A geomorphic, hydrologic, and geochemical study of Hamilton Creek Watershed, Travis County, Texas

Environmental sensitivity of a small, karst watershed in the Texas Hill Country was evaluated through characterization of basin geomorphology, hydrologogy, and geochemistry. The susceptibility of the Hamilton Creek watershed and Hamilton Pool to water quality degradation from erosion and surface runoff was highlighted by this interdisciplinary approach. Geomorphic and geochemical findings indicate a mechanism for the formation of the waterfall-fed grotto. Evaluation of geomorphology indicated that much of the watershed is on terrane that would contitute a water contamination hazard if the area were urbanized or land-use changes occurred. Those portions of the watershed with the highest erosion and surface runoff potentials were identified from evaluation of surface features (such as soils, geology, and slope) using a Geographic Information System. The watershed is subject primarily to Hortonian flow, thus there is very little potential for attenuation of storm-water contaminants before they enter surface water. The surface-feature analysis included identification of regional lineament orientations. Analysis of these orientations suggested that the rectilinear pattern of Hamilton Creek and the location of Hamilton Pool are due to lineament intersections. The hydrology of the watershed was investigated during steady-state and stormflow conditions. Analysis of a storm hydrograph indicated that the bulk of storm runoff is due to Hortonian flow. A stage-discharge relation was developed using direct and indirect methods of determining discharge, and used to construct a portion of the annual hydrograph. The dependence of flow regime on seasonal controls was indicated by comparison of the annual hydrograph and a seasonal water balance for the watershed. Investigation of the aqueous geochemistry suggested that the watershed may be vulnerable to degradation from groundwater sources in addition to surface water sources. Chemical analyses of springs and surface water indicated that groundwater is contributed to the creek system by both diffuse and conduit flow mechanisms. Those inflows controlled by conduit flow are highly susceptible to contamination, due to flow velocity and lack of filtration. The physical differences between conduit and diffuse flow, notably flow velocity, were represented by differences in magnesium, calcium, and bicarbonate concentrations, pH, and responses of calcium concentration and conductivity to rainfall. Investigation of the geochemistry indicated that dissolution processes could have played a role in pool formation. Determination of dissolved calcite mass flow rate during storms allowed estimation of the time for removal of the required volume of calcite. The contrast in storm and baseflow residence times within the pool suggested that dissolution is still ongoing because several weeks may elapse before flood water, initially undersaturated with respect to calcite, is displaced with baseflow water.