Geohydrology of the Flathead Indian Reservation, northwestern Montana

The geohydrology of the Flathead Indian Reservation was studied to provide information needed to formulate groundwater development plans. Bedrock in the area consists predominantly of slightly metamorphosed carbonate and fine-grained clasitc rocks of the Precambrian Belt Supergroup. Valley-fill deposits consist of alluvium of Holocene age; glacial and glaciolacustrine boulders, cobbles, gravel, sand, silt, and clay of Pleistocene age; and siltstone, fine-grained sandstone, and coal of probable Tertiary age. Most wells in the reservation are completed in valley-fill aquifers--for which well discharge ranges from 0.5 to 1,600 gal/minute and transmissivity ranges from 3.2 to 45,600 sq ft/day. Groundwater flow follows the trend of the respective valleys. Water level in valley-fill aquifers fluctuates seasonally in response to recharge from streams and irrigation canals and discharge from wells. Recharge to valley-fill aquifers occurs by direct infiltration of snowmelt and rainfall, leakage from streams and irrigation canals, subsurface inflow, and irrigation return flow. Discharge from the valley-fill aquifers occurs through evaporation, transpiration by plants, withdrawals from wells, leakage to rivers and streams, and subsurface outflow. Water in bedrock is available from fracture zones of secondary permeability; discharge from wells completed in bedrock ranges from 2.5 to 40 gal/min. Water from wells and springs is calcium bicarbonate or sodium bicarbonate type and contained dissolved-solids concentrations of 42 to 1,100 mg/L. All dissolved constituents in most samples were within Federal drinking water standards. (USGS)