

where

L is suspended-sediment load, in tons per day; 

and 

Q is daily mean discharge, in cubic feet per 

second.

R2 for the regression equation was 0.87; RSE was 

427 pounds per day. The residual plot on figure 14 shows 

regression residuals (as a percentage of the measured 

suspended-sediment load) plotted as a function of the 

measured suspended-sediment load. The distribution of the 

error residuals was relatively uniform, indicating a reasonable 

regression model.

The regression equation was then used to estimate 

suspended-sediment loads during 2001–8. The estimated 

daily suspended-sediment load computed using measured 

streamflow and sediment data was used as a calibration 

target to compare with simulated suspended-sediment loads 

at Calallen. Model sediment-transport parameter values for 

RCHRESs downstream from the Calallen gage were adjusted 

on the basis of this comparison. 

Table 7. Comparison of literature estimates and simulation 

results for sediment yields from selected land-cover and land-use 

types in the coastal area of south Texas and the lower Nueces 

River watershed.

[--, not applicable]

Land-cover/

land-use type

Sediment yield estimate 

from literature

(pounds per 

acre per year) 

and reference

Average 

1989–2008 

simulated 

sediment yield1

(pounds per 

acre per year)

Cropland 610 (Ockerman and 

Petri, 2001); 139 and 

522 (Ockerman and 

Fernandez, 2010)

200–530

Rangeland (pasture, 

shrub, grassland)

28 (Ockerman, 2002) 39–55

Developed (commercial, 

transportation, 

residential, industrial)

52 (Baird and others, 

1996)

38–75

Open/undeveloped -- 120–340

1

Ockerman and Heitmuller, 2010.
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 31

Calibration results for the Calallen gage (table 9) include 

a comparison of the average monthly streamflow volumes and 

suspended-sediment loads simulated using the HSPF model 

with the average monthly streamflow volumes measured at 

the gage and average monthly suspended-sediment loads 

estimated using equation 3. Table 9 lists the annual, monthly, 

and daily model-fit statistics for 2001–8. During 2001–8, 

average monthly suspended-sediment loads at the Calallen 

gage were undersimulated by 4.2 percent (table 9) compared 

with the loads estimated by regression (equation 3). Model-fit 

statistics indicated the model simulates daily, monthly, and 

annual sediment loads reasonably well. The NSE was 0.80 for 

daily simulations, increasing to 0.95 and 0.97 for simulation of 

monthly and annual, respectively, suspended-sediment loads 

(table 9). 

Suspended-sediment calibration results for individual 

samples collected at the Calallen gage are listed in 

table 10. Table 10 compares simulated suspended-sediment 

concentrations and loads with estimated suspended-sediment 

concentrations and loads computed from measured streamflow 

and suspended-sediment concentrations obtained from 11 

samples collected during 2006–7, and 2010. Similar to 

comparison of measured and simulated loads for individual 

samples collected at the Bluntzer gage, simulation errors 

for instantaneous suspended-sediment loads were at times 

relatively large, simulation errors for longer periods (for 

example, monthly or yearly) were deemed acceptable. The 

percent error of the sum of simulated sediment loads for 

all 11 samples (12,370 tons/d), compared with the sum of 

the measured (computed) sediment loads for all 11 samples 

(11,720 tons/d) was 5.5 percent. 

Donigian and others (1984) present general guidelines 

for characterizing HSPF sediment calibrations, similar to the 

guidelines for streamflow calibration. For annual and monthly 

sediment loads, model calibration is considered very good 

when the error is less than 15 percent, good when the error is 

15–25 percent, and fair when the error is 25–35 percent. By 

these guidelines, calibration results for the sum of simulated 

suspended-sediment loads at the Calallen gage (table 10) 

are considered very good. The R2

 and NSE values were 

considered acceptable for annual, monthly, and daily statistics 

(table 9).

Table 8. Measured and simulated streamflows, measured suspended-sediment particle size, and measured and simulated suspendedsediment concentrations and loads for selected suspended-sediment sampling events at 08211200 Nueces River at Bluntzer, Texas, 

2006–7, 2010. 

[ft3/s, cubic feet per second; mm, millimeters; mg/L, milligrams per liter; tons/day, tons per day]

Sample date

Measured 

streamflow

(ft3

/s)

Simulated 

streamflow

(ft3/s)

Measured 

sediment particle 

diameter less 

than 0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Simulated 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment 

load 

(tons/day)

Simulated 

suspendedsediment 

load 

(tons/day)

Error in 

suspendedsediment load

(percent)2

May 17, 2006 121 136 97 48 22 16 8.3 -48

July 20, 2006 116 118 88 51 25 16 7.9 -51

Sept. 5, 2006 136 129 97 52 23 19 8.1 -57

Sept. 19, 2006 1,050 1,370 97 1,070 468 3,030 1,730 -43

Nov. 20, 2006 82 71 89 25 35 5.5 6.8 24

Jan. 4, 2007 153 344 81 293 344 121 238 97

Jan. 25, 2007 243 970 99 760 455 498 1,190 139

May 26, 2007 471 405 100 94 38 119 41 -66

May 29, 2007 1,330 1,170 97 54 44 194 140 -28

July 1, 2007 5,980 6,290 92 45 33 726 553 -24

July 11, 2007 15,000 14,400 79 40 64 1,620 2,500 54

Sept. 7, 2010 160 197 78 78 306 34 101 197

Sept. 19, 2010 1,500 3,640 85 540 402 2,190 4,200 92

Sept. 20, 2010 4,270 3,060 94 206 226 2,370 1,900 -20

1

Percent by weight of sediment sample that passes through a 0.0625 millimeter sieve.

2

Error is [(simulated load-measured load)/measured load] × 100.
32 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 14. Relation between streamflow and suspended-sediment load and plot of regression error residuals based on 10 streamflowload data pairs from 08211500 Nueces River at Calallen, Texas, 2006–7.

1

10

100

1,000

10,000

-250

250

0

-100

 50

 200

 -200

Suspended-sediment load, in tons per day

Error residuals of suspended-sediment load, in percent

(estimated daily load minus measured daily load

divided by measured daily load)

 150

 100

-150

-50

10 100 1,000

Measured daily suspended-sediment load, in tons per day

1 10,000

10 100 1,000 10,000 100,000

Streamflow, in cubic feet per second

L = 0.119 x Q1.061 

Load (L) estimated by least-squares regression

Load computed from measured streamflow (Q) and

 sample suspended-sediment concentration

EXPLANATION
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 33

Table 9. Suspended-sediment calibration results for the Hydrological Simulation Program—FORTRAN model of the lower Nueces 

River watershed, south Texas, 2001–8.

