Streamflow trends in the Spokane River and tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington

A clear understanding of the aquifer and river dynamics within the Spokane Valley/Rathdrum Prairie is essential in making proper management decisions concerning ground-water and surface-water appropriations. Management of the Spokane Valley/Rathdrum Prairie aquifer is complicated because of interstate, multi-jurisdictional responsibilities, and by the interaction between ground water and surface water.

Kendall?s tau trend analyses were completed on monthly mean (July through December) and annual 7-day low streamflow data for the period 1968?2002 from gaging stations located within the Spokane Valley/Rathdrum Prairie. The analyses detected trends of decreasing monthly mean streamflow at the following gaging stations: Spokane River near Post Falls, Idaho (August and September); Spokane River at Spokane, Washington (September); and Little Spokane River at Dartford, Washington (September and October); and decreasing annual 7-day low streamflows at the following gaging stations: Spokane River near Post Falls, Idaho and Spokane River at Spokane, Washington. Limited analyses of lake-level, precipitation, tributary inflow, temperature, and water-use data provided little insight as to the reason for the decreasing trends in streamflow.

A net gain in streamflow occurs between the gaging stations Spokane River near Post Falls, Idaho and Spokane River at Spokane, Washington. Significant streamflow losses occur between the gaging stations Spokane River near Post Falls, Idaho and Spokane River at Greenacres, Washington; most, if not all, of the gains occur downstream from the Greenacres gaging station. Trends of decreasing net streamflow gains in the Spokane River between the near Post Falls and at Spokane gaging stations were detected for the months of September, October, and November.