U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3019

Revised April 2012

Science To Support the Understanding of South Texas SurfaceWater and Groundwater Resources in a Changing Landscape

Printed on recycled paper

Introduction

Against a backdrop of constant cycles of 

extreme hydrologic conditions ranging from 

oppressive droughts to life-threatening floods, 

the water-resource landscape of south Texas is 

undergoing constant change. Demands on water 

resources are increasing because of changes 

related to population growth, energy demands, 

agricultural practices, and other human-related 

activities. In south Texas, the Nueces, San 

Antonio, and Guadalupe River Basins cover 

approximately 50,000 square miles and include 

all or part of 45 counties (fig. 1). These stream 

systems transect the faulted and fractured 

carbonate rocks of the Edwards aquifer recharge 

zone and provide the largest sources of recharge 

to the aquifer. As the streams make their way 

to the Gulf of Mexico, they provide water for 

communities and ecosystems in south Texas and 

deliver water, sediment, and nutrients to the south 

Texas bays and estuaries. 

The U.S. Geological Survey (USGS) works 

in cooperation with other local, State, and Federal 

agencies to provide timely access to water 

data, publications, and information to foster a 

better understanding of the water resources of 

south Texas. The USGS and our cooperators 

are involved in a wide variety of programs 

for collecting hydrologic data and scientific 

information in the changing landscape of south 

Texas to help our cooperators effectively address 

water-resource issues in this part of the State. 

This fact sheet provides an overview of our 

collaborative scientific endeavors in the basins of 

the Nueces, San Antonio, and Guadalupe Rivers 

and lower Rio Grande. An overview of USGS 

capabilities pertaining to water resource issues 

in Texas, including recently completed and 

ongoing studies in south Texas, is available at 

http://tx.usgs.gov/Capabilities/index.html. 

SOUTH

TEXAS Study area basins

TEXAS

UNITED STATES

MEXICO

Nueces River Basin

San Antonio River Basin

Rio Grande River Basin

Guadalupe River Basin

WEBB

COUNTY

DUVAL

COUNTY

FRIO

COUNTY

BEE

COUNTY

KERR

COUNTY

EDWARDS

COUNTY

UVALDE

COUNTY

STARR

COUNTY

BEXAR

COUNTY

KENEDY

COUNTY

KINNEY

COUNTY

HIDALGO

COUNTY

DIMMIT

COUNTY

ZAVALA

COUNTY

LA SALLE

COUNTY

MEDINA

COUNTY

REAL

COUNTY

ZAPATA

COUNTY

HAYS

COUNTY

LAVACA

COUNTY

MAVERICK

COUNTY

LIVE OAK

COUNTY

FAYETTE

COUNTY

ATASCOSA

COUNTY

GOLIAD

 COUNTY

BROOKS

COUNTY JIM HOGG

COUNTY

DEWITT

COUNTY

GONZALES

COUNTY

MCMULLEN

COUNTY

NUECES

 COUNTY

WILSON

COUNTY

KLEBERG

COUNTY

VICTORIA

COUNTY

CAMERON

COUNTY

KARNES

COUNTY

REFUGIO

COUNTY

BASTROP

COUNTY

JACKSON

COUNTY

BANDERA

COUNTY

COMAL

COUNTY

JIM WELLS

COUNTY

KENDALL

COUNTY

WILLACY

COUNTY

GUADALUPE

 COUNTY

BLANCO

COUNTY

CALDWELL

COUNTY

SAN

PATRICIO

COUNTY

CALHOUN

COUNTY

WHARTON

COUNTY

COLORADO

COUNTY

ARANSAS

COUNTY

Frio

Medina

San 

Nueces

Rio Grande

 River

 River

River

 Antonio

 River

 River

Lake

Corpus

Christi

Choke

Canyon

Reservoir

Canyon

Lake

International

 Falcon

 Reservoir

Creek

Oso

 Gulf of Mexico

Guadalupe

Corpus Christi

Laredo

San

Antonio

San

Marcos

Oso Creek

Basin

29°

28°

27°

26°

100°

97°

98° 99°

EXPLANATION

Major aquifer (Ashworth

 and Hopkins, 1995)

