SOUTH-CENTRAL TEXAS STUDY UNIT
The SCTX study unit is located in south-central Texas, encompassing the Guadalupe, San Antonio, and Nueces River Basins, two minor coastal basins, and the Trinity, Edwards, Carrizo-Wilcox, and Gulf Coast aquifers where they underlie the three river basins (fig. 1). This region of Texas contains a diversity of both surface-water and aquifer habitats. In addition to lakes and streams of various sizes and geomorphic types, the Edwards aquifer and associated springs provide habitat to a variety of unique aquatic species.
Surface-Water Basins and Ecoregions
The SCTX study unit, with a drainage area of about 30,000 square miles (mi2), encompasses parts of five ecoregions (fig. 1). The ecoregions, as described by Omernik (1987), are the Edwards Plateau, Texas Blackland Prairies, East Central Texas Plains, Western Gulf Coastal Plain, and Southern Texas Plains. The study unit has a wide variety of climatic, geologic, topographic, and hydrologic conditions. The proximity of ecoregions with different characteristics (fig. 1) makes the study unit a "convergent zone" of soil, climatic, topographic, and biotic features. Detailed descriptions of the flora, fauna, and land use are in Blair (1950) and Gould (1975).
The biology of the study-unit streams is determined mostly by the characteristics of the ecoregions they transect. Aquatic life is similar in the upper reaches of each of the three major river basins, as these reaches are within the Edwards Plateau ecoregion. The mostly spring-fed streams in this ecoregion have stable bottom substrates, well-vegetated streambanks, and cool, clear water year round. Invertebrate taxa richness and other measures of aquatic life health used by the TNRCC are consistently greater in central Texas than other regions of the State (Hornig and others, 1995). Invertebrate samples collected from the streams in this area have included more than 50 taxa from 3 square feet (ft2) of stream bottom (Bayer and others, 1992).
The habitat and accompanying biota of the streams in the study-unit basins change substantially in the downstream reaches. The SCTX study unit extends into the Southern Texas Plains (to the southwest), the East Central Texas Plains and Western Gulf Coastal Plain (to the southeast), and the Texas Blackland Prairies (to the east). The streams in these ecoregions are characterized by warm, turbid water; dominated by soft-bottom runs and pools (with only occasional riffles); and bordered by highly erodible streambanks (Bayer and others, 1992). Warm-water, stress-tolerant species predominate in the streams of these ecoregions.
The Guadalupe River originates in Kerr County at about 1,800 feet (ft) above mean sea level and joins the San Antonio River 11 miles (mi) upstream from Guadalupe Bay (part of San Antonio Bay) (fig. 1). The river is about 410 mi long with a drainage area of about 6,000 mi2. The 30-year (1961–91) normal precipitation in the basin ranges from about 30 inches (in.) near the headwaters to about 40 in. near the coast (Dallas Morning News, Inc., 1997, p. 113–118). Annual mean discharge of the Guadalupe River into Guadalupe Bay is 1,867 cubic feet per second (ft3/s) (on the basis of 1935–97 water-year records at USGS streamflow-gaging station 08176500 Guadalupe River at Victoria (Gandara and others, 1998)). Canyon Dam, forming Canyon Lake (fig. 1), was completed in 1964 for flood control, water storage, hydroelectric power generation, and recreational uses. With the closing of the dam, the Guadalupe River became a regulated river over much of its length, rarely subject to the wide range of natural flows that are typical of this region. Daily mean discharge from Canyon Dam ranges from 0.80 to 5,680 ft3/s, and annual mean discharge is 457 ft3/s (on the basis of 1963–97 water-year records, the period of regulated streamflow, at USGS streamflow-gaging station 08167800 Guadalupe River at Sattler (Gandara and others, 1998)). The San Marcos River, with its confluence to the Guadalupe River in Gonzales County (fig. 1) provides the only regular input of substantial flow below Canyon Dam. The San Marcos is a spring-fed river; annual mean discharge from the springs is 170 ft3/s (on the basis of 1957–94 water-year records at USGS streamflow-gaging station San Marcos River springflow at San Marcos (Gandara and others, 1995)). During periods of little or no precipitation, which result in low streamflows in other streams in the basin, the San Marcos River is the major contributor of streamflow in the Guadalupe River.
