The Blanco River of central Texas provides an important hydrologic link between surface and groundwater as it traverses two major karst aquifer systems—the Trinity and Edwards Aquifers. The Blanco River is characterized by alternating gaining and losing stretches due to the presence of springs that discharge water into the river and swallets that drain water from the river. The region consists primarily of Lower Cretaceous limestone, dolomite, and marls. One of the more significant springs along the Blanco River is Pleasant Valley Spring. During below-average flow conditions, Pleasant Valley Spring becomes the headwaters of the Blanco River even though the headwaters, under wet conditions, are about 50 km upstream. Water that enters the Edwards Aquifer from the Blanco River can eventually discharge at both San Marcos Springs to the south and Barton Springs to the north. During periods of extreme drought, when other recharging streams are dry, the Blanco River can provide enough water to the Edwards Aquifer that will help maintain flow at Barton Springs where endangered species of salamanders need sufficient flow of high-quality groundwater. In the western part of the study area, increasing rates of pumping from the Trinity Aquifer, combined with impact from drought, are reducing heads in the aquifer and are subsequently reducing springflows (such as from Pleasant Valley Spring) that sustain the Blanco River. Decreasing flow in the Blanco River can lead to less recharge to the Edwards Aquifer and less discharge from San Marcos and Barton Springs. A better understanding of these aquifer systems and how they are influenced by the Blanco River is important for management of groundwater in an area undergoing significant population growth.
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