Geology and Geography
Lake Coeur d'Alene, like other lakes surrounding the Spokane Valley and Rathdrum Prairie, was formed by the Missoula Floods, most recently 12,000 to 15,000 years ago. The Purcell Lobe of the Cordilleran Ice Sheet flowed south from Canada, carving the basin of present-day Lake Pend Oreille and damming the Clark Fork river. The impounded river repeatedly filled to form Glacial Lake Missoula and broke through the ice dam, resulting in massive floods that filled the Rathdrum Prairie area with sand, gravel, and boulders. Large eddy bars formed downstream from bedrock obstructions, thereby damming tributary valleys and creating lakes. Lake Coeur d'Alene is fed primarily by two rivers, the Coeur d'Alene River and the Saint Joe River. The outflow is via the Spokane River.
The elevation of the lake varies from 2,128 feet (649 m) above sea level in the summer to up to 7 feet (2.1 m) lower in the winter, controlled by the Post Falls Dam 9 miles (14 km) below the lake on the Spokane River.
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