The San Antonio River (fig. 1) originates in metropolitan San Antonio (1996 estimated population 1.1 million (Dallas Morning News, Inc., 1997)) at about 690 ft above mean sea level. The river flows southeasterly for about 240 mi from the headwaters to its confluence with the Guadalupe River north of Guadalupe Bay, and has a drainage area of about 4,300 mi2. The 30-year normal precipitation in the basin is similar to that in the Guadalupe River Basin. The annual mean discharge of the San Antonio River to the Guadalupe River near Guadalupe Bay is about 723 ft3/s (on the basis of 1924–97 water-year records at USGS streamflow-gaging station 08188500 San Antonio River at Goliad (Gandara and other, 1998)). Stream quality of the San Antonio River is affected a short distance downstream of the headwaters by numerous municipal and industrial wastewater discharges and by urban runoff. During low-flow conditions, flow is predominantly treated wastewater. The Medina River (fig. 1) is a major tributary of the San Antonio River. Annual mean discharge of the Medina River is 206 ft3/s (on the basis of 1939–97 water-year records at USGS streamflow-gaging station 08181500 Medina River at San Antonio (Gandara and others, 1998)). Salado, Leon, and Cibolo Creeks (fig. 1) are minor tributaries that contribute little to the base flow of the San Antonio River. Cibolo Creek begins as a spring-fed creek in the Edwards Plateau, contributing recharge to the Edwards aquifer as it flows across the aquifer recharge zone.
The Nueces River Basin (fig. 1) is the largest of the three major basins in the study unit. The Nueces River originates in Edwards County at about 1,600 ft above mean sea level and flows about 440 mi from the headwaters to its mouth at Nueces Bay. The 30-year normal precipitation (1961–90) in the basin ranges from 21 in. in the upper basin to 35 in. near the coast (Dallas Morning News, Inc., 1997, p. 113–118). Although the Nueces River has a large drainage area (about 17,000 mi2), it has the smallest annual mean discharge of the three major rivers in the study unit—135 ft3/s (on the basis of 1939–97 water-year records at USGS streamflow-gaging station 80192000 Nueces River below Uvalde (Gandara and others, 1998)). The Nueces River and its upstream tributaries, including the Frio and Sabinal Rivers and Seco and Hondo Creeks, originate from seeps and springs in the Edwards Plateau. As the streams cross the Balcones fault zone to the south (fig. 1), a substantial amount of flow from these streams enters the Edwards aquifer. The Nueces River is the only stream in the basin that regularly maintains some flow beyond the recharge zone. Mostly erratic rainfall provides much of the streamflow for the Nueces River and its tributaries south of the Balcones fault zone, with periods of no flow in the lower reaches of the Nueces River.
Two minor basins in the SCTX study unit drain coastal areas directly into the Gulf of Mexico. Streamflows in these basins are primarily dependent on precipitation. The San Antonio-Nueces Coastal Basin has a drainage area of about 2,600 mi2, and the Nueces-Rio Grande Coastal Basin has a drainage area of about 280 mi2. The 30-year (1961–90) normal precipitation in the coastal basins ranges from about 30 to 40 in. (Dallas Morning News, Inc., 1997, p. 113–118).
The Edwards Plateau ecoregion, encompassing about 6,500 mi2 (25 percent of the study unit), also is known locally as the Edwards Plateau or Texas Hill Country. The topography is hilly with elevations from 800 ft to more than 1,800 ft above mean sea level and is commonly incised by streams. The Edwards Plateau receives 16 to 33 in. of precipitation annually, increasing from west to east (Gould, 1975). Soils are mostly shallow, underlain by limestone or caliche. Typical land use is grazed open woodland, grazed forest, and woodland; some subhumid grassland; and semiarid grazing (Anderson, 1970).
Texas Blackland Prairies Ecoregion
The Texas Blackland Prairies ecoregion encompasses about 2,700 mi2 (8.7 percent of the study unit). The topography of the region is gently rolling to relatively flat, with elevations from 300 to 800 ft above mean sea level. The region is well dissected by streams, which allow for rapid drainage. Soils associated with this region are fairly uniform, dark-colored calcareous clays interspersed with some gray, acid sandy loams. Annual precipitation varies from 30 in. for the western part to more than 40 in. for the eastern part (Gould, 1975). Land use is primarily cultivated cropland (Anderson, 1970).
East Central Texas Plains Ecoregion
The East Central Texas Plains ecoregion encompasses about 5,500 mi2 (18 percent of the study unit). The region consists of rolling to hilly landscapes with elevations from about 300 to 800 ft above mean sea level. Annual precipitation varies from 35 to 45 in. (Gould, 1975). Soils range from acid sandy loams or sands to clays. Land use is typically woodland with some cropland and pasture (Anderson, 1970).
