THE ROLE OF KARST GROUNDWATER IN A SNOWMELT-DOMINATED HYDROLOGIC SYSTEM: THE KAWEAH RIVER, CALIFORNIA, USA

By volume, most water leaving mountain rivers in the western United States is derived directly from snowmelt, but baseflow is often maintained by delayed release from other storage components; primarily groundwater in a variety of aquifer systems. Little work has been done to assess the role of karstic groundwater in these mountain systems. We addressed this knowledge gap by taking four approaches: 1) comparing seasonality of water chemistry from karst springs and river outlets, 2) comparing baseflow recessions of springs and rivers, 3) directly measuring the amount of water discharging from karst springs, and 4) conducting 3-component end-member mixing models to determine the relative contributions of karst and non-karst groundwater to river discharge in the Kaweah River and its five forks (North, Marble, Middle, East, and South), in the Sierra Nevada, California, USA.

The river and springs have statistically similar baseflow recession coefficients (F_{1, 63}= 2.799, p= 0.099) and river waters in basins with significant karst are geochemically similar to the karst springs within each basin. The percentage of total discharge that is derived from karst groundwater varied between forks depending on the amount of karst present, and between years, depending on precipitation. Measured contributions by karst springs varied from 3.5% to 16% during high flow and 20% to 65% during baseflow between forks and 3.5% and 6% during high flow and 20% and 36% during baseflow between years for the entire river. Modeled results were comparable to direct measurements.

These results show that karst aquifers may be the single most important non-snow storage component in the Kaweah River basin: mapped karst represents just 1.4% of the surface area, but water stored in karst represents a much larger portion of river discharge during both high flow and baseflow conditions. This suggests that when karst aquifers are present in mountain systems, even as a spatially small component, they can be a large component of the hydrologic system and provide substantial water storage.