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.To this day, we still see lots of evidence of the Cordilleran Ice Sheet and the Missoula Floods here in North Idaho.The Missoula floods (aka Spokane floods or Bretz floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. These floods were the result of periodic sudden ruptures of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each ice dam rupture, the waters of the lake would rush down the Clark Fork and the Columbia River, flooding much of eastern Washington and the Willamette Valley in western Oregon. After the lake drained, the ice would reform, creating Glacial Lake Missoula again.
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.
- Car-sized boulders embedded in ice floated some 500 miles; they can still be seen today!
- There is no evidence of fish in the glacial lake, but there may have been in the tributaries
- No human relics have been found but native oral history suggests people may have witnessed the floods.
What might have caused those enormous ice dams to melt and collapse, then reform, collapse again, reform again, over centuries?