GROUND WATER RESOURCES IN THE QUATERNARY DEPOSITS OF THE COLVILLE RIVER WATERSHED, STEVENS COUNTY, WASHINGTON

A study of the ground-water system in the Quaternary deposits of the Colville River Watershed provided the local watershed planners with a description of the hydrogeologic framework, preliminary determinations of ground- and surface-water interactions, and descriptions of water-quantity characteristics. The 2,608-square-kilometer (1,007-square-mile) watershed, located in Stevens County in northeastern Washington, is closed to further surface-water appropriations. The information provided by this study will assist local planners in assessing the status of water resources within the watershed and determining if there is water available for further allocation.

The hydrogeologic framework consists of Quaternary alluvium, glaciolacustrine deposits, till, outwash, Lake Missoula flood deposits, and loess overlying Tertiary to Middle Proterozoic bedrock. A map of the surficial geology was used with logs for more than 350 wells to construct 28 hydrogeologic sections. Seven hydrogeologic units were delineated: the Upper outwash aquifer, the Till confining unit, the Older outwash aquifer, the Colville Valley confining unit, the Lower aquifer, the Lower confining unit, and Bedrock.

Along the Colville Valley floor, upward head gradients in the Lower aquifer indicate that the Lower aquifer and the shallow ground-water system in that area, mostly shallow coarse lenses in the Colville Valley confining unit, may act as fairly independent systems. However, most reaches on the Colville River lost water to the shallow ground-water system in September 2001. There may be some connection with the losing stream reaches and the Lower aquifer near the edges of the valley where the Valley confining unit is thin or absent. Total water withdrawals during 2001 were about 35,400 million liters (9,340 million gallons); surface water supplied 65 percent of the total and ground water supplied 35 percent. An approximate annual water budget for the watershed shows that there are 69 centimeters (27 inches) of precipitation balanced by 11 centimeters (4.2 inches) of streamflow discharge, 1 centimeter (0.3 inches) of ground-water discharge, and 57 centimeters (22.5 inches) of evapotranspiration.