 Trinity aquifer

 Edwards aquifer—Recharge zone

 Edwards aquifer—Subcrop

Basin boundary

0 25 50 MILES

0 25 50 KILOMETERS

Base modified from U.S. Geological Survey

1:24,000-scale digital data

Texas Albers

North American Datum of 1983

Basins modified from Texas Water

 Development Board, 2009, and

 Ockerman and Fernandez, 2010

Figure 1. South Texas basins, aquifers, and major hydrologic features.
Streamflow Data Available Online

The USGS—in cooperation with numerous local, State, and Federal cooperators—operates 

a network of more than 100 streamflow-gaging stations in a 25-county area of south Texas. These 

stations typically record data at 15-minute intervals and transmit the data to USGS offices every 

hour. Within minutes, the near real-time streamflow data are then posted to the Internet and available 

for viewing. In addition to these data, streamflow statistics, annual peak streamflows, and field 

measurement data from each station are also available on the Internet. Some of the uses of streamflow 

data include flood forecasting, water supply allocation, and hydraulic analyses. 

http://waterdata.usgs.gov/tx/nwis/rt/

USGS South Texas Water Science Activities

Groundwater Data for Texas and the Nation

The USGS groundwater database consists of more than 850,000 records of wells, springs, test 

holes, tunnels, drains, and excavations in the United States. Available site information includes well 

location information such as latitude and longitude, well depth, and the aquifer in which the well 

is completed. The USGS annually monitors groundwater levels in thousands of wells in the United 

States. Groundwater-level data are collected and stored either as discrete water-level measurements 

or as continuous time-series data from automated recorders. Data from some of the continuous 

record stations are relayed to USGS offices nationwide, providing public access to near real-time 

groundwater data. 

http://waterdata.usgs.gov/nwis/gw

Near Real-Time Water-Quality Monitoring

The USGS operates a network of near real-time water-quality monitoring stations that provides 

in-stream and groundwater-quality measurement data, including temperature, specific conductance, 

pH, dissolved oxygen, and turbidity. These data are needed for decision making regarding drinking 

water, water treatment, regulatory programs, recreation, healthy ecosystems, and public safety. 

Continuous, near real-time information on water quality is a vital asset that helps safeguard lives and 

property and ensures adequate water resources. 

http://water.usgs.gov/owq/topics.html

Assessment of Streamflow Gains and Losses in the Guadalupe River Basin

The USGS—in cooperation with the Edwards Aquifer Authority—has evaluated data from 

USGS streamflow-gaging stations to determine streamflow gains and losses on reaches of the 

Guadalupe River and its major tributaries. The assessment also determined the relative contribution 

of major springs to streamflow in selected reaches of the Guadalupe River. During the selected 

short-term periods of August 2000 and August 2006 (periods of relatively low flow), springflow in 

defined sections of the Guadalupe River accounted for as much as 77 percent in 2000 and 78 percent 

in 2006 of total flow, compared with a springflow contribution of 18 percent during the long-term 

period of 1987–2006. 

http://pubs.usgs.gov/sir/2008/5165/

Trinity Aquifer Springs Survey

The USGS—in cooperation with the Trinity Glen Rose Groundwater Conservation District, the 

San Antonio River Authority, and the Edwards Aquifer Authority—is documenting the location and 

the physicochemical properties of the springs issuing out of the Trinity aquifer in northern Bexar 

County. The karstic Trinity aquifer is a major source of water for central Texas. Springs issuing from 

the Trinity aquifer ultimately supply recharge either to deeper portions of the Trinity aquifer or to the 

Edwards aquifer that is exposed downgradient. 

