Area Geology

The Ice Age Floods

During the last 1 to 2 million years, there have been a large number of cooling periods resulting in extensive periods of glaciations over large portions of North America. The last cooling began about 30,000 years ago and ended about 15,000 years ago. There is evidence of Ice Age flooding from a number of these periods of glaciations.

Recent Glaciations Consequences

This last glaciations produced an ice sheet that covered much of Canada. The average depth of ice at the U.S. / Canadian border was 5,000 feet thick. A portion of the Cordilleran Ice Sheet crept south into the U.S. and blocked the Clark Fork River Valley, near Sandpoint, Idaho. The resultant lake backed up behind this ice dam which contained as much as 530 cubic miles of water. Periodically, the ice dam would fail, and the flood waters would begin their race to the sea.

Lake Missoula in Ice

The water emptied from glacial Lake Missoula in as few as 48 hours. The finger of the ice sheet would continue to move south and would again fill in the valley of the Clark Fork River. Again, the water would back up forming another glacial Lake Missoula. The ice dam would eventually fail, and this flood sequence would be repeated. Some geologists believe that this situation repeated itself as many as 100 times during this last glacial cycle alone.

Glacial Water Flood

The flood waters from glacial Lake Missoula generally came down the Grand Coulee, Telford-Crab Creek drainage, and the Cheney-Palouse route. Regardless of which way the flood waters went, they all met at Wallula Gap by Pasco. These converging flood waters overwhelmed Wallula Gap's ability to accommodate the deluge. The flood waters quickly backed up to a depth of 900 feet. These waters backed up the Snake River valley over 100 miles. The waters flooded:

  • Othello and Quincy Basins
  • Pasco Basin
  • Walla Walla river valleys
  • Yakima

This enormous ponding is known as Lake Lewis. Lake Lewis only lasted about a week or less, before these flood waters could drain through Wallula Gap. During this week or less, however, the muddy water deposited a graded layer of sand and silt on the floors of this lake. The multiple floods, with their multiple pondings from Lake Lewis, are responsible for much of the fertile top soils found here in the Quincy Basin.

Geological Models Speculations

Recent geological models suggest that as much as three times more water than existed in glacial Lake Missoula is needed to produce the flood features found in the Channeled Scablands. There is a growing body of evidence showing how considerable amounts of melt water can accumulate under ice sheets and the release of this water is often cataclysmic. Perhaps we will find that the giant Cordilleran Ice Sheet to the north would periodically release great amounts of melt water that could augment the floods from glacial Lake Missoula.

Clearly, there is much to learn about the ice age floods story. It will be fascinating to see this story unfold, as more research uncovers additional details about this great geological event.