Western Gulf Coastal Plain Ecoregion
The Western Gulf Coastal Plain ecoregion encompasses about 3,400 mi2 (11 percent of the study unit). This poorly drained plain is less than 150 ft above mean sea level and is dissected by streams flowing into the Gulf of Mexico. Annual precipitation varies from about 20 in. for western areas to about 50 in. for eastern areas (Gould, 1975). Soils are acid sands, sandy loams, and clays. Cropland and cropland with grazing are the dominant land uses (Anderson, 1970).
Southern Texas Plains Ecoregion
Encompassing about 12,000 mi2 (35 percent of the study unit), the Southern Texas Plains is the largest ecoregion in the study unit. The topography is level to rolling hills with elevations from about 0 to 1,000 ft above mean sea level. Annual precipitation varies from 16 in. for the western part to 35 in. for the eastern part (Gould, 1975). Soils range from clays to clay loams. Predominant land use is grazed open woodland, subhumid grassland, and semiarid grazing land (Anderson, 1970).
The western part of the Edwards aquifer, known as the San Antonio region, extends from Hays County to Kinney County within the SCTX study unit. The deposition of the material that became the carbonate rocks of the Edwards aquifer began almost 100 million years ago in a shallow sea. Repeated submergence and exposure of the carbonate rocks allowed early formation of cavernous porosity. Hundreds of feet of sediments were deposited over this early aquifer, and as the North American continent was slowly uplifted, the Cretaceous seas began to recede, allowing streams to cut into the sediments and expose the underlying Edwards aquifer. A period of extensive faulting during the Miocene (12 to 17 million years ago) resulted in the formation of the Balcones fault zone. With the changes imposed by the new faults, new ground-water movement was manifested in some areas as recharge points and in other areas as resurgence points or springs (Longley, 1986).
The high permeability of the Edwards aquifer results from the freshwater diagenesis of faulted and fractured carbonate rocks. After the rocks were broken and displaced during the Balcones faulting, large quantities of freshwater infiltrated strata that previously had been isolated from the surface (Kastning, 1983). Subsequent faulting processes were initiated that eventually provided an extremely transmissive (fast-moving) ground-water-flow system (Abbott, 1975). The present-day aquifer is riddled with joint cavities and solution channels (caverns) that have evolved through erosional unloading and dissolution. The outcrop area has a porous, honeycombed, or Swiss cheese appearance because of the preferential leaching of soluble materials (Barker and Ardis, 1996).
Many wells penetrate caverns in the area around San Antonio (Livingston, 1947; Petitt and George, 1956). It is estimated that in 1975, wells and springs in Bexar County discharged 259.0 thousand acre-feet (acre-ft) of water from the Edwards aquifer, with about 15 percent of this discharge from springs (Rappmund, 1976, p. 5). In reviewing publications on the hydrology of the Bexar County area, Petitt and George (1956) noted that the well logs of a large percentage of the wells in the San Antonio area included some cavernous areas. These areas could provide sufficient space for propagation of aquatic organisms.
The USGS and various Texas water agencies have conducted analyses on the chemical quality of the Edwards aquifer in the San Antonio region (Garza, 1962; Pearson and Rettman, 1976; Reeves, 1976; Reeves and others, 1972). In general these publications provide information on the geochemistry of the area.
Other publications give insight into how the water movement occurs within the Edwards aquifer in the San Antonio region (Abbott, 1977; Maclay and Small, 1976; Pearson and Rettman, 1976; Pearson and others, 1975; Puente, 1976). In general, the movement in the aquifer is from the west to the east or northeast. Numerous publications discuss the hydrology of the aquifer specifically and include water levels, recharge, discharge, amounts of precipitation, and other hydrologic properties (Follett, 1956; Garza, 1966; Lang, 1954; Maclay and Rettman, 1973; Puente, 1974; Rappmund, 1975, 1977; Rettman, 1969; Sieh, 1975). Hydrologic models have been developed for predictive purposes on the basis of increased population and subsequent increased water use. These models indicate that without additional recharge, the average water level in the aquifer will continue to drop in the future (Wanakule, 1988). Other than a reduction in springflow, it is not clear how water-level declines would affect the availability of habitats for spring and aquifer organisms in the region.
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