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5165

In cooperation with the Edwards Aquifer Authority

Streamflow Conditions in the Guadalupe River Basin, 

South-Central Texas, Water Years 1987–2006—An 

Assessment of Streamflow Gains and Losses and 

Relative Contribution of Major Springs to Streamflow

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USGS South Texas Water Science Activities

Understanding the Complex Geology of the Trinity Aquifer

As part of the National Cooperative Geologic Mapping Program, the USGS will use 

multidisciplinary science to help characterize the geologic framework of the Trinity aquifer of south 

Texas. The southern part of the Trinity aquifer forms the catchment area for the Edwards aquifer and 

might contribute to the water budget of the Edwards aquifer by subsurface flow across formation 

boundaries at considerable depths. The study area will be in south Texas north of San Antonio. In this 

area, the Trinity aquifer system is the primary source of water.

Hydrogeologic and Geophysical Investigations of the Edwards Aquifer in Northeastern 

Bexar County

The USGS—in cooperation with the San Antonio Water System—investigated hydrogeologic 

and geophysical properties to characterize the hydrostratigraphic zones and hydrogeologic features of 

the Edwards aquifer in a 16-square-kilometer area in northeastern Bexar County. Some areas on the 

Edwards aquifer recharge zone are undergoing rapid development, and the aquifer can be susceptible 

to contamination because of the presence of karst features, such as sinkholes and caves, that provide 

direct paths for urban runoff to move from the surface to the aquifer. The study identified areas where 

the Edwards aquifer might be susceptible because of urban development in northeastern Bexar County. 

http://pubs.usgs.gov/sir/2008/5181/

Water Budget Model for the Lower San Antonio River Basin

The USGS—in cooperation with the San Antonio River Authority, the Evergreen Underground 

Water Conservation District, and the Goliad County Groundwater Conservation District—configured, 

calibrated, and tested a watershed model for a study area consisting of about 2,150 square miles of the 

lower San Antonio River Basin in Bexar, Guadalupe, Wilson, Karnes, DeWitt, Goliad, Victoria, and 

Refugio Counties in south Texas. The model simulated streamflow, evapotranspiration, and groundwater 

recharge by using rainfall, meteorological data, and streamflow data. Future uses of the model could 

include simulating the effects of changes in land-cover and water use on water availability. 

http://pubs.usgs.gov/sir/2010/5027/

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5226

In cooperation with the U.S. Department of Agriculture Natural Resources Conservation Service, 

the Edwards Region Grazing Lands Conservation Initiative, the Texas State Soil and Water 

Conservation Board, the San Antonio River Authority, the Edwards Aquifer Authority, Texas 

Parks and Wildlife, the Guadalupe Blanco River Authority, and the San Antonio Water System

Effects of Brush Management on the Hydrologic Budget and 

Water Quality In and Adjacent to Honey Creek State Natural 

Area, Comal County, Texas, 2001–10

A Paired Watershed Study on the Effects of Brush Management on the Hydrologic 

Budget

The USGS—in cooperation with the Natural Resources Conservation Service and numerous 

others—is studying the effects of removing woody vegetation that might have encroached into historical 

savanna grassland ecosystems in Texas. In this study, a paired watershed approach was implemented 

to evaluate the effects of brush control on the hydrologic budget. In addition to daily rainfall and 

streamflow, evapotranspiration data were collected from 2001 to 2010 by using the Bowen ratio method. 

After measuring hydrologic conditions at both sites for the first 2 years of the study, the treatment 

watershed had selected brush removal of Juniperus ashei. The subsequent data were analyzed to detect 

changes in the hydrologic budget as a result of brush removal. 

http://pubs.usgs.gov/sir/2011/5226/

Sources of Recharge to San Marcos Springs

The USGS—in cooperation with the San Antonio Water System—has collected groundwater, 

surface-water, and water-quality data to define and characterize the sources of recharge to springs in 

San Marcos, a main discharge point for the Edwards aquifer and the second largest spring complex 

in Texas. From December 2008 to December 2010, geochemical and hydrologic data were collected 

from groundwater, surface-water, and spring sites at or near San Marcos. Numerical and statistical 

methods, including geochemical modeling with PHREEQC, were used to evaluate the data to define 

and characterize local and regional sources of recharge to springs in San Marcos. 
USGS South Texas Water Science Activities

Sources of Streamflow in the Upper Nueces River Basin

The USGS—in cooperation with The Nature Conservancy and the U.S. Army Corps of Engineers–

Fort Worth District—is measuring streamflow to help identify springs in the upper reaches of the 

Nueces River Basin, where streams are sustained by flow from springs located primarily in the 

uppermost reaches of the basin. Understanding of the location and quantities of springflow is important 

for preserving springs and the habitat they provide. The streamflow measurements are supplemented 

by analysis of water-quality samples, collected from groundwater wells, spring orifices, and streams, to 

help characterize groundwater contributions to streamflow.

