SUMMER TIME RESPONSE OF STREAMFLOW TO VARIATION IN LITHOLOGY AND SNOWPACK IN THE GREATER YELLOWSTONE REGION

Understanding hydrologic contributions to baseflow during the late summer is becoming increasingly important as ecosystems in the Western U.S. may suffer from greater water stress due to the combined effects of drought and climate change. Low flows during the summer dry season function to sustain ecological communities and allow humans to meet their water needs. The delayed release of groundwater contributes significantly to baseflow ensuring that low flows persist through the dry season (Godsey et al., 2014). Previous hydrogeomorphic studies have shown that permeable volcanic landscapes in the Cascade Range function to create groundwater dominated basins (Jefferson et al., 2010; Mayer and Naman 2011). The Rhyolite flows of the Greater Yellowstone Region (GYR) provide an example of a geomorphic setting that may also exhibit similar groundwater dominated basins related to the high permeability of a volcanic landscape. Here, we address the following questions How do the various streams in the GYR respond over the summer on average due to variation in rock type? How does the age of rhyolite flows per basin influence summer time hydrograph character? How does summer streamflow vary based on snowpack year? We hypothesize that: Basins with a greater proportion of rhyolite will exhibit a higher baseflow relative to peak flow, basins with a greater percentage of younger rhyolite flow will show higher baseflow relative to peak flow, and in a higher snowpack year we will see a higher peak flow and a more rapid decrease in discharge over the summer. It is predicted that the high permeability of rhyolite and geomorphic processes working over time will contribute to basins with more rhyolite and basins with younger rhyolite showing less of a decrease in streamflow over the summer. Additionally, we predict that a higher snowpack year would contribute to a more saturated subsurface environment and lead to more rapid outflow of groundwater. The fundamental step in testing our hypotheses will be the creation of a GIS map, where basins will be delineated using U.S. Geological Survey gauging stations. This work contrasted with other studies (Burnett, et al., 2018 Unpublished data) of spring-fed streamflow in the GYR may help in elucidating the ecological importance of delayed groundwater as a source of relief for drought and climate change stressed biota.