Detailed hydrogeologic maps of the Comal and San Marcos Rivers for endangered species habitat definition, Texas

The Edwards aquifer of Texas is a regionally-extensive carbonate aquifer, and Comal and San Marcos springs are its largest natural discharge points. They issue from faults in the confined portion of the aquifer to create Comal and San Marcos Rivers, which create habitat for a number of endangered species including the fountain darter (etheostoma fonticola), Texas wild rice (zinzania texana), the San Marcos gambusia (gambusia georgei), and the San Marcos salamander (eurycea nana). Protection of the endangered species living in the spring systems provided the impetus for this study. Pumping from wells in the Edwards has been increasing over the past few decades, and if this trend continues, natural springs in the aquifer will eventually cease to flow. The species in the rivers are not only endangered because of low populations, but also by the possibility of a loss of spring discharge. There are many different endangered species habitats present in the Comal and San Marcos systems, and this thesis presents maps and data on their characteristics. Hydrochemical, substrate, soil, vegetation, and flow information are included in these characteristics. Substrates have been separated into size classes to create maps of the river beds. Hydrochemical parameters in the rivers were examined showing remarkable uniformity in their measurements along the entire lengths of both rivers, including a year-round nearly constant temperature. The soils surrounding each river were mapped and their hydrologic characteristics were examined. Vegetation in the rivers was also mapped, because of its importance to the habitats. The plants provide a structure that protects the endangered species from high flow velocities in the rivers, and they help stabilize the substrate. Flow characteristics, locations of seeps and springs, and velocity measurements, have been mapped for each river. Habitats are defined through the combination of vegetation, substrate, soil, hydrochemistry, and flow type for different areas of both rivers. The flow velocities in the rivers are the most important characteristic, because it is the flow rate that controls the morphology of the entire river and the substrate that is present on the river bed. Through control of the substrate, the flow rate indirectly controls vegetation and habitat distributions in the rivers. Ranges of flow velocities necessary to maintain habitats in both rivers are estimated, because a change in the flow rate would alter the distribution of substrate on the river beds, and the current habitats would no longer exist. The Comal and San Marcos Rivers are unique because of their combinations of habitats, which is due primarily to the flow velocity variations in the rivers