MOUNTAIN FRONT RECHARGE IN A SEMI-ARID CLIMATE; SOUTHWEST MONTANA

Groundwater recharge to regional valley benches originates largely from adjacent Mountain Front Recharge (MFR). The Madison Valley in Southwest Montana is a semi-arid mediterranean climate that receives 12.5in annual valley precipitation. In this study, we compare two drainages looking at focused underflow at the mountain-valley transition. We are i) calculating the amount of groundwater recharge that is occurring from the losing section of the two streams, and ii) targeting focused underflow below two different streams exiting a mountain canyon into a larger valley. This study will characterize groundwater surface-water interactions along Daylight Creek (DC) and along North Meadow Creek (NMC). The focus is on creating a physical mass balance recharge estimate from the losing stream sections near the mountain-valley transition. DC that runs through Virginia City, MT is within a small highland pass watershed with peak elevations of 6,800ft and does not receive much precipitation. NMC is located west of McAllister. The watershed reaches elevations up to 10,000ft and receives snowfall averaging 19.4in SWE. To help answer valley recharge i) we will utilize a physical mass balance equation,

P = ET + R + G

Where P is precipitation; ET is evapotranspiration; R is surface runoff and G is groundwater. Methods used to answer focused underflow ii) is the installation of shallow wells (6-9ft) which are instrumented with pressure transducers and temperature loggers set vertically in wells. Staff gauges measure surface runoff and will help characterize vertical groundwater movement between the stream and shallow streambed sediments. From April through June, the smaller DC is showing a signal of a losing stream. The larger NMC has three unique zones. The upper mountainous zone is a gaining stream; middle zone is a transition from gaining to losing; and the lower zone is a losing stream. Temperature profiles indicate a downward vertical flow for DC and NMC transitions from an upward flow to a downward flow as the creek progresses onto the Madison Valley. Initial mass balance budgets for April and June indicate subsurface underflow is ten times the magnitude of surface flow on both streams. Hydrogen and oxygen isotope samples from stream water and groundwater show that a mix of precipitation and groundwater is occurring during the spring runoff.