Missoula Flood
Glacial Lake Missoula was formed during the last Ice Age, when a section of the Cordilleran ice sheet moved south into the Idaho Panhandle, blocking the Clark Fork River. As the waters accumulated and rose behind the 2,000-foot ice dam, the valleys of western Montana became flooded.
Glacial Lake Missoula was formed during the last Ice Age. Shortly after it was created it stretched east for 200 miles, forming an inland sea.
Once in a while the ice dam that created the lake would fail, causing disastrous large-scale flooding of ice- and dirt-filled water that would run down the Columbia River drainage, across northern Idaho, eastern and central Washington, through the Columbia River Gorge, through Willamette Valley in Oregon, finally emptying into the Pacific Ocean at the mouth of the Columbia River.
Glacial Lake Missoula contained more than 500 cubic miles of water at its largest. When it broke through the ice dam and flowed downstream, it moved at a speed 10 times the combined flow of all the rivers in the world. Moving at such a high rate, the water and ice actually shook the ground as it traveled toward the Pacific Ocean. As it moved it stripped away thick soils, and cut deep canyons in the bedrock underneath. The floodwaters would have reached speeds of about 65 miles per hour, draining the lake in only 48 hours.
As the Cordilleran ice sheet continued to move south it blocked the Clark Fork River several times, which would cause more Glacial Lake Missoulas to emerge. This constant flooding and draining occurred over a period of thousands of years; the lake would fill, the dam would fail, and flooding would repeat over and over again. The Ice Age floods created many unique features you can still see today. The formation of Glacial Lake Missoula, along with the floods, are called the "Ice Age Floods."
The Missoula floods are the largest known floods that ever happened on earth in the past two million years. But geologic evidence indicates that upwards of 40 floods may have occurred during the Pleistocene Age, with the last flood dated at about 13,000 years ago.
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