08211200 Nueces River at Calallen, Texas

Average monthly streamflow volumes 

and suspended-sediment loads

Measured/

estimated1 Simulated Error2

(percent)

Flow volume (acre-feet) 58,500 56,400 -3.6

Suspended-sediment load (tons) 5,910 5,660 -4.2

Model-fit statistics for suspended-sediment loads 2001–8 Annual Monthly Daily

Coefficient of determination (R2

) 0.98 0.96 0.82

Nash-Sutcliff coefficient of model-fit efficiency (NSE) 0.97 0.95 0.80

Mean absolute error (tons) 12,200 1,640 84.4

Percent mean absolute error (percent) 17.1 27.8 43.4

Root mean square error (tons) 15,500 4,270 444

1

Streamflow volumes at 08211500 Nueces River at Calallen, Tex., are measured values. Suspended-suspended loads are estimated by regression of measured 

streamflow and suspended-sediment loads calculated from streamflow and measured suspended-sediment concentrations for 10 sampling events during 2006–7.

2

Error is [(simulated-measured/measured)] × 100.

Table 10. Measured and simulated streamflows, measured suspended-sediment particle size, and measured and simulated 

suspended-sediment concentrations and loads, for selected sampling events at 08211500 Nueces River near Calallen, Texas, 2006–7, 

2010. 

[ft3/s, cubic feet per second; mm, millimeters; mg/L, milligrams per liter; tons/day, tons per day]

Sample date

Measured 

streamflow

(ft3

/s)

Simulated 

streamflow

(ft3/s)

Measured 

sediment particle 

diameter less 

than 0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Simulated 

suspendedsediment concentration 

(mg/L)

Measured 

suspendedsediment 

load 

(tons/day)

Simulated 

suspendedsediment 

load 

(tons/day)

Error in 

suspendedsediment 

load

(percent)2

May 16, 2006 15 22 82 58 27 2.3 1.4 -39

Sept. 19, 2006 1,150 1,340 100 429 316 1,330 992 -25

Jan. 5, 2007 66 299 98 19 129 3.4 104 2,960

Jan. 25, 2007 163 455 89 32 98 14 161 1,050

Mar. 15, 2007 221 170 32 134 74 80 40 -50

July 2, 2007 5,100 7,360 94 231 44 3,180 871 -73

July 4, 2007 6,530 7,070 93 50 43 882 814 -7.7

July 5, 2007 8,960 10,200 84 64 61 1,550 1,670 7.7

July 11, 2007 19,800 17,400 49 45 80 2,410 3,750 56

Aug. 16, 2007 4,050 4,610 94 22 24 241 296 23

Sept. 20, 2010 1,970 4,330 89 381 300 2,030 3,670 81

1

Percent by weight of sediment sample that passes through a 0.0625 millimeter sieve.

2

Error is [(simulated load-measured load/measured load)] × 100.
34 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Parameter Calibration Values

Calibration and testing of the HSPF model resulted 

in a final set of model parameter values for simulation of 

streamflow and suspended-sediment loads for the study area. 

Calibrated values (or ranges of values) for selected parameters 

related to hydrology simulation are listed in table 11, and 

calibrated values (or ranges of values) for selected sedimentrelated parameters used in the HSPF model are listed in 

table 12.

Sensitivity Analysis

Calibrated values of selected HSPF process-related 

parameters were further evaluated by doing a sensitivity 

analysis to determine the effects that changes in the 

selected parameters would have on simulated streamflow 

and suspended-sediment loads. Each sensitivity simulation 

was made by adjusting a single parameter of the model by 

relatively large amounts (increased by 40 percent) while 

keeping other model parameters unchanged. The sensitivity 

analysis model runs were performed for 2001–8. The resulting 

changes in streamflow and suspended-sediment loads were 

evaluated at the inflow to RCHRES 97 (fig. 9), which is 

considered the inflow to the Nueces Estuary. The changes 

in streamflow and suspended-sediment loads resulting from 

adjustments of selected parameters are listed in table 13.

Simulated streamflow was relatively insensitive to 

adjustments of any of the selected parameters. One reason is 

that most of the streamflow, which is simulated in the model 

as an input boundary condition, originates as inflow from 

Lake Corpus Christi releases. In the sensitivity analysis, the 

parameters that had the largest effects on simulated suspendedsediment loads were the index to infiltration capacity of 

soil (INFILT), lower-zone nominal storage (LZSN), and 

coefficient of detached-sediment washoff equation (KSER). 

Overall, changes in suspended-sediment loads, when these 

parameters were adjusted, were comparatively small compared 

with the changes applied to the parameter values. The INFILT 

parameter had the largest effect on simulated suspendedsediment loads; a 40-percent increase in the INFILT 

parameter resulted in a 7.1-percent decrease in the simulated 

suspended-sediment load. For the other parameters examined 

for sensitivity, increases in parameter values of 40 percent 

resulted in changes to the simulated suspended-sediment load 

ranging from -3.2 to 4.9 percent (table 13).

Table 11. Summary of calibrated values for selected hydrologic parameters for the Hydrological Simulation Program—FORTRAN 

model of the lower Nueces River watershed, south Texas.

[PERLND, pervious land surface; --, none; IMPLND, impervious land surface]

Parameter

(appendix 1) Land surface Description Value Unit

AGWETP PERLND Fraction of available evapotranspiration from active groundwater 0 --

AGWRC PERLND Base groundwater recession rate 0.90–0.92 1/day

BASETP PERLND Fraction of available evapotranspiration from baseflow 0.01 --

CEPSC PERLND Interception storage capacity 0.10–0.30 inch

DEEPFR PERLND Fraction of groundwater inflow to deep recharge 0.2 --

INFEXP PERLND Infiltration equation exponent 2.0 --

INFILD PERLND Ratio of maximum to mean infiltration rate 2.0 --

INFILT PERLND Index to infiltration capacity of soil 0.20–0.30 inch/hour

INTFW PERLND Index to interflow 2.0 --

IRC PERLND Interflow recession coefficient 0.50 1/day

KVARY PERLND Groundwater outflow modifier 2.0 1/inch

LSUR PERLND or IMPLND Average length of assumed overland-flow plane 250 foot

LZETP PERLND Lower-zone evapotranspiration 0.2–0.80 --

LZSN PERLND Lower-zone nominal storage 3.2–3.4 inch

NSUR PERLND or IMPLND Manning's n for assumed overland-flow plane 0.2 --

RETSC IMPLND Retention storage capacity of impervious areas 0.1 inch

SLSUR PERLND or IMPLND Average slope of assumed overland-flow plane 0.01–0.04 --

UZSN PERLND Upper-zone nominal storage 0.25–0.30 inch
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River Watershed 35

Table 12. Summary of calibrated values for selected sediment-related parameters for the Hydrological Simulation Program—

FORTRAN model of the lower Nueces River watershed, south Texas.

