Investigation of the inflow to the Rathdrum Prairie-Spokane Valley aquifer

Copious flows of water discharge into Spokane River in a reach extending from a few miles west of the Idaho-Washington State line to Some distance below Spokane. These flows and the water stored in the aquifer are of considerable economic importance for hydroelectric power, irrigation, and industrial and municipal uses.

The large ground-water flew is fed by water from an area which includes the Spokane Valley Rathdrum Prairie plains east and northeast of Spokane, the Little Spokane River valley north of Spokane, and possibly, to a lesser extent, Hangman Creek valley to the south. The water sources include deep percolation from precipitation and irrigation on the plains, leakage from lakes at the margins of the plains, and seepage from numerous ephemeral and perennial streams which flow from the surrounding mountains and hills.

Prior to 1959 available surface-water records were not adequate to define, even roughly, the contribution to ground water from the scores of small watersheds totaling more than 3,000 square miles of drainage area. In 1959, the surface-water record collection was broadened to determine more closely the volumes of water from the various sources. Miscellaneous measurements of the discharge at 13 small representative drainage basins furnished information for correlative estimates of the mean annual flow of all subbasins upstream from the city of Spokane. Precipitation records and consumptive use data furnished a basis for estimating percolation from subbasins having no surface runoff.

The total inflow to the aquifer from sources above Spokane was determined to be 1,200 cfs (cubic feet-per second) during 1959. Of this total, leakage from Coeur d'Alene and Hayden Lakes accounted for 140 and 100 cfs, respectively. Seepage from other lakes and streams-above Spokane adjacent to the Rathdrum Prairie Spokane Valley plains, but not including leakage from Pend Oreille Lake, was 770 cfs. Average total inflow from the sources above Spokane for the period 1951-59 was determined to he 1,100 cfs. On the basis of gaging station records, total inflow from groundwater sources to Spokane River between "near Otis Orchards" and "at Lona Lake" averaged about 1,450 cfs for both the 1959 water year and the 1951-59 period.

The investigation shows that by far the largest part of the effluent seepage is generated in the Spokane River basin itself. This contradicts assumptions of earlier investigators who concluded that the largest. single source of the flow is seepage from Pend Oreille Lake.