Surface Water/Groundwater Interaction Between the Spokane River and SVRP Aquifer

The 2015 drought in the Inland Northwest produced the hottest and driest summer on record in the Spokane area. The extremely low snowpack in the headwaters of the Spokane River resulted in the lowest daily average flow ever recorded in the Spokane River for the first few weeks of the summer, low flow season. The hydrograph for the summer months showed very little variability with no significant rain events and consistently low flows. The seven day low flow did not reach historic lows thanks to the 2009 FERC license issued to Avista for their dams on the Spokane River.

One consequence of the drought conditions was that Avista did not open the gates on their Post Falls dam on the Tuesday after Labor Day (which is the first day their FERC license allows them to begin drawing down the Lake Coeur d’Alene) like they do most years. In fact, they waited until mid-October to open the gates. This month-long delay in changing the flow regime in the Spokane River provided an opportunity for the hydraulics of the system to reveal an interaction between the Spokane River and the Spokane Valley Rathdrum Prairie Aquifer that had not been recognized in the 100+ years of streamflow data.

Data from the largest water purveyor in the aquifer suggests that summer pumping for all municipal uses from the aquifer was likely around 450 cfs in mid-August. As the daily maximum air temperature dropped near the end of August from the 95 ^oF range to 55 ^oF over a two week period, pumping stress on the aquifer started a month long decline. By the end of September, daily pumping from the aquifer likely dropped by 160 cfs. During the month of September, flows at the downstream gage on the Spokane River gradually climbed by 90 cfs while the flows at the upstream gage remained steady.

Continuous water level measurements collected at observation wells in the aquifer reveal a pronounced change in slope within a couple of days of the beginning of the break in groundwater pumping. The water table in the aquifer began rising at on August 29, 2015. 29 hours later, flow in the Spokane River began to rise.

A 160 cfs decrease in groundwater pumping from the aquifer resulted in a gradual increase in head in the aquifer of around half a foot over the month of September. This lead to a 90 cfs increase in discharge of water out of the aquifer and back into the river in the gaining reach.

This increase in discharge of groundwater back into the river must occur every year as the weather transitions from summer to fall conditions and groundwater pumping from the aquifer subsides. Normal river operations prevent us from seeing this gradual rebound in groundwater discharge. But the 2015 drought allowed us to see this surface water/groundwater relationship in the river flow for the first time in 100+ years of monitoring.