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How did these floods happen?

During the most recent episode of major ice-sheet expansion, between about 18,000 and 13,000 years ago, a lobe of the Cordilleran ice sheet advanced into the Idaho Panhandle to the area that is now occupied by Lake Pend Oreille, thus blocking the Clark Fork River drainage and causing Glacial Lake Missoula to form. At its largest, the lake was deeper than 2,000 feet deep at the dam and held over 500 cubic miles of water–as much as Lake Erie and Lake Ontario combined. The ice dam, however, was subject to repeated failure.

The actual failure of the ice dam is inferred from observations of modern glacial floods. Research on glacial floods is relatively new and the physical processes are not yet fully understood, but one scenario is that, when the height and pressure of the lake’s water against the ice dam reached a critical stage, the dam would develop significant subglacial leaks, eventually leading to a sudden and complete collapse.

When the dam broke, a towering mass of water and ice was released and swept across parts of Idaho, Washington, and Oregon on its way to the ocean. The peak rate of flow was ten times the combined flow of all the rivers of the world. The huge lake may have emptied in as little as two or three days. Over a period of years the glacier would advance, once again blocking the river, and the dam and the lake would form again. This process was repeated scores of times, until the ice sheet ceased its advance and receded to the north at the end of the Ice Age. It is assumed that the same processes would have occurred earlier during other glacial advances throughout the Ice Age, although most of the evidence for the earlier events may have been removed by the flooding that occurred during the last glacial advance.

Along the floodwaters’ course, more than 50 cubic miles of earth and rock were removed, and some of this was transported and then deposited as new landforms. The floods built gravel bars as tall as 400 feet and moved boulders weighing many tons as if they were pebbles. Some of the eroded material was deposited along the path of the floods, but most of the eroded material was carried out onto the floor of the Pacific Ocean, where extensive deposits of flood sediment have recently been identified hundreds of miles from the current mouth of the Columbia River.

more:
What are the Ice Age Floods?
How did these floods happen?
Phenomena related to the Ice Age Floods

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Last updated 12/10/03. Contact the Sitemaster.