HYDROGEOLOGY OF THE LITTLE SPOKANE RIVER BASIN, WASHINGTON

The Little Spokane River Basin includes an area of about 675 mi² in northeastern Washington State covering parts of Spokane, Stevens, and Pend Oreille Counties. Streams originate in the northern part of the basin and contribute flow to the Little Spokane River which flows about 49 miles to its confluence with the Spokane River, near Spokane. The basin relies on spring snowmelt from the higher elevation areas of the basin and groundwater discharge to the river to maintain stream flows during the drier months.

Spokane County is concerned about the effects of future groundwater development that may occur throughout the basin. With increased subdivision and development, an increase in exempt groundwater use is expected to continue, but the potential effects of this growth on the Little Spokane River and the basin’s aquifers are unknown. In order to obtain information necessary to evaluate these concerns, the USGS is conducting a study to describe the hydrogeologic framework of the basin. This work is intended to be followed by subsequent studies that will provide the remaining information needed for the eventual construction and calibration of a numerical groundwater flow model. Such a model could then be used by the County to evaluate the possible regional impacts of different groundwater-use and climate scenarios on the ground and surface-water system of the basin.

An analysis of the hydrogeologic framework has been completed using well logs, geologic mapping, and field observations. Existing and new groundwater data were compiled and used to evaluate the flow system. A simplified geologic map will be presented with hydrogeologic cross sections constructed for the study area. Principal hydrogeologic units are illustrated, including maps of the tops and extents of the major units. Hydraulic parameters estimated for hydrogeologic units using available data from aquifer tests, drillers’ reports and published values are provided in table format. Water-level maps for the basin illustrate hydraulic head in the basin and general groundwater flow directions.