[PERLND, pervious land surface; IMPLND, impervious land surface; RCHRES, stream/reservoir reach]

Parameter

(appendix 1) Model unit Parameter description Value Unit

KRER PERLND Coefficient of soil-detachment equation 0.07–0.45 complex

JRER PERLND Exponent of soil-detachment equation 2.0 complex

KSER PERLND Coefficient of detached-sediment washoff equation 0.05–0.65 complex

JSER PERLND Exponent of detached-sediment washoff equation 2.0 complex

AFFX PERLND Fraction by which detached sediment decreases daily through soil 

compaction

0.05 1/day

COVER PERLND Fraction of land surface shielded from rainfall erosion 0.10–0.85 none

NVSI PERLND Rate at which sediment enters detached-sediment storage from 

atmosphere

0.3 pound/acre-day

KEIM IMPLND Coefficient of solids washoff equation 0.40 complex

JEIM IMPLND Exponent of solids washoff equation 2.0 complex

ACCSDM IMPLND Solids accumulation rate 0.015 ton/acre-day

RHO RCHRES Density of sediment particle 2.0–2.6 gram/cubic centimeter

M (silt) RCHRES Erodibility coefficient of sediment 0.07–0.12 pound/square foot-day

M (clay) RCHRES Erodibility coefficient of sediment 0.09–0.14 pound/square foot-day

W (silt and 

clay)

RCHRES Settling velocity of sediment particle in still water 0.00001–0.0005 inch/second

TAUCD 

(silt)

RCHRES Critical bed shear stress for sediment deposition 0.03–0.08 pound/square foot

TAUCS 

(silt)

RCHRES Critical bed shear stress for sediment scour 0.10–0.22 pound/square foot

TAUCD 

(clay)

RCHRES Critical bed shear stress for sediment deposition 0.015–0.04 pound/square foot

TAUCS 

(clay)

RCHRES Critical bed shear stress for sediment scour 0.09–0.14 pound/square foot

Table 13. Sensitivity of simulated streamflow volumes and suspended-sediment loads to changes in selected process-related 

parameters for the lower Nueces River watershed model, south Texas.

[DEEPFR, fraction of groundwater inflow to deep recharge; INFILT, index to infiltration capacity of soil; LZSN, lower zone nominal storage; KRER, coefficient 

of soil-detachment equation; KSER, coefficient of detached-sediment washoff equation; M (silt), erodibility coefficient of silt sediment; M (clay), erodibility 

coefficient of clay sediment]

Parameter

(appendix 1) Initial value Adjusted value

Change in 

parameter value1

(percent)

Change in 

streamflow volume1

(percent)

Change in suspendedsediment load1

(percent)

DEEPFR 0.20 0.28 40 -0.6 -0.5

INFILT 0.20–0.30 0.28–0.42 40 -0.2 -7.1

LZSN 3.20–3.40 4.48–4.76 40 -1.0 -3.2

KRER 0.07–0.45 0.10–0.63 40 0 0

KSER 0.05–0.65 0.07–0.91 40 0 4.9

M (silt) 0.07–0.12 0.10–0.17 40 0 2.1

M (clay) 0.09–0.14 0.13–0.20 40 0 1.1

1

Simulation period for sensitivity analyses, 2001–8. 
36 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Model Limitations

Errors in model calibration can be classified as systematic 

or measurement errors (Raines, 1996). Systematic errors are 

those that arise because of the model’s failure to adequately 

represent the hydrologic and water-quality processes of the 

study watershed. As a result, there are limits to how well 

model parameters and equations can replicate the complex 

physical properties of streamflow and water-quality processes. 

Measurement errors are those that are introduced as a 

result of inaccurate or missing data. The degree to which 

available rainfall data represent actual rainfall is potentially 

the most serious source of measurement error associated with 

this model; limitations in the amount of available rainfall 

data are often a serious source of measurement error for a 

watershed modeling study (Ockerman and Roussel, 2009). In 

the lower Nueces River watershed model, rainfall was applied 

to the study area using data from two locations—Mathis and 

Robstown NWS stations. Long-term rainfall totals from these 

two stations are likely representative of long-term study-area 

conditions. However, the lack of additional rainfall gages 

severely limits spatial resolution of rainfall input to the 218-

mi2

 study area, especially for relatively large runoff events. 

Typically, measurements of extreme values of rainfall at a 

single location, when used to simulate average rainfall for 

larger areas (such as the lower Nueces River watershed), tend 

to overestimate the average rainfall of the watershed (Asquith, 

1999). Also, the overestimation increases as the size of the 

watershed increases; the fewer rain gages available for a study 

area, the greater the probability that rainfall in the study area is 

overestimated during large storms. 

Another limitation of the rainfall data used for the model 

was that all data from the Mathis NWS station and much of 

the data from the Robstown NWS station were only available 

as daily totals. These daily totals were disaggregated to hourly 

values based on theoretical temporal distributions. As a result, 

some loss of accuracy in the timing and intensity of rainfall 

might have been incorporated into the model results. 

Overall, streamflow was simulated with reasonable 

accuracy at the streamflow-gaging stations in the study area. 

Most of the streamflow originating in the study area was 

from Lake Corpus Christi releases and was measured by the 

Mathis gage. The Bluntzer gage was a partial-record station 

and did not record streamflows greater than 2,750 ft3

/s. 

Similar to streamflow at the Mathis gage, streamflow at the 

Bluntzer gage was largely controlled by releases from Lake 

Corpus Christi. The accuracy of simulated streamflow at the 

Bluntzer gage might have been diminished during periods 

when additional runoff downstream from Lake Corpus Christi 

was occurring. Streamflow simulation results at the Calallen 

gage were generally very good. Low flows at the Calallen 

gage (streamflows of less than about 10 ft3

/s) exhibited greater 

percentage error compared with higher flows (table 5). The 

errors in low flows at the Calallen gage were possibly a result 

of withdrawals upstream from the gage, which were only 

available as monthly totals. Because only monthly totals 

were available to represent the withdrawals upstream, hourly 

and daily streamflows at the Calallen gage during low-flow 

periods were subject to greater uncertainty compared with 

monthly or annual flows.

Suspended-sediment simulations also were affected 

by uncertainties associated with rainfall and streamflow. 

For example, overestimation of rainfall (in particular, 

rainfall intensity) would increase soil particle detachment 

and washoff of sediment from PERLNDs, resulting in 

possible oversimulation of runoff and suspended-sediment 

concentrations and loads.

How well the model represents basinwide suspendedsediment yields for various land types is somewhat uncertain. 

Calibration of basinwide suspended-sediment yields depends 

on reasonable estimates of sediment yields from various 

land types. Several studies of suspended-sediment yields for 

specific land types provided guidance for establishing target 

yields for calibration. The target yields were reasonably 

simulated by the model—albeit the sediment-yield studies 

used to establish the targets were limited in scope compared 

with the size of the study watershed and variety of land uses 

and land covers in the lower Nueces River watershed. 

Overall, suspended-sediment calibration results were 

very good for monthly and annual suspended-sediment 

loads (table 10). Relatively large errors were associated 

with simulated daily suspended-sediment loads and with 

suspended-sediment load estimated for individual samples 

(tables 8 and 9).

Estimated Suspended-Sediment Loads, 

by Source, to the Nueces Estuary, 

1958–2010

The calibrated model of the lower Nueces River 

watershed was used to estimate suspended-sediment loads 

to the Nueces Estuary for 1958–2010, as well as sediment 

loading rates (mass per unit area) and sediment loads 

simulated for the different land types and within different 

geographic areas. 

