Scientists believe these floods may have occurred dozens of times. The aquifer lies below the land surface of 325 square miles of northern Idaho and eastern Washington, and is the sole source of drinking water for the region's 450,000 people. It is extremely permeable, high in groundwater velocity and susceptible to contamination. Coeur d'Alene Lake and the Spokane River contribute about one-third of the water flow in the aquifer.
Precipitation contributes another third.
The lake watersheds of Hayden, Spirit, Twin, Hauser and Pend Oreille provide most of the additional flow crossing the state line. The movement of water particles ranges from less than a foot to almost 50 feet per day, as it flows west from Idaho to Washington. The depth to the water table varies from 450 feet in Idaho to only 50 feet at some points in Washington. Water discharges from the aquifer into the Spokane River via springs just west of Spokane. The vulnerability of the resource was originally proven in the mid-1970s with detections of elevated nitrates down gradient of densely populated areas relying on septic systems for disposal of wastewater.