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)