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.

These glacial lake outburst floods, or jökulhlaups, were the result of periodic sudden ruptures of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each ice dam rupture, 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 peak flow of the largest floods is estimated to be 40 to 60 cubic kilometers per hour (9.5 to 15 cubic miles per hour).^[1]^[2] The maximum flow speed approached 36 meters/second (80 miles per hour).^[3] Up to 1.9×10¹⁹ joules of potential energy were released by each flood, the equivalent of 4500 megatons of TNT.^[4] The cumulative effect of the floods was to excavate 210 km³ (50 mi³) of loess, sediment and basalt from the channeled scablands of eastern Washington and to transport it downstream.^[1]

The mechanism by which the ice dam failed was as follows. Under pressure, the melting point of water decreases. As the depth of the water in Lake Missoula increased, the pressure at the bottom increased enough to lower the freezing point of water below the temperature of the ice forming the dam. This allowed liquid water to seep into minuscule cracks present in the ice dam. Over a period of time, the friction from water flowing through these cracks generated enough heat, which gradually began to melt the ice walls and enlarge the cracks. This allowed more water to flow through the cracks, generating more heat, allowing even more water to flow through the cracks. This feedback cycle eventually weakened the ice dam so much that it could no longer support the pressure of the water behind it, and it failed catastrophically. This same process triggered a similar event in Iceland on November 5, 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 mi (1.6 km) wide narrows near Kalama, Washington. Some temporary lakes rose to an elevation of more than 400 ft (120 m), 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. By studying the canyon of the Flathead River, he estimated that flood waters in excess of 45 miles per hour would be required to roll the largest of the boulders moved by the flood. He estimated the water flow was nine cubic miles per hour, more than the combined flow of every river in the world.^[5] Some estimates place the flow at ten times the flow of all current rivers combined.

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

^ ^a ^b Allen, John Eliot; Burns, Marjorie and Sargent, Sam C. (c1986). Cataclysms on the Columbia : a layman's guide to the features produced by the catastrophic Bretz floods in the Pacific Northwest. Portland, OR: Timber Press, p. 104. ISBN 0881920673.
^ Bjornstad, Bruce N. (c2006). On the trail of the Ice Age floods : a geological field guide to the mid-Columbia basin / Bruce Bjornstad.. Sandpoint, Idaho: Keokee Books, p. 2. ISBN 9781879628274.
^ Bjornstad, Bruce N. (c2006). On the trail of the Ice Age floods : a geological field guide to the mid-Columbia basin / Bruce Bjornstad.. Sandpoint, Idaho: Keokee Books, p. 1. ISBN 9781879628274.
^ Allen, John Eliot; Burns, Marjorie and Sargent, Sam C. (c1986). Cataclysms on the Columbia : a layman's guide to the features produced by the catastrophic Bretz floods in the Pacific Northwest. Portland, OR: Timber Press, pp. 199-200. ISBN 0881920673.
^ Alt, David; Hundman, Donald W. (1995). Northwest Exposures: A Geologic History of the Northwest. Mountain Press, 381-390. ISBN 0-87842-323-0.

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