The location chosen for modeling delivery of the 

suspended-sediment load to the Nueces Estuary was USGS 

streamflow-gaging station 08211502 Nueces River near 

Odem, Tex. (hereinafter the Odem gage) (site 11, fig. 10; 

table 4. The Odem gage is located in the tidal reach of 

the Nueces River at Interstate Highway 37, about 1.1 mi 

downstream from the Calallen gage and 0.2 mi downstream 

from the confluence of Hondo Creek and the Nueces River 

(fig. 2) (near the confluence of RCHRESs 95 and 96 or the 

inflow to RCHRES 97; fig. 9). 

Estimated annual suspended-sediment loads, in tons per 

day, transported during 1958–2010 to the Nueces Estuary are 

shown on figure 15 and listed in table 14. The annual mean 

suspended-sediment load was highly variable, ranging from 
Estimated Suspended-Sediment Loads, by Source, to the Nueces Estuary, 1958–2010 37

an estimated 3.8 tons/d in 1962 to 2,490 tons/d in 1967. The 

daily mean suspended-sediment load during 1958–2010 was 

an estimated 297 tons/d. The median suspended-sediment load 

was an estimated 133 tons/d.

Estimated annual mean sediment loads at the Nueces 

Estuary (Odem gage) were compared with estimated sediment 

loads at the Mathis gage (outflow of Lake Corpus Christi) 

and at the Bluntzer and Calallen gages (table 15). These 

comparisons give an indication of sources of sediment 

delivered to the Nueces River and Nueces Estuary. 

Estimated annual suspended-sediment loads in the lower 

Nueces River simulated by the model for each major source of 

sediment in the study area are listed by category in table 16. 

On average, an estimated 288 tons/d of suspended sediment 

were delivered to the lower Nueces River; an estimated 278 

tons/d were delivered to the Nueces Estuary. Releases from 

Lake Corpus Christi delivered an estimated 94 tons/d of 

suspended sediment or 33 percent of the 288 tons/d estimated 

to have been delivered to the lower Nueces River. This 

indicated that, on average, about 194 tons/d of sediment, or 

67 percent of the estimated 288 tons/d total, were generated 

from erosion of land surfaces and stream-channel bed and 

banks. The largest source of sediment originating from within 

the study area was generated from cropland, about 113 tons/d, 

or about 39 percent of the total estimated for all sources each 

year, on average. Erosion of stream-channel bed and banks 

accounted for, on average, 45 tons/d, or about 16 percent 

of the estimated total suspended-sediment load. All other 

land categories, except cropland, accounted for an estimated 

36 tons/d, or 12 percent of the total. An estimated 10 tons/d of 

suspended sediment or 3 percent of the suspended-sediment 

load delivered to the lower Nueces River were removed by 

water withdrawals before reaching the Nueces Estuary. 

Model results indicate most of the sediment load in the 

Nueces River consists of silt and clay, defined as particle sizes 

less than 0.0625 millimeter. At the Bluntzer gage, simulated 

silt and clay loads composed about 99 percent of the total 

suspended-sediment load. At the Calallen gage, simulated 

silt and clay loads composed about 98 percent of the total 

suspended-sediment load. 

Figure 15. Estimated annual streamflow and suspended-sediment loads to the Nueces Estuary, south Texas, 1958–2010.

0

 500

1,000

1,500

2,000

2,500

3,000

Annual mean suspended-sediment load, in tons per day

0

1,000

2,000

3,000

4,000

Annual mean streamflow, in cubic feet per second

1960 1970 1980 1990 2000 2010

Year

Streamflow

Suspended-sediment load

EXPLANATION
38 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Annual streamflows and suspended-sediment loads to 

the Nueces River and Nueces Estuary varied depending 

on rainfall and streamflow conditions. Annual suspendedsediment loads delivered to the estuary ranged from 3.8 tons/d 

in 1962 to 2,490 tons/d in 1967. Large rainfall and runoff 

events contributed most of the streamflow and suspendedsediment loads to the Nueces River. Whether the runoff 

occurred upstream or downstream from Lake Corpus Christi 

affected the amount of annual suspended-sediment loads that 

were contributed by each of the main sources shown in table 

16. During some years with relatively low annual average 

streamflow, net stream-channel erosion was negative (1962, 

1984, 1988, 1989, 1995, and 2009), indicating net annual 

sediment deposition to the stream channel. Simulation results 

indicate that the largest sediment loads transported to the 

Nueces Estuary as a result of stream-channel bed and bank 

erosion occurred during years with relatively large annual 

mean streamflows (for example, 1958, 1967, and 1971). 

During low-flow years, relatively large percentages of the 

total suspended sediment transported to the Nueces River 

were removed by water withdrawals. For example, 2008 

and 2009 were low-flow years; the annual mean streamflow 

of the Nueces River to the Nueces Estuary was 64 and 

27 ft3/s, respectively, compared with annual mean streamflow 

of 717 ft3/s measured during 1958–2010. About 44 percent 

and 50 percent of the total annual suspended-sediment load 

was removed in 2008 and 2009, respectively, by water 

withdrawals.

Table 14. Estimated annual streamflows and suspended-sediment loads to the Nueces Estuary, south Texas, 1958–2010.

[tons/day, tons per day]

Year

Annual mean 

streamflow 

(cubic feet 

per second)

Annual 

suspendedsediment 

load 

(tons)

Annual mean 

suspendedsediment 

load 

(tons/day)

1958 2,110 215,000 588

1959 617 70,100 192

1960 742 334,000 914

1961 508 108,000 296

1962 38 1,390 3.8

1963 58 2,820 7.7

1964 421 50,000 137

1965 524 48,400 133

1966 601 141,000 386

1967 2,730 909,500 2,490

1968 1,040 101,000 277

1969 472 327,000 895

1970 559 60,400 165

1971 3,810 452,910 1,240

1972 462 33,400 91

1973 1,690 292,000 799

1974 575 42,100 115

1975 509 39,000 107

1976 1,480 108,000 296

1977 737 40,000 110

1978 321 24,500 67

1979 589 48,600 133

1980 863 189,000 517

1981 1,610 158,000 433

1982 238 12,500 34

1983 115 11,100 30

1984 139 15,800 43

Year

Annual mean 

streamflow 

(cubic feet 

per second)

Annual 

suspendedsediment 

load 

(tons)

Annual mean 

suspendedsediment 

load 

(tons/day)

1985 835 304,000 832

1986 131 4,880 13

1987 1,150 138,000 378

1988 118 61,300 168

1989 91 2,220 6.1

1990 454 22,700 62

1991 256 16,300 45

1992 1,450 177,000 485

1993 291 22,300 61

1994 254 11,600 32

1995 193 11,100 30

1996 113 9,720 27

1997 491 105,000 287

1998 507 67,400 185

1999 255 14,600 40

2000 154 19,500 53

2001 373 29,000 79

2002 3,160 272,000 745

2003 703 35,700 98

2004 1,280 68,200 187

2005 303 31,700 87

2006 73 3,700 10

2007 1,550 101,000 277

2008 64 2,320 4.8

2009 27 1,760 14

2010 174 10,400 28

1958–2010 

annual average

717 101,000 278
Estimated Suspended-Sediment Loads, by Source, to the Nueces Estuary, 1958–2010 39

Table 15. Estimated annual suspended-sediment loads at selected stations, lower Nueces River watershed, south Texas, 1958–2010.

