INVESTIGATION OF HYDROGEOLOGIC INFLUENCES ON WATER SUPPLY AND TEMPERATURE IN THE BIG HOLE RIVER, SOUTHWESTERN MT

Drought and elevated temperatures are major concerns for many river basins, and the Big Hole River in southwestern Montana is no exception. The Big Hole River is a blue-ribbon trout fishery and an important water resource for the agricultural economy. Low summer stream flows limit the water available for irrigation and can contribute to elevated temperatures that often threaten the fishery. Community stakeholders are interested in managing the river resource in a manner that supports the fishery and agriculture. This prompted a groundwater/surface-water study by the Montana Bureau of Mines and Geology (MBMG) Ground Water Investigation Program (GWIP) to better understand the factors influencing streamflow and temperature in the Big Hole River near Glen, MT.

Groundwater and surface water were monitored starting in fall 2022. The monitoring network includes 47 wells and 16 surface-water sites, which are monitored monthly for water level and stream stage/discharge, respectively. Water-quality samples were collected at a subset of wells to analyze for inorganic compounds and stable-water isotopes. Areas of groundwater discharge to the river were identified with a hand-held thermal infrared camera; these areas typically had other visual indicators including saturated banks, biofilms, and iron staining. Temperature was monitored at eighteen groundwater discharge sites and the four surface-water sites in summer-fall 2023.

Most wells are located in shallow alluvium, where groundwater is well mixed, and interconnected with the river. Seasonal groundwater levels increased near flood irrigated fields and next to irrigation canals, but less near pivot irrigation. Groundwater inputs to the river were generally observed to be cooler than the river; however, some inputs had long surficial flow paths that caused the water to warm up before reaching the river. Maximum daily temperature differences between the downstream and upstream sites coincide with maximum solar radiation, suggesting solar radiation has the greatest influence on stream temperature in the river. Our preliminary findings indicate the river and groundwater are interconnected, the type of irrigation influences the amount of local aquifer recharge, and surface-water and groundwater inputs are not substantially affecting river temperatures.