GROUND-WATER/SURFACE-WATER INTERACTIONS, FLOOD IRRIGATION, AND TROUT HABITAT, BIG HOLE RIVER, BEAVERHEAD COUNTY, MONTANA

Streamflow in the upper Big Hole River basin in southwestern Montana has provided irrigation water for the ranching community for more than 100 years and supported a blue-ribbon wild trout fishery. Multiple years of drought have left the fisheries and ranchers competing for surface water. Critical to the issue is that the Big Hole River is home to the last native population of the fluvial arctic grayling in the lower 48 states. A dwindling population of the grayling coupled with low streamflows and high water temperatures due to drought and irrigation withdrawals along with other factors have placed the grayling as a candidate species under the Endangered Species Act. Late-season flows have commonly been reduced to the point that negotiated voluntary reductions in diversions have saved fish populations.

To better understand the impact of irrigation on river flow, a study was initiated to examine ground-water/surface-water interactions, to determine the magnitude and timing of irrigation return flows to the river, and to interpret how land use affects the area’s hydrology. A 16 square mile study area was selected that includes a reach of the river, irrigation ditches, and adjacent ranch land with 16 monitoring wells. Synoptic measurements of ground-water levels and surface-water flows and climatic information were made. Ground-water hydrographs showed an increase in water levels as flood irrigation commenced and an abrupt decline at the end of the irrigation season. Preliminary results indicate that flood irrigation return flow to the river is prevalent primarily after the growing season has ended. During the growing season, evapotranspiration appeared to consume any excess water in shallow ground water storage. Future modeling will simulate field conditions and determine how sensitive the basin is to variations in evapotranspiration, hydraulic gradient, aquifer properties, and recharge. The model will be used to simulate how altering irrigation management practices will affect the shallow ground-water system. Understanding the ground-water/surface-water interactions and characterizing the magnitude and timing of irrigation return flows to the river can help stakeholders make informed management decisions on how best to utilize the water resources.