LOOKING AT STREAMBED SEEPAGE IN GREAT BASIN, NEVADA STREAMS: WHAT MULTIPLE METHODS TELL US ABOUT SURFACE WATER – GROUNDWATER INTERACTIONS

Mountain front recharge comprises a large fraction of the water that replenishes Great Basin aquifers on an annual basis. Whereas some of this recharge occurs through fractured rock and percolation, much of it occurs through course-grained and cobble streambeds. In an effort to fully quantify these surface and ground water interactions in one small stream in the southern Snake Range, multiple methods were applied: differential discharge measurements yielded net gains or losses between measurement sites on a single day, streambed piezometers were used to assess local hydraulic conductivity (slug tests), hydraulic gradients, and to house thermal and water level loggers. Time series of temperature at multiple depths were used to estimate time series of streambed seepage rates and combined with hydraulic gradients to estimate time series of hydraulic conductivity at 12 locations along an 8-kilometer reach. Seepage and hydraulic conductivity estimates distributed in both time and space allowed us to characterize the seasonality of recharge, and to identify hydraulic properties of the streambed. Finally, evidence from surface and groundwater chemistry provided clarity in delimiting the relative contributions of source water to the stream, and to confirm the mechanism of calcite precipitation that results in streambed clogging. Although each of the methods contributed an important piece of the puzzle, none alone could fully explain the interactions between surface water and groundwater observed along the stream.