GROUNDWATER RESPONSE TO SNOWMELT IN A MOUNTAINOUS WATERSHED

Snowmelt recharge to shallow groundwater systems is the primary source of streamflow in many mountainous watersheds, but characteristics of these systems are not well understood, and their contribution to streamflow is often not appreciated. Data from a detailed study on the Upper Sheep Creek Watershed located within the Reynolds Creek Experimental Watershed in southwestern Idaho were analyzed to characterize the interactions between snowmelt, groundwater and streamflow. Response time between snowmelt, groundwater levels and streamflow was drastically different from year to year depending on the extent of the snowpack. Response time to snowmelt for piezometers and weirs located 135 m downslope from an isolated drift was 3-5 days during an average snow year and up to 70 days for a year with snow accumulation that was 40% of normal. The primary aquifer is believed to be unconfined during low snowmelt years and confined when normal or above-normal snowmelt causes high groundwater levels. Snowmelt from an isolated drift enters the primary aquifer upslope of the confining layer. Rapid response during years with normal snow accumulation is therefore primarily a pressure pulse through the confined aquifer. Recharge during years of low snow accumulation is insufficient to fill the primary aquifer to the confining layer, and response time is indicative of travel time through the aquifer.