Characterization of Stormwater Runoff in the Edwards Aquifer Recharge Zone

The USGS—in cooperation with the San Antonio Water System—is continuing to study the 

potential effect of runoff water quality on groundwater recharge for intermittent stream reaches within 

an active Edwards aquifer recharge zone. The primary goal of the study is to understand the influence 

of urban development on water quality within the Edwards aquifer, the major water source for the San 

Antonio area. The stream reaches being studied flow only during large rainfall events and for short 

periods of time. Therefore, sampling and data collection take place immediately following storm events 

to capture representative surface water that is likely to enter the Edwards aquifer. This information can 

be used by natural resource managers to protect groundwater quality in the Edwards aquifer.

Edwards Aquifer Groundwater Modeling

The USGS is active in the development and evaluation of numerical groundwater-flow models 

of the Edwards aquifer in south Texas. Regional-scale models have been developed by using 

MODFLOW, which emphasizes (1) conduit development and conduit flow (conduit-flow model) and 

(2) flow predominantly through a network of numerous small fractures and openings (diffuse-flow 

model). A local-scale model with 12 hydrogeologic units and 92 model layers was completed in 

2010 to evaluate the movement of water and solutes to a selected public-supply well. The USGS 

also has evaluated various models with respect to accessibility and ease of use, range of conditions 

simulated, accuracy of simulations, agreement with dye-tracer tests, and limitations of the models. 

http://pubs.usgs.gov/sir/2009/5183/

Sediment Monitoring in the Lower San Antonio and Guadalupe Rivers

The USGS—in cooperation with the San Antonio River Authority and the Texas Water 

Development Board—initiated a study to characterize sediment dynamics in the lower San Antonio 

River. The study’s first objective is to evaluate sediment transport in the lower San Antonio River and 

the Guadalupe River through collection of suspended-sediment, bedload, turbidity, and ancillary data. 

The second objective is to develop a sediment model for the lower San Antonio River to characterize 

sediment loading in these basins. This study will aid in understanding how urban development in the 

San Antonio area might be contributing excess sediment in the upper part of the basin. This study will 

also contribute to the understanding of coastal sediment delivery and how sediment loads are related to 

in-stream flows.

Quantities and Sources of Suspended Sediment to Coastal Bays and Estuaries

The 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 

watershed model. The model simulated the streamflow, suspended-sediment concentrations, and 

suspended-sediment loads during 1958–2008 in the Nueces River Basin, downstream from Lake 

Corpus Christi in south Texas. The model estimated contributions of suspended-sediment loads from 

various sources. Cropland in the basin was the largest source of suspended sediment, providing about 

38 percent of the total suspended-sediment load to the estuary. 

http://pubs.usgs.gov/sir/2010/5194/
USGS South Texas Water Science Activities

Agricultural Runoff in the Texas Coastal Bend

The USGS—in cooperation with Texas State Soil and Water Conservation Board, Coastal Bend Bays 

and Estuaries Program, and the Texas AgriLife Research and Extension Center at Corpus Christi—studied 

cropland runoff quality in the Oso Creek Basin in the Texas Coastal Bend. Land use surrounding Oso Creek 

is largely agricultural. Runoff samples collected during and after storm events were analyzed for nutrients, 

major inorganic ions, pesticides, and bacteria. Quantities of constituents in runoff were compared with 

quantities of fertilizers and pesticides applied to croplands. A better understanding of runoff patterns will 

contribute to future studies that focus on potential impacts to regional bays and estuaries. 

