Compilation of geologic, hydrologic, and ground-water flow modeling information for the Spokane Valley-Rathdrum Prairie aquifer, Spokane County, Washington, and Bonner and Kootenai Counties, Idaho

The U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources and Washington Department of Ecology compiled and described geologic, hydrologic, and ground-water flow modeling information about the Spokane Valley-Rathdrum Prairie (SVRP) aquifer in northern Idaho and northeastern Washington. Descriptions of the hydrogeologic framework, water-budget components, ground- and surface-water interactions, computer flow models, and further data needs are provided. The SVRP aquifer, which covers about 370 square miles including the Rathdrum Prairie, Idaho and the Spokane valley and Hillyard Trough, Washington, was designated a Sole Source Aquifer by the U.S. Environmental Protection Agency in 1978. Continued growth, water management issues, and potential effects on water availability and water quality in the aquifer and in the Spokane and Little Spokane Rivers have illustrated the need to better understand and manage the region's water resources.

The SVRP aquifer is composed of sand, gravel, cobbles, and boulders primarily deposited by a series of catastrophic glacial outburst floods from ancient Glacial Lake Missoula. The material deposited in this high-energy environment is coarser-grained than is typical for most basin-fill deposits, resulting in an unusually productive aquifer with well yields as high as 40,000 gallons per minute. In most places, the aquifer is bounded laterally by bedrock composed of granite, metasedimentary rocks, or basalt. The lower boundary of the aquifer is largely unknown except along the margins or in shallower parts of the aquifer where wells have penetrated its entire thickness and reached bedrock or silt and clay deposits. Based on surface geophysics, the thickness of the aquifer is about 500 ft near the Washington-Idaho state line, but more than 600 feet within the Rathdrum Prairie and more than 700 feet in the Hillyard trough based on drilling records. Depth to water in the aquifer is greatest in the northern Rathdrum Prairie (about 500 feet) and least near the city of Spokane along the Spokane River (less than about 50 feet). Ground-water flow is south from near the southern end of Lake Pend Oreille and Hoodoo Valley, through the Rathdrum Prairie, then west toward Spokane. In Spokane, the aquifer splits and water moves north through the Hillyard Trough as well as west through the Trinity Trough. From the Trinity Trough water flows north along the western arm of the aquifer. The aquifer's discharge area is along the Little Spokane River and near Long Lake, Washington.

A compilation of estimates of water-budget components, including recharge (precipitation, irrigation, canal leakage, septic tank effluent, inflow from tributary basins, and flow from the Spokane River) and discharge (withdrawals from wells, flow to the Spokane and Little Spokane Rivers, evapotranspiration, and underflow to Long Lake) illustrates that these estimated values should be compared with caution due to several variables including the area and time period of interest as well as methods employed in making the estimates.

Numerous studies have documented the dynamic ground-water and surface-water interaction between the SVRP aquifer and the Spokane and Little Spokane Rivers. Gains and losses vary throughout the year, as well as the locations of gains and losses. September 2004 streamflow measurements indicated that the upper reach of the Spokane River between Post Falls and downstream at Flora Road lost 321 cubic feet per second. A gain of 736 cubic feet per second was measured between the Flora Road site and downstream at Green Street Bridge. A loss of 124 cubic feet per second was measured for the reach between the Green Street Bridge and the Spokane River at Spokane gaging station. The river gained about 87 cubic feet per second between the Spokane River at Spokane gaging station and the TJ Meenach Bridge. Overall, the Spokane River gained about 284 cubic feet per second between the Post Falls,