Geography

Groundwater in the Spokane Valley-Rathdrum above the valley . Prairie Aquifer flows beneath a broad valley The Bitterroot Mountains east of Rathdrum that slopes southwest from Lake Pend Oreille Prairie and the Selkirk Mountains along the to downtown Spokane, where the Aquifer turns Washington - Idaho border also form the north-northwest to discharge groundwater Aquifer edges (or “basin”) . These mountains into the Little Spokane and Spokane Rivers, are more than 2,000 feet higher than the basalt losing almost 700 feet of elevation during the plateau to the southwest, so clouds carried by journey . The basalt formation that creates the prevailing southwesterly winds must rise Spokane Falls and Five Mile Prairie divides the when they reach the mountains . As the clouds Aquifer flow near downtown Spokane, splitting climb they release more precipitation, often as the Aquifer into the Hillyard Trough to the east snow, which provides significant recharge to and on down the Spokane River gorge to the the Aquifer during the spring . west . North of downtown Spokane the surface Several lakes are situated in the mountains elevation rises and then drops steeply at the around the borders of the Aquifer, and contribute confluence of the Spokane and Little Spokane water to the Aquifer through subsurface flow . Rivers . In general, the higher the surface The surface outlet for the largest lake, Lake elevation, the greater the depth to the Aquifer . Pend Oreille, is the Pend Oreille River that The valley floor above the Aquifer covers drains to the north, away from the Aquifer . approximately 250 square miles in Idaho, The surface outlet for Lake Coeur d’Alene is the including the Rathdrum Prairie, and 120 Spokane River, which is the only watercourse square miles in Washington, including the over the Aquifer that remains on the surface for Spokane Valley . The valley walls are composed an extended distance . Only Hangman (Latah) of massive rocks and clay that continue below Creek and the Little Spokane River flowing out the ground surface to form the impervious of the surrounding highlands actually reach basin that holds the Aquifer gravels . Relatively the Spokane River . Other streams contact the flat basalt plateaus such as Five Mile Prairie coarse, gravelly soils overlying the Aquifer and and the Columbia Plateau rise hundreds of feet disappear, percolating downward .

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