Texas Water Atlas

2. Surface Water and Groundwater

UNLIKE MANY STATES, TEXAS IS NOT dominated by one or two river systems—fifteen major river systems meander through the state. Although the Rio Grande forms the nearly 1,000 mile-long southwestern border of the state, the modest flow of the river limits its influence to a narrow band roughly 20 miles wide until its floodplain expands to nearly 50 miles across in the lower Rio Grande Valley. A series of rivers flowing in a radial pattern into the Gulf of Mexico drain the remainder of the state, except the Trans-Pecos, the northern Texas Panhandle, and the Red River Valley. Examples of major streams that follow this radial pattern flowing in a southeasterly direction into the Gulf of Mexico are the Nueces, San Antonio, Guadalupe, Colorado, Brazos, Trinity , Neches, and Sabine. The Canadian River, a tributary of the Arkansas, created its valley by slicing west to east through the northern Texas Panhandle. The Red River defines the Texas-Oklahoma border for more than 200 miles, yet its small northward-flowing tributaries affect only a narrow ribbon in north and northeast Texas (USGS 2001; TWDB 2002). The thirty-seven hundred named rivers and their tributaries have a total stream length of approximately 80,000 miles and enter the Gulf of Mexico via seven major estuaries to form more than 2 million surface acres of saltwater bays from north to south: Sabine Lake and Sabine-Neches Estuary, Galveston Bay and Trinity-San Jacinto Estuary , Matagorda Bay and Lavaca-Colorado Estuary, San Antonio Bay and Guadalupe Estuary, Aransas Bay and Mission-Aransas Estuary, Corpus Christi Bay and Nueces Estuary, and Baffin Bay and Upper Laguna Madre, each rich biological environments that contain a large percentage of the state’s wetlands. Human activities are increasingly impairing Texas surface waters in serious ways (TPWD 2006; TCEQ 2006). Since the late 1800s, many of Texas’ streams have been dammed, creating more than two hundred major reservoirs. Dams were initially built for generation of hydroelectricity, flood control, and the storage of irrigation waters. More recently, these dams and reservoirs have focused on recreation and aesthetics. All of the early hydroelectric dams, such as those on the lower Guadalupe River system, are small, raising the water surface less than 50 feet. Today, more than 1.2 million acres of lakes cover the Texas landscape (TPWD 2006). Wherever large amounts of water seep into the ground from which the water can eventually be withdrawn, a potential subterranean reservoir is formed. These water-storing strata, or aquifers, provide more than half of all the water consumed in Texas. Most of the state, with the exception of the bedrock mountains of the TransPecos , are underlain by usable amounts of groundwater. Aquifers can be composed of either permeable bedrock or permeable sediments such as sand and gravel. Lining most streams are sedimentary aquifers composed of water-saturated river sediment or alluvium that provides seepage inputs for streams. Aquifers can be classified in several ways, but it is useful to place Texas’ aquifers into three groups. First, unconfined aquifers have water levels directly dependent on recharge and flow to the surface whenever their upper saturated layer, the water table, intersects with the surface . Perhaps the most well-known unconfined aquifer is the Ogallala, which extends north beneath the High Plains from the Texas Panhandle all the way to South Dakota. Second, confined aquifers are saturated layers bounded above and below by largely impervious rocks called aquicludes, which place the contained water under pressure. Confined aquifers, such as those that feed the San Solomon Springs in West Texas, flow under artesian pressure if the confined layer intersects the surface and the water table is sufficiently high. Finally, karst aquifers are contained in limestone and marble rocks that often form “rock sponges” filled with numerous small channels and, in some cases, large underground caverns and “streams.” One of the most famous aquifers in the state, the Edwards 2 Surface Water and Groundwater SURFACE WATER AND GROUNDWATER 19 Aquifer, is a confined karst aquifer from which water freely discharges from a number of springs along the Balcones Escarpment (Petersen 1995; TWDB 2003; Red River Authority 2004; Edwards Aquifer Authority 2004; Save Our Springs, Inc. 2004). The amount of water that can be obtained from an aquifer is dependent on both the total capacity of the aquifer —the ability of water to flow through the aquifer, that is, transmissivity—and the rate of recharge...