GROUNDWATER AND SURFACE WATER INTERACTIONS IN THE BEAVERHEAD RIVER, MT

Many intermountain basins in arid and semi-arid regions are experiencing increased demands on water. Characterizing the groundwater and surface water interactions is crucial for managing present and future demands. Groundwater and surface water were investigated in the Beaverhead Basin near Dillon, MT. The study area encompasses 81,200 acres with about 44% irrigated from groundwater, the Beaverhead River and its tributaries or surface water diverted through canals. Stream stage and flows, water quality parameters and stable water isotopes (δD and δ¹⁸O) were collected in the Beaverhead River at four locations and in eight tributaries within the study area. Bi-monthly measurements were collected through the irrigation season from March through November 2010 and monthly measurements were collected from November 2010 until April 2011. Five mountain snow samples and 32 groundwater samples were also collected throughout the duration of the study. Water quality parameters and stable isotopes reveal at least three water types. They include, shallow groundwater near surface water, intermediate groundwater, and regional groundwater. All but deep groundwater appears to mix with surface water. Stable isotopes show that groundwater and snow samples plot along the local meteoric water line with little to no evaporation and surface water samples fall along local evaporation lines. Data indicates a significant amount of regional groundwater flows to the Beaverhead River where the valley is constricted, and highly evaporated water mixes with the River during the irrigation season between these pinch points. The highly evaporated water appears to primarily be irrigation return flow. The isotopic data also shows an increase in irrigation return flow in tributaries as they flow from the Tertiary benches to the Beaverhead River flood plain. Isotopes sampled in a tributary flowing near the flood plain/Tertiary bench boundary reveal that the stream is almost entirely fed by regional groundwater. Although surface water flow measurements and groundwater elevations are important for characterizing groundwater/surface water interactions in the Beaverhead River basin, the collection of isotopes was crucial for enhancing the conceptual understanding of groundwater and surface water interactions.