http://pubs.usgs.gov/sir/2010/5136/

Contaminants in Streambed and Suspended Sediments in the San Antonio Area

Elevated concentrations of sediment-associated contaminants are typically found in urban areas, 

including San Antonio, the seventh most populous city in the United States. During 2007–9, the USGS—in 

cooperation with the San Antonio River Authority and the San Antonio Metropolitan Health District Public 

Center for Environmental Health—collected streambed sediment samples during base flow and suspendedsediment samples during stormwater runoff at 20 sites in the San Antonio River Basin. The samples were 

analyzed for major and trace elements and organic compounds including halogenated compounds and 

polycyclic aromatic hydrocarbons. 

http://pubs.usgs.gov/sir/2011/5097/

Assessing the San Antonio River Watershed for Possible Contaminants from Hydraulic 

Fracturing Fluid and Water Produced by Oil and Gas Activities

The recent increase in hydraulic fracturing of gas and oil plays has contributed to a surge in oil and gas 

development, and in Texas there has been rapid development in the San Antonio River watershed. These 

activities have elicited concerns from the public about the potential risks to human and environmental 

health. The USGS—in cooperation with the San Antonio River Authority and the Guadalupe–Blanco 

River Authority—will evaluate a suite of water-quality constituents to help detect and monitor for possible 

hydraulic fracturing-derived contaminants in surface water and sediments in the lower San Antonio River 

Basin. This study will provide much needed baseline data for a broad spectrum of potential contaminants that 

might be associated with oil and gas production there.

Pharmaceutical Compounds, Hormones, and Other Emerging Contaminants in the San 

Antonio River Basin, South Texas

The USGS—in cooperation with the San Antonio River Authority—is assessing the occurrence and 

concentrations of approximately 100 different emerging contaminants at selected locations in the San 

Antonio River Basin. Emerging contaminants represent a new category of environmental pollutants that are 

derived from municipal, agricultural, and industrial wastewater sources. These chemicals include human and 

veterinary drugs, antibiotics, hormones, detergents, disinfectants, plasticizers, fire retardants, insecticides, 

and antioxidants. This study will provide information on the presence of detectable concentrations of those 

compounds in the San Antonio River Basin. These data can be used by resource professionals to assess 

vulnerability of public water supplies and ecosystems to pharmaceutical, hormone, and other emerging 

contaminants. 

National Water-Quality Assessment Program

The USGS National Water-Quality Assessment (NAWQA) Program provides comprehensive 

information about water-quality conditions, evaluates changes in water quality over time on a national 

scale, and assesses how the environment and human activities affect water quality. Regional and national 

assessments are possible because of a consistent study design and uniform methods of data collection and 

analysis. Monitoring data are integrated with geographic information concerning hydrological characteristics, 

land use, and other landscape features in models to enhance our understanding of water-quality issues. Local, 

State, and national stakeholders use NAWQA Program information to design and implement strategies for 

managing, protecting, and monitoring water resources in many different hydrologic and land-use settings 

across the Nation. 

http://water.usgs.gov/nawqa/
—Darwin J. Ockerman, Travis J. Garcia, and Stephen P. Opsahl

This fact sheet is available online at http://pubs.usgs.gov. 

For additional information, contact 

Mark Null, Office Chief, jmnull@usgs.gov

U.S. Geological Survey Texas Water Science Center

South Texas Program Office

5563 De Zavala Rd., Suite 290

San Antonio, TX 78249

Phone: 210–691–9200 Fax: 210–691–9270

http://tx.usgs.gov/

Publishing support provided by

Lafayette Publishing Service Center

References

Ashworth, J., and Hopkins, J., 1995, Aquifers of Texas: Austin, Texas Water 

Development Board, Report 345.

Banta, J.R., and Slattery, R.N., 2011, Effects of brush management on the 

hydrologic budget and water quality in and adjacent to Honey Creek State 

Natural Area, Comal County, Texas, 2001–10: U.S. Geological Survey Scientific Investigations Report 2011–5226, 35 p., appendixes available online. 