[suspended-sediment loads in tons per day]

Year 08211000 Nueces River 

near Mathis, Tex.

08211200 Nueces River 

at Bluntzer, Tex.

08211500 Nueces River 

at Calallen, Tex.

08211502 Nueces River 

near Odem, Tex.

1958 304 502 608 588

1959 70 180 200 192

1960 70 630 789 914

1961 45 236 277 296

1962 5.9 8.3 8.1 3.8

1963 6.3 13 13 7.7

1964 46 133 143 137

1965 64 126 141 133

1966 50 315 359 386

1967 616 1,651 2,166 2,491

1968 134 224 279 277

1969 33 602 756 895

1970 61 131 171 165

1971 646 805 1,188 1,238

1972 45 85 102 91

1973 203 545 726 799

1974 61 108 118 115

1975 59 100 120 107

1976 164 261 301 296

1977 93 102 121 110

1978 30 62 68 67

1979 57 106 133 133

1980 111 350 460 517

1981 202 353 416 433

1982 28 28 43 34

1983 9.0 30 31 30

1984 7.6 28 41 43

1985 72 567 709 832

1986 13 18 18 13

1987 142 290 350 378

1988 10 129 145 168

1989 10 8.8 10 6.1

1990 50 63 67 62

1991 18 33 47 45

1992 156 372 444 485

1993 23 58 62 61

1994 19 34 35 32

1995 15 26 33 30

1996 10 27 28 27

1997 33 214 256 287

1998 43 134 166 185

1999 24 38 44 40

2000 10 26 52 53

2001 39 65 80 79

2002 578 569 736 745

2003 90 90 115 98

2004 154 171 192 187

2005 26 74 82 87

2006 6.7 11 13 10

2007 221 221 282 277

2008 5.9 6.8 11.1 6.4

2009 7.3 10 4.0 4.8

2010 6.8 27 28 29

1958–2010 average 94 207 260 278
Table 16. Estimated annual streamflows and suspended-sediment loads, by sediment source, simulated by the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, south Texas, 1958–2010—Continued.

Year

Annual mean 

streamflow, 

Nueces River 

to Nueces 

Estuary 

(cubic feet 

per second)1

Suspended-sediment load 

(tons per day)

Inflow 

associated 

with Lake 

Corpus 

Christi 

releases

Erosion and 

washoff from 

cropland

Erosion and 

washoff from 

other land 

categories

Erosion 

from streamchannel bed 

and banks2

Total 

transported 

to lower 

Nueces River

Removal 

by water 

withdrawals

Total 

transported 

to Nueces 

Estuary

1958 2,110 304 26 18 240 588 20 608

1959 617 70 31 17 89 207 15 192

1960 742 70 677 114 82 943 29 914

1961 508 45 140 44 80 309 13 296

1962 38 5.9 0.0 4.7 -1.8 9 5.0 3.8

1963 58 6.3 0.1 7.1 0.1 14 5.9 7.7

1964 421 46 15 9.0 78 148 11 137

1965 524 64 6.2 5.0 68 143 10 133

1966 601 50 171 57 123 401 15 386

1967 2,730 616 1,236 416 234 2,500 12 2,490

1968 1,040 135 39 15 99 288 11 277

1969 472 33 668 201 3.0 905 10 895

1970 559 61 39 16 64 180 15 165

1971 3,810 647 240 74 295 1,260 16 1,240

1972 462 45 6.4 6.6 45 103 12 91

1973 1,690 203 384 132 101 820 21 799

1974 575 61 22 7.5 32 123 7.7 115

1975 509 59 37 11 20 127 20 107

1976 1,480 164 38 13 94 309 13 296

1977 737 93 4.1 3.2 22 122 12 110

1978 321 30 18 10 15 73 6.3 67

1979 589 58 37 15 33 143 9.5 133

1980 863 112 304 88 26 530 13 517

1981 1,610 202 124 44 77 447 14 433

1982 238 28 4.3 4.4 6.2 43 8.9 34

1983 115 9.0 19 5.2 3.3 37 6.5 30

1984 139 7.6 32 11 -2.0 49 5.6 43

1985 835 72 567 189 23 851 19 832

1986 131 13 0.1 2.5 2.5 18 5.1 13

1987 1,150 142 147 52 46 387 9.3 378

1988 118 10 155 19 -6.1 178 9.9 168

1989 91 10 0.2 2.9 -2.6 11 4.4 6.1

1990 454 50 12 4.7 2.6 69 7.3 62

1991 256 18 12 7.9 14 52 6.5 45

1992 1,450 156 215 75 51 497 12 485

1993 291 23 24 11 11 69 7.7 61

40 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Table 16. Estimated annual streamflows and suspended-sediment loads, by sediment source, simulated by the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, south Texas, 1958–2010. 
Estimated Suspended-Sediment Loads, by Source, to the Nueces Estuary, 1958–2010 41

Estimated Suspended-Sediment Yields by 

Subwatershed for Selected Runoff Events during 

2009–10 

Sediment yield, a measure of the load-producing 

characteristics of a watershed, is computed by dividing the 

sediment load of the watershed by the drainage area of the 

watershed:

Y = L/A (4)

where

Y is the suspended-sediment yield, in tons per 

acre;

L is the suspended-sediment load, in tons; and

A is the drainage area of the watershed, in acres.

The updated model was used to estimate suspendedsediment yields for each of the subwatersheds represented 

in the model (fig. 9) for selected events during 2009–10. 

Figures 16, 17, and 18 show sediment yield (tons per acre), 

by subwatershed, for the runoff events of November 20–21, 

2009; September 7–8, 2010; and September 20–21, 2010; 

respectively.

The three runoff events during 2009–10 (figs. 16–18) 

were characterized by relatively intense rainfall centered 

near the study area, generating runoff mostly in the lower 

Nueces watershed, with very little contribution of streamflow 

upstream from Wesley E. Seale Dam; therefore, nearly 

the entire suspended-sediment load delivered to the lower 

Nueces River during these events originated downstream from 

the dam. 

Table 16. Estimated annual streamflows and suspended-sediment loads, by sediment source, simulated by the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, south Texas, 1958–2010—Continued.

