KEN SAUNDERS, KARIM AZIZ, ROY KLEINSASSER, KEVIN MAYES, RANDY MOSS, GORDON LINAM, Texas Parks and Wildlife Department, River Studies Program, P.O. Box 947, San Marcos, Texas 78667, USA

The upper San Marcos River is fed by thermally constant springs emanating from the Edwards Aquifer and has historically been home to four federally listed species: Texas wild rice Zizania texana, fountain darter Etheostoma fonticola, San Marcos Gambusia Gambusia georgei, and San Marcos salamander Eurycea nana. As a result of increased pumping from the aquifer and the resulting concerns about preserving springflows, the Texas Parks and Wildlife Department initiated an instream flow study in April 1993. A combination of hydrologic modeling, habitat mapping, and biological survey data are being used along with water quality data to define flow requirement for the affected species. A series of 27 transects were place in the river and data has been collected during three different flow regimes. Information collected along each transect included depth, water surface elevation, velocity, substrate types, and coverage by aquatic macrophytes. Faunal characterization occurred with a series of grid quadrats.