Year Established: 2017 Start Date: 2017-03-01 End Date: 2018-02-28
Total Federal Funds: $2,000 Total Non-Federal Funds: $670
Principal Investigators: Charles Shama
Abstract: The town of Ennis is located in southwest Montana along the upper Madison River valley. Residential development is increasing along the western valley benches. The residents are concerned that there is a limited groundwater supply and believe that more domestic pumping wells added to the bench will lower the ground water table. The magnitude of sub-surface flow is important in groundwater source protection, regional development planning, and water rights disputes for basin-fill aquifers such as the Upper Madison Valley near Ennis, yet existing estimates carry large uncertainties. Southwest Montana is a semi-arid environment and Ennis, elevation 4,950ft, receives 12.5in annual precipitation. Most of the groundwater recharge to the valley benches comes from the mountains during spring runoff. This Mountain Front Recharge (MFR) is an important water source to basin aquifers in semi-arid regions and the least understood component to balancing a groundwater budget. MFR provides groundwater recharge to regional aquifers located at the valley margin and adjacent mountain sides. Water budget estimates are generally based on precipitation data from rain gauges within the valley and snow water equivalent stations in the mountains. The benches along the valley south of Ennis have a highland pass, elevation 6,800ft, that does not receive much precipitation or significant snowfall and does not have rain gauge or SNOTELL site to measure precipitation. Mountains northwest of Ennis, elevations of 10,000ft, receive significant snowfall averaging 19.4in snow water equivalent (SWE) over the past 35 years from Montana SNOTEL site #603 at elevation 7900ft. Groundwater that enters these benches originates from either direct recharge from rain events or from surface/sub-surface water from the highland pass. Flow paths in the highlands are at greatest magnitude at the stream valleys where diffusive flow has been focused through narrow valleys and contributes the most towards MFR. This shallow sub-surface water is transmitted by streambed sediments and while the stream channel may be dry, there is often significant groundwater discharge to the valley benches. Infiltration from perennial and ephemeral streams and the sub-surface flow underneath these streams focus water flow from the mountain block (>7,000ft) to the benches (<7,000ft) and are the largest component to MFR.