The Aquifer underlies about 370 square miles in two states. It has one of the fastest flow rates in the United States, flowing as much as 60 feet per day in some areas. The volume of the entire Aquifer is about 10 trillion gallons, making it one of the most productive aquifers in the country. Note, however, that the Aquifer is not simply an "underground river” — a common misconception. It is a combination of gravels, cobbles, and boulders — the result of the rapid draining of Glacial Lake Missoula when ice dams broke. Water from adjacent lakes, mountain streams, the Spokane River, and precipitation flow through these flood deposits supplying the Aquifer.
As the sole source of water for most people in Spokane County in Washington and Kootenai County in Idaho, the Aquifer was designated as a "sole source aquifer” by the EPA in 1978. It was the second Aquifer in the nation to receive this special designation, and it increased public awareness for Aquifer protection and supported the development of special management practices by local agencies. Practices such as eliminating septic tanks and pre-treating stormwater over the Aquifer have greatly improved water quality over the last forty years.
The surface outlet for Lake Coeur d'Alene is the Spokane River, which is the only watercourse over the Aquifer that remains on the surface for an extended distance. Only Hangman Creek and the Little Spokane River flowing out of the surrounding highlands actually reach the Spokane River. Other streams contact the coarse, gravelly soils overlying the Aquifer and disappear, percolating downward.