The Short Story
About 12,000 years ago, the valleys of western Montana lay beneath a lake nearly 2,000 feet deep. Glacial Lake Missoula formed as the Cordilleran Ice Sheet dammed the Clark Fork River just as it entered Idaho. The rising water behind the glacial dam weakened it until water burst through in a catastrophic flood that raced across Idaho, Oregon, and Washington toward the Pacific Ocean. Thundering waves and chunks of ice tore away soils and mountainsides, deposited giant ripple marks, created the scablands of eastern Washington and carved the Columbia River Gorge. Over the course of centuries, Glacial Lake Missoula filled and emptied in repeated cycles, leaving its story embedded in the land.
 Flood Facts:

The ice dam was over 2000 feet tall.

Glacial Lake Missoula was as big as Lakes Erie and Ontario combined. 

The flood waters ran with the force equal to 60 Amazon Rivers. 

http://www.glaciallakemissoula.org