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Based on the presence of multiple interglacial calcretes interbedded with flood deposits, magnetostratigraphy, optically stimulated luminescence dating, and unconformity truncated clastic dikes, it has been estimated that the oldest of the Pleistocene Missoula floods happened before 1.5 million years ago.Because of the fragmentary nature of older glaciofluvial deposits, which have been largely removed by subsequent Missoula floods, within the Hanford formation, the exact number of older Missoula floods, which are known as Ancient Cataclysmic Floods, that occurred during the Pleistocene cannot be estimated with any confidence.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.His most compelling argument for separate floods was that the Touchet bed deposits from two successive floods were found to be separated by two layers of volcanic ash (tephra) with the ash separated by a fine layer of windblown dust deposits, located in a thin layer between sediment layers ten rhythmites below the top of the Touchet beds. By analogy, since there were 40 layers with comparable characteristics at Burlingame Canyon, Waitt argued they all could be considered to have similar separation in deposition time.The two layers of volcanic ash are separated by 1–10 centimetres (0.4–3.9 in) of airborne nonvolcanic silt. The controversy whether the Channeled Scabland landforms were formed mainly by multiple periodic floods, or by a single grand-scale cataclysmic flood from late Pleistocene Glacial Lake Missoula or from an unidentified Canadian source, continued through 1999.Over a period of time, the friction from water flowing through these cracks generated enough heat 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.
He estimated the water flow was nine cubic miles per hour, more than the combined flow of every river in the world.
As the water emerged from the Columbia River gorge, it backed up again at the 1 mile (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.
Further, Shaw and team proposed that the rhythmic Touchet beds are the result of multiple pulses, or surges, within a single larger flood.
In 2000, a team led by Komatsu simulated the floods numerically with a 3-dimensional hydraulic model.
As the depth of the water in Lake Missoula increased, the pressure at the bottom of the ice dam increased enough to lower the freezing point of water below the temperature of the ice forming the dam.