Ice Age Floods, An Introduction

Link: Ice Age Floods Institute (IAFI)


During the most recent Ice Age (18,000 to 13,000 years ago), and probably in previous Ice Ages, cataclysmic floods inundated portions of the Pacific Northwest. These Ice Age Floods originated primarily from Glacial Lake Missoula, but also from Lake Bonneville and perhaps as sub-glacial outbursts from under the continental ice sheet.

When part of that Cordilleran Ice Sheet pushed into the Lake Pend Oreille area of the Idaho Panhandle, it created an ice dam over 40 miles wide and 3000 feet thick, that blocked the Clark Fork River drainage and impounded Glacial Lake Missoula. At its largest, the lake was more than 2,000 feet deep at the ice dam and held over 500 cubic miles of water—as much as Great Lakes Erie and Ontario combined.

Eventually, the impounded lake water broke through the ice dam, sending a towering mass of water and ice, up to 1000 feet deep, sweeping across parts of Idaho, Washington, and Oregon on its way to the ocean. The huge lake likely emptied in as little as two to four days. With a peak flow rate ten times the combined flow of all the current rivers of the world, the floods stripped and locally redeposited downstream as much as 50 cubic miles of sediment and bedrock, and created the astounding landforms that still attest to this story.

Over a period of years after each flood, the ice sheet would readvance, once again damming the river and causing the lake to reform. This process repeated scores of times, perhaps every 40 to 140 years until the ice sheet ceased its advance and receded northward at the end of the Ice Age.

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