HYDROGEOLOGIC DELINEATION OF HABITATS FOR ENDANGERED SPECIES - THE COMAL SPRINGS RIVER SYSTEM/

We delineate 18 distinct habitats for the endangered fountain darter i n the Comal Springs/River system based upon a combination of vegetativ e, geologic, and hydrologic criteria. Comal Springs are the largest na tural discharge from the Edwards aquifer of central Texas; they issue along faults which penetrate the confined portion of the aquifer. The springs are the head of the Comal River which is an important economic and recreational resource to the local community. The spring/river sy stem is the home of the fountain darter, a federally listed endangered species. Previous studies of this system were on a large scale and la cked detail necessary for characterization. The fountain darter and ot her fauna in the Comal Springs/River system are threatened by the poss ibility of diminished flows and the concomitant habitat change. The 18 habitats are defined by the dominant species of vegetation, the subst rate, and the flow conditions. Human alterations and spring discharge control the morphology of the river and the substrates. Stream velocit ies determined substrate conditions and, thus indirectly, the distribu tion of vegetation and habitats. In this system, hydrochemical paramet ers were uniform as were the soils adjacent to the stream;these factor s were not important to the habitat delineation. The range of stream v elocities necessary to maintain the habitats can be estimated with the Hjulstrom diagram. The darters were associated with a number of these habitats. We surmise that significant alteration of the system's habi tats will occur if discharge conditions change.