Year

Annual mean 

streamflow, 

Nueces River 

to Nueces 

Estuary 

(cubic feet 

per second)1

Suspended-sediment load 

(tons per day)

Inflow 

associated 

with Lake 

Corpus 

Christi 

releases

Erosion and 

washoff from 

cropland

Erosion and 

washoff from 

other land 

categories

Erosion 

from streamchannel bed 

and banks2

Total 

transported 

to lower 

Nueces River

Removal 

by water 

withdrawals

Total 

transported 

to Nueces 

Estuary

1994 254 19 6.4 4.4 8.0 38 5.8 32

1995 193 15 17 7.1 -2.5 37 6.6 30

1996 113 10 14 6.5 2.4 33 5.9 27

1997 491 33 181 49 35 298 11 287

1998 507 43 108 31 10 192 7.2 185

1999 255 24 8.4 5.5 8.2 46 6.1 40

2000 154 10 29 10 10 59 6.1 53

2001 373 39 20 8.4 17 85 5.6 79

2002 3,160 578 63 16 96 753 8.4 745

2003 703 90 0.7 3.9 19 114 16 98

2004 1,280 154 14 8.5 18 195 7.5 187

2005 303 26 44 17 6.1 93 6.1 87

2006 73 6.7 1.6 3.9 2.3 15 4.5 10

2007 1,550 221 14 7.4 41 284 7.7 276

2008 64 5.9 0.7 3.6 1.5 12 5.3 6.4

2009 27 7.3 1.5 5.1 -4.6 9 4.5 4.8

2010 174 6.8 2.1 9.8 14 33 4.5 28

1958–2010 

average 

annual

717 94 113 36 45 288 10 278

1

Simulated streamflow at the Nueces River at IH-35, near Corpus Christi, Tex.

2

Negative number indicates a net suspended-sediment deposition to stream reaches.
42 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 16. Model-estimated sediment yield by subwatershed for the runoff event of November 20–21, 2009, lower Nueces River 

watershed, south Texas.

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

LIVE OAK COUNTY

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

88

100

83.9 43.9

57.5

65.3

46.1

21.1 7.8

26.6

10.5

22.9 57.4

27 33

43.6 54.4 26.1

87

36.3 31.6

4.4

89.1

70.8

1.5

3.8

18.1

27

23

47.3 36.8

1.1

0.2

0.2

46.9

25.1

28.9

0.5

5.5

0.3

1.6

0.2

0.6

0.9 0.3

6.1

4.8 9.6

28.4

33.1

9.3

11.3

8.6

7.7 1

82.6

73.3

7

34.8

0.4

8.2

7.6

0.5

4

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Sediment yield, in tons per acre—Value 

 is model-estimated sediment yield,

 by subwatershed

 0.20 to 10

 10.01 to 20

 20.01 to 40

 40.01 to 60

 60.01 to 100

Lower Nueces River watershed boundary

1.5

73.3

57.5

36.8

10.5
Estimated Suspended-Sediment Loads, by Source, to the Nueces Estuary, 1958–2010 43

Figure 17. Model-estimated sediment yield by subwatershed for the runoff event of September 7–8, 2010, lower Nueces River 

watershed, south Texas.

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

LIVE OAK COUNTY

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

87.8

100

82.7 43.3

56.4

62.3

45.6 17.4 7.1

25.2

2.6

3.4

56.8

26.5 32.4

43.1 54 25.5

3

2.2

3 3.3

1.8

1.4

2.7

2.9

3.1

2.7

10.1

0.5 0.9

1.8

0.2

0.3

2.7

0.2

2.6

0.9

3.4

0.5

2.9

0.2

1.2

1.6 0.3

11.2

8.8 17.7

1

2

3.8

3.4

3.8

4 1.8

81.8

6.9

12.8

33.4

0.8

15

13.9

1

3

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Sediment yield, in tons per acre—Value 

 is model-estimated sediment yield,

 by subwatershed

 0.20 to 10

 10.01 to 20

 20.01 to 40

 40.01 to 60

 60.01 to 100

Lower Nueces River watershed boundary

0.5

87.8

56.8

33.4

17.4
44 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Figure 18. Model-estimated sediment yield by subwatershed for the runoff event of September 20–21, 2010, lower Nueces River 

watershed, south Texas.

Nueces 

River

Rincon 

Lake

Corpus

Christi

Bayou

BEE COUNTY

SAN PATRICIO COUNTY

JIM WELLS COUNTY

NUECES COUNTY

LIVE OAK COUNTY

97°40'

97°45'

97°50'

97°55'

28°10'

28°5'

28°0'

27°55'

Base modified from U.S. Geological Survey digital data, 1:24,000

Texas Centric Mapping System, Albers Equal Area Projection 

North American Datum of 1983

37.5

1.5

111.3

126

105.4 58

73.9

81.8

60.5

27.9 14.4

37.1 17.3

35.1

74.6

38.6 45.6

58 71.3

126.8

55 48.2

7.5

130.2

104.7

7.4

7.4

28.7

40.6

33.5

71.1 54.9

3.5

1.1

1.8

69.7

38.8

43.7

9

11.3

5.1

9.6

0.2

5.7

4.1

0.6

22.8

20.5 33.3

43

50.8

15.6

17.3

14.9

5.1 12.9

104.6

105.9

24.6

46.5

28.2

26.1

1.9

6.3

0 2 4 8 MILES

0 2 4 8 KILOMETERS

6

6

EXPLANATION

Sediment yield, in tons per acre—Value 

 is model-estimated sediment yield,

 by subwatershed

 0.20 to 10

 10.01 to 20

 20.01 to 40

 40.01 to 60

 60.01 to 100

 100.01 to 140

Lower Nueces River watershed boundary

7.4

105.9

81.8

55

33.5

17.3
Summary 45

Sediment yields during the three runoff events were 

influenced by the quantity, intensity, and distribution of 

rainfall. The average sediment yield for the entire lower 

Nueces River watershed was 27.2 tons/acre during the 

November 20–21, 2009, runoff event; 16.3 tons/acre during 

the September 7–8, 2010, event; and 40.3 tons/acre during 

the September 20–21, 2010, event. During all three events, 

sediment yields generally were larger in the subwatersheds 

that are primarily cropland. Inspection of figures 16–18 shows 

that the Bayou Creek subwatersheds (model subwatersheds 

9–18, fig. 9), in the northern part of the study area consistently 

produced some of the greatest yields of sediment. Analysis 

of model results for the November 2009 runoff event shows 

that the Bayou Creek subwatersheds contributed about 54 

percent of the simulated suspended-sediment load at the 

primary Nueces River withdrawal locations for Corpus Christi 

area industrial and public supply (subwatersheds 82 and 84, 

fig. 9). Runoff from the Bayou Creek subwatersheds during 

November 2009 was likely the primary source of suspended 

sediment that contributed to high concentrations of suspended 

sediment in the lower Nueces River during the period when 

the City of Corpus Christi failed to meet treatment standards 

for turbidity in drinking water.

Summary

The loss of sediment loading is an ecological problem 

in the Nueces River watershed in south Texas. The reduction 

in sediment loads to the Nueces Estuary is the result of 

sedimentation in large impoundments, notably Lake Corpus 

Christi, a reservoir whose storage volume was greatly enlarged 

in 1958 compared to its original (1935) impoundment 

capacity; construction in 1958 of Wesley E. Seale Dam 

expanded the existing conservation storage volume of Lake 

Corpus Christi from 43,800 to 257,260 acre-feet. The U.S. 

