Glacial Lake Missoula National Natural Landmark

Ice Age Floods National Geologic Trail

Glacial Lake Missoula was the largest of several lakes impounded by the Cordilleran Ice Sheet in the Northern Rocky Mountains during the Quaternary period. The lake was approximately 2,900 square miles in size and contained more than 500 cubic miles of water. It occupied the Mission, Jocko and Little Bitterroot valleys, drained by the Flathead River; and the Missoula, Ninemile and Bitterroot valleys, drained by the Clark Fork River. The lake was formed by an ice dam across the mouth of the Clark Fork River at Lake Pend Oreille, Idaho, a result of successive advances of a lobe of the Cordilleran Ice Sheet. The site is located 12 miles north of Perma, Montana.

At least three and possibly more than 100 separate collapses of the ice dam, possibly coincident with periods of ice retreat, caused the lake to drain, producing catastrophic floods across the Columbia Plateau and down the Columbia River Basin. It is estimated that the waters of Glacial Lake Missoula, 1,000 feet deep at Eddy Narrows, were withdrawn at a peak discharge rate of 10 cubic miles per hour. Approximately 550 cubic miles of water passed through those narrows during withdrawal. Evidence of this catastrophic withdrawal can be found in scourings, high eddy deposits of flood debris, flood bars of boulders and course gravel, and successions of giant arcuate ripples or ridges of gravel resting on bedrock surfaces.