(Also available at http://pubs.usgs.gov/sir/2011/5226/.)

Lindgren, R.J., Taylor, C.J., and Houston, N.A., 2009, Description and evaluation of numerical groundwater flow models for the Edwards aquifer, 

south-central Texas: U.S. Geological Survey Scientific Investigations Report 

2009–5183, 25 p. (Also available at http://pubs.usgs.gov/sir/2009/5183/.)

Lizarraga, J.S., and Ockerman, D.J., 2010, Simulation of streamflow, evapotranspiration, and groundwater recharge in the lower San Antonio River 

watershed, south-central Texas, 2000–2007: U.S. Geological Survey 

Scientific Investigations Report 2010–5027, 41 p. (Also available at 

http://pubs.usgs.gov/sir/2010/5027/.)

Ockerman, D.J., and Fernandez, C.J., 2010, Hydrologic conditions and water 

quality of rainfall and storm runoff for two agricultural areas of the Oso 

Creek watershed, Nueces County, Texas, 2005–08: U.S. Geological Survey 

Scientific Investigations Report 2010–5136, 63 p. (Also available at 

http://pubs.usgs.gov/sir/2010/5136/.)

Ockerman, D.J., and Heitmuller, F.T., 2010, Simulation of streamflow and suspended-sediment concentrations and loads in the lower Nueces River watershed, downstream from Lake Corpus Christi to the Nueces Estuary, South 

Texas, 1958–2008: U.S. Geological Survey Scientific Investigations Report 

2010–5194, 50 p. (Also available at http://pubs.usgs.gov/sir/2010/5194/.)

Ockerman, D.J., and Slattery, R.N., 2008, Streamflow conditions in the 

Guadalupe River Basin, south-central Texas, water years 1987–2006—An 

assessment of streamflow gains and losses and relative contribution of 

major springs to streamflow: U.S. Geological Survey Scientific Investigations Report 2008–5165, 22 p. (Also available at http://pubs.usgs.gov/

sir/2008/5165/.)

Shah, S.D., Smith, B.D., Clark, A.K., and Payne, J.D., 2008, An integrated 

hydrogeologic and geophysical investigation to characterize the hydrostratigraphy of the Edwards aquifer in an area of northeastern Bexar County, Texas: 

U.S. Geological Survey Scientific Investigations Report 2008–5181, 26 p. 

(Also available at http://pubs.usgs.gov/sir/2008/5181/.)

Texas Water Development Board, 2009, Major river basins in Texas: accessed 

February 27, 2012, at http://www.twdb.state.tx.us/mapping/maps/pdf/

mrb_8X11.pdf. 

U.S. Geological Survey, 2011, Selected water-quality topics—Learn more about 

water quality: accessed February, 9, 2012, at http://water.usgs.gov/owq/

topics.html.

U.S. Geological Survey National Stream Quality Accounting Network, 2009, 

Monitoring the water quality of the Nation’s large rivers, Rio Grande 

NASQAN Program: accessed February 9, 2012, at http://water.usgs.gov/

nasqan/docs/riogrndfact/riogrndfactsheet.html.

U.S. Geological Survey National Water Information System, 2011a, Web interface—USGS groundwater data for the Nation: accessed February 9, 2012, at 

http://waterdata.usgs.gov/nwis/gw.

U.S. Geological Survey National Water Information System, 2011b, Web interface—USGS real-time water data for Texas: accessed February 9, 2012, at 

http://waterdata.usgs.gov/tx/nwis/rt/.

U.S. Geological Survey National Water-Quality Assessment Program, 2012, 

Web page—National Water-Quality Assessment (NAWQA) Program: 

accessed February 9, 2012, at http://water.usgs.gov/nawqa/.

Wilson, J.T., 2011, Assessment of selected contaminants in streambed- and 

suspended-sediment samples collected in Bexar County, Texas, 2007–09: 

U.S. Geological Survey Scientific Investigations Report 2011–5097, 57 p. 

(Also available online at http://pubs.usgs.gov/sir/2011/5097/.)