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

Army Corps of Engineers–Fort Worth District (USACE), and 

the City of Corpus Christi, developed, calibrated, and tested 

a Hydrological Simulation Program—FORTRAN (HSPF) 

watershed model to simulate streamflow and suspendedsediment concentrations and loads during 1958–2008 in the 

lower Nueces River watershed, downstream from Lake Corpus 

Christi to the Nueces Estuary in south Texas. The original 

model and report of the lower Nueces River watershed model 

were published in 2010. 

Another issue related to suspended sediment is periods of 

high suspended-sediment concentration during and after runoff 

events. In November 2009, the City of Corpus Christi failed to 

meet State water-treatment standards for turbidity in drinking 

water because of high concentrations of suspended sediment 

in raw water withdrawn from the Nueces River near Calallen. 

At the time, the Nueces River was experiencing runoff 

caused by heavy rainfall in the study area. During this event, 

releases from Lake Corpus Christi were minimal and runoff 

originated almost entirely from the subwatersheds downstream 

from Lake Corpus Christi and the Wesley E. Seale Dam. 

Accordingly, the USGS, in cooperation with the USACE; 

City of Corpus Christi; Guadalupe-Blanco River Authority; 

San Antonio River Authority; and San Antonio Water System 

completed a study using an updated version of the previously 

developed watershed model to characterize sources of 

suspended sediment during a November 2009 runoff event 

and during selected runoff events in 2010 and characterized 

sediment yields from selected subwatersheds in the lower 

Nueces River watershed. During 2010, additional suspendedsediment data were collected and the model simulation period 

was extended through 2010. The original model calibration 

parameters were used along with 2009–10 input data to 

simulate streamflow and suspended-sediment concentrations 

and loads during 1958–2010. 

Data available to simulate suspended-sediment 

concentrations and loads consisted of historical sediment data 

collected during 1942–82 by the Texas Water Development 

Board (TDWB) and its predecessor agencies at sites in the 

study area, and suspended-sediment concentration data 

collected periodically by the USGS during 2006–7 and 2010 

at three sites—USGS streamflow-gaging station 08211000 

Nueces River near Mathis, Tex. (Mathis gage), station 

08211200 Nueces River at Bluntzer, Tex. (Bluntzer gage), 

and station 08211500 Nueces River at Calallen, Tex. 

(Calallen gage), and during 2010 at 08211050 Bayou Creek 

near Mathis, Tex.

During 1942–82, TWDB and its predecessor agencies 

monitored suspended-sediment loads of the Nueces River at a 

site on the Nueces River at the Mathis gage. The Mathis gage 

is downstream from Wesley E. Seale Dam and was completed 

in 1958 to impound Lake Corpus Christi. Suspended-sediment 

data collected before and after completion of Wesley E. Seale 

Dam provide insights to the effects of the dam and reservoir 

on suspended-sediment loads transported by the lower Nueces 

River downstream from the dam to Nueces Bay. Suspendedsediment loads measured at the Mathis gage decreased after 

completion of the dam in 1958. Annual suspended-sediment 

loads exceeded 100,000 tons during 9 of the 16 years 

before completion of the dam but only during 2 years after 

completion of the dam. In the 21 years following completion 

of the dam, for which reliable suspended-sediment data were 

available (1962–82), annual suspended-sediment loads were 

considerably lower at the Mathis gage for a given annual mean 

discharge than before the dam was completed.

Most of the suspended sediment transported by the 

Nueces River downstream from Wesley E. Seale Dam 

occurred during high-flow releases from the dam or during 

floods. During October 1964–September 1971, about 536,000 

tons of suspended sediment were transported by the Nueces 

River past the Mathis gage. Of this amount, about 473,000 

tons, or about 88 percent, were transported by large runoff 

events (mean streamflow exceeding 1,000 cubic feet per 

second). Furthermore, the suspended-sediment transport rate 

increases with higher magnitude flows.
46 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

To develop the watershed model to simulate suspendedsediment concentrations and loads in the lower Nueces River 

watershed, streamflow simulations were calibrated and tested 

with available data for 2001–10. Streamflow data measured 

during 2005–8 at the Calallen gage were used to calibrate 

the model and streamflow data for 2001–4 and 2009–10 

were used to test the calibrated model. Streamflow data for 

2005–6 from the Bluntzer gage were used to test streamflow 

simulations at that site. Streamflow data from the Mathis gage 

were used as input to the model at the upstream boundary of 

the model. Simulated streamflow volumes for the Bluntzer 

and Calallen gages showed good agreement (within 2 and 3 

percent, respectively) with measured streamflow volumes. 

Annual, monthly, and daily coefficients of determination 

of the linear regression between measured and simulated 

streamflow and Nash-Sutcliffe coefficients of model-fit 

efficiency are considered acceptable for both gages.

The HSPF model was calibrated to simulate suspended 

sediment using suspended-sediment data collected at the 

Mathis, Bluntzer, and Calallen gages during 2006–7. Soil 

erosion and sediment washoff from various land types 

simulated by the model were calibrated by comparing 

simulated suspended-sediment loads from the various land 

types to estimates of suspended-sediment runoff yields 

determined from other studies. Parameters related to sediment 

transport were calibrated primarily by comparing measured 

suspended-sediment concentrations and loads at the three 

gages with simulated suspended-sediment concentrations and 

loads. Evaluation of model simulation results showed that 

2009–10 model simulations of streamflow and suspendedsediment concentrations and loads compared favorably with 

measured streamflow and suspended-sediment data.

The watershed model was used to estimate streamflow 

and suspended-sediment loads for 1958–2010, including 

suspended-sediment loads transported to the Nueces Estuary. 

During 1958–2010, on average, an estimated 288 tons per 

day (tons/d) of suspended sediment were delivered to the 

lower Nueces River; an estimated 278 tons/d were delivered 

to the estuary. The annual suspended-sediment load was 

highly variable, depending on the occurrence of runoff events 

and high streamflows. During 1958–2010, the annual total 

sediment loads to the estuary varied from an estimated 3.8 

to 2,490 tons/d. On average, 113 tons/d, or about 39 percent 

of the estimated annual suspended-sediment contribution, 

originated from cropland in the study watershed. Releases 

from Lake Corpus Christi delivered an estimated 94 tons/d 

of suspended sediment or about 33 percent of the 288 tons/d 

estimated to have been delivered to the lower Nueces River. 

Erosion of stream-channel bed and banks accounted for 

45 tons/d or about 16 percent of the estimated total suspendedsediment load. All other land categories, except cropland, 

accounted for an estimated 36 tons/d, or about 12 percent of 

the total. An estimated 10 tons/d of suspended sediment or 

about 3 percent of the suspended-sediment load delivered to 

the lower Nueces River were removed by water withdrawals 

before reaching the Nueces Estuary. 

Annual streamflows and suspended-sediment loads to the 

Nueces River and Nueces Estuary varied depending on rainfall 

and streamflow conditions. Large rainfall and runoff events 

contributed most of the streamflow and suspended-sediment 

loads to the Nueces River. 

