Missoula Floods

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Glacial Lake Columbia (west) and Glacial Lake Missoula (east) are shown south of Cordilleran Ice Sheet. The areas inundated in the Columbia and Missoula floods are shown in red.

During the Ice Age Floods, Dry Falls was under 300 feet of water approaching at a speed of 65 miles per hour. (Image ©2004 Teri J. Pieper, courtesy of byways.org)

The Missoula Floods (also known as the Spokane Floods or the Bretz Floods) refer to the cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age.

The floods were the result of the periodic sudden rupture of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each rupture of the ice dam, the waters of the lake would rush down the Clark Fork and the Columbia River, inundating much of eastern Washington and the Willamette Valley in western Oregon. After the rupture, the ice would reform, recreating Glacial Lake Missoula once again.

The mechanism by which the ice dam failed was as follows. The boiling and melting points of water vary depending on pressure; under very high pressures – such as those that would build at the bottom of Lake Missoula as water volume in the lake increased – the freezing point of water drops. Liquid water therefore seeps into minuscule cracks in the ice dam. Over a period of time, the interaction of these flows of liquid water and the ice walls creates friction, which gradually begins to melt the ice walls. The cracks therefore enlarge, permitting more water, thus more friction, and thus ever larger cracks. Eventually, the ice dam becomes so weakened that it can no longer support the pressure of the water behind it, and catastrophic failure occurs. This same process triggered a similar event in Iceland on November 5th 1996.

Geologists estimate that the cycle of flooding and reformation of the lake lasted on average of 55 years and that the floods occurred approximately 40 times over the 2,000 year period between 15,000 and 13,000 years ago.

As the water emerged from the Columbia River gorge, it backed up again at the 1 mile wide narrows near Kalama, Washington. Some temporary lakes rose to an elevation of more than 400 feet flooding the Willamette Valley to Eugene, Oregon and beyond. Iceberg rafted glacial erratics and erosion features are evidence of these events. Lake-bottom sediments deposited by the Missoula Floods are the primary reason for the agricultural richness of the Willamette Valley.

Geologist J Harlen Bretz first recognized evidence of the catastrophic floods, which he called the Spokane Floods, in the 1920s. He was researching the Channeled scablands in Eastern Washington, the Columbia Gorge and the Willamette Valley of Oregon. Bretz, however was not able to explain the source of the huge volume of water and his hypothesis was controversial, partly due to the popularity at that time of the principle of uniformitarianism in geologic processes. In 1925 another geologist, Joseph Pardee, suggested that the water came from the failure of the glacial dam holding back the waters of Lake Missoula. Further research confirmed this hypothesis and the cause of the floods was finally explained. The Missoula Floods are also called the Bretz Floods in honor of Bretz.

[edit] References

J Harlen Bretz, (1923), The Channeled Scabland of the Columbia Plateau. Journal of Geology, v.31, p.617-649.
J Harlen Bretz, (1925), The Spokane flood beyond the Channeled Scablands. Journal of Geology, v.33, p.97-115, 236-259.
John Eliot Allen and Marjorie Burns, with Sam C. Sargent. Cataclysms on the Columbia. Portland, Oregon: Timber Press, 1986. ISBN 0-88192-215-3