Simulation results indicate that the largest sediment loads 

transported to the Nueces Estuary as a result of stream-channel 

bed and bank erosion occurred during high-flow years and that 

during low-flow years, large percentages of total suspended 

sediment transported to the Nueces River were removed by 

water withdrawals.

To help understand sources of suspended sediment during 

local runoff events, the updated model was used to estimate 

suspended-sediment yields for each of the subwatersheds 

represented in the model for selected events during 2009–

10 (November 20–21, 2009, September 7–8, 2010, and 

September 20–21, 2010). These events were characterized by 

intense rainfall that occurred mostly downstream from Wesley 

E. Seale Dam; therefore, streamflow and suspended sediment 

originating from the Nueces River watershed upstream from 

the study area were minimal compared with runoff and 

associated sediment loads from the study area. Subwatersheds 

with the greatest sediment yield (tons per acre) during these 

events generally were cropland. The cropland areas of the 

Bayou Creek subwatersheds, in the northern part of the study 

area, typically had relatively large sediment loads during all 

three events. Analysis of model results for the November 

2009 runoff event shows that the Bayou Creek subwatersheds 

contributed about 54 percent of the simulated suspendedsediment load at the primary Nueces River withdrawal 

locations for Corpus Christi industrial and public supply 

use. Runoff from the Bayou Creek subwatersheds during 

November 2009 was likely the primary source of suspendedsediment that contributed to high concentrations of suspended 

sediment in the lower Nueces River during the period when 

the City of Corpus Christi failed to meet treatment standards 

for turbidity in drinking water.

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Appendix 1—Hydrological Simulation Program—FORTRAN 

(HSPF) Parameters Used to Simulate Hydrologic and Sediment 

Processes in the Lower Nueces River Watershed, South Texas

Appendix 1

Appendix 1 53

Appendix 1

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas.

[--, none; HSPF, Hydrological Simulation Program—FORTRAN]

Pervious Land (PERLND)

Secondary 

module Parameter Unit Description

Water balance

Interception storage

PWATER CEPSC inch Interception storage capacity 

CEPS inch Initial interception storage

Surface and subsurface storages

UZSN inch Upper-zone nominal storage; an index to amount of depression and surface-layer storage of a 

pervious area

LZSN inch Lower-zone nominal storage; an index to soil-moisture-holding capacity

SURS inch Initial surface storage

IFWS inch Initial interflow storage

UZS inch Initial upper-zone storage

LZS inch Initial lower-zone storage

AGWS inch Initial active-groundwater storage

Evapotranspiration

LZETP -- Lower-zone evapotranspiration; an index to density of deep-rooted vegetation on a pervious area

AGWETP -- Fraction of available potential evapotranspiration demand from active groundwater

BASETP -- Fraction of available potential evapotranspiration demand from baseflow (groundwater outflow)

Recession rates

KVARY 1/inch Groundwater outflow modifier; an index of how much effect recent recharge has on groundwater 

outflow

AGWRC 1/day Basic groundwater recession rate if KVARY is zero and there is no inflow to groundwater 

IRC 1/day Interflow recession coefficient

GWVS inch Index to groundwater slope

Infiltration

INFILT inch/hour Index to infiltration capacity of soil

INFILD -- Ratio of maximum to mean infiltration rate of a pervious area

INFEXP -- Infiltration equation exponent

INTFW -- Index to amount of water that infiltrates and flows as interflow (shallow subsurface runoff)

DEEPFR -- Fraction of groundwater inflow to deep recharge
54 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas—Continued.

Pervious Land (PERLND)—Continued

Secondary 

module Parameter Unit Description

Water balance—Continued

Overland flow

LSUR foot Average length of overland-flow plane

SLSUR -- Average slope of overland-flow plane

NSUR -- Average roughness coefficient of overland-flow plane

Soil erosion

SEDMNT SMPF -- Management factor to account for use of erosion-control factors

KRER complex Coefficient of soil-detachment equation

JRER complex Exponent of soil-detachment equation

AFFIX 1/day Fraction by which detached sediment decreases daily through soil compaction

COVER -- Fraction of land surface shielded by vegetation or mulch from erosion by direct rainfall impact

NVSI pound/acre/

day

Rate at which sediment enters detached-sediment storage from atmosphere

KSER complex Coefficient of detached-sediment washoff equation

JSER complex Exponent of detached-sediment washoff equation

KGER complex Coefficient of matrix soil scour equation

JGER complex Exponent of matrix soil scour equation 
Appendix 1 55

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas—Continued.

Impervious Land (IMPLND)

Secondary 

module Parameter Unit Description

Water balance

IWATER LSUR foot Average length of overland-flow plane

SLSUR -- Average slope of overland-flow plane

NSUR -- Average roughness coefficient of overland-flow plane

RETSC inch Retention storage capacity of impervious areas

RETS inch Initial retention storage

SURS inch Initial overland-flow storage

Sediment washoff

SOLIDS KEIM complex Coefficient of solids washoff equation

JEIM complex Exponent of solids washoff equation

REMSDP 1/day Fraction of solids removed on each day without runoff

ACCSDP ton/acredaySolids accumulation rate

SLDS ton/acre Initial storage of solids

Stream Reaches (RCHRES)

Secondary 

module Parameter Unit Description

Water balance

HYDR FTABNO -- Number of F-table that contains RCHRES geometric and hydraulic properties

LEN mile Length of stream reach

DELTH foot Drop in water elevation within stream reach

STCOR foot Correction in reach depth to calculate stage

KS -- Weighting factor for flow routing

DB50 millimeter Median diameter of bed sediment

ADCALC CRRAT -- Ratio of maximum velocity to mean velocity in reach cross section under typical flow conditions
56 Sources of Suspended-Sediment Loads in the Lower Nueces River Watershed, Downstream from Lake Corpus Christi

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas—Continued.

Stream Reaches (RCHRES)—Continued

Secondary 

module Parameter Unit Description

Sediment transport

SEDTRN BEDWID foot Width of streambed

BEDWRN foot Depth of streambed

POR -- Porosity of streambed

D inch Effective diameter of sediment particle

W inch/second Settling velocity of sediment particle in still water

RHO gram/cubic 

centimeter

Density of sediment particle

KSAND complex Coefficient of HSPF sand-load equation

EXPSND complex Exponent of HSPF sand-load equation

TAUCD pound/square foot Critical bed shear stress for sediment deposition

TAUCS pound/square foot Critical bed shear stress for sediment scour

M pound/square 

foot-day

Erodibility coefficient of sediment

BEDDEP foot Initial thickness of bed material

SSAND milligram/liter Initial concentration of sand in suspension

SSILT milligram/liter Initial concentration of silt in suspension

SCLAY milligram/liter Initial concentration of clay in suspension

FRACSAND -- Initial fraction by weight of sand in bed material

FRACSILT -- Initial fraction by weight of silt in bed material

FRACCLAY -- Initial fraction by weight of clay in bed material

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