Little by little evidence mounted for an old theory that was rejected by the scientific establishment. It involved glaciers, volcanoes, a relatively minor river and a prodigious amount of water. It was the greatest flood of the past two million years, and it posed a giant scientific riddle. A maverick geologist became convinced that thousand-foot-deep floodwaters had scoured out vast areas of the American northwest near the end of the last ice age. Mainstream scientists scorned his theory while he searched patiently for answers to what could have triggered such an inconceivably violent event. Finally, an ingenious solution silenced the skeptics: traces of an enormous ice dam half a mile high, which had blocked a valley in present-day Montana and created an enormous lake behind it. With the help of stunningly realistic animation, this documentary takes viewers back to the Ice Age to reveal what happened when the dam broke, unleashing a titanic flood that swept herds of woolly mammoth and everything else into oblivion. In geomorphology, an outburst flood, which is a type of megaflood, is a high-magnitude, low-frequency catastrophic flood involving the sudden release of water. During the last deglaciation, numerous glacial lake outburst floods were caused by the collapse of either ice sheets or glaciers that formed the dams of proglacial lakes. Examples of older outburst floods are known from the geological past of the Earth and inferred from geomorphological evidence on Mars. Landslides, lahars, and volcanic dams can also block rivers and create lakes, which trigger such floods when the rock or earthen barrier collapses or is eroded. Lakes also form behind glacial moraines, which can collapse and create outburst floods. 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 glacial flood events have been researched since the 1920s. These glacial lake outburst floods 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, flooding much of eastern Washington and the Willamette Valley in western Oregon. After the rupture, the ice would reform, creating Glacial Lake Missoula again. Geologists estimate that the cycle of flooding and reformation of the lake lasted an average of 55 years and that the floods occurred several times over the 2,000-year period between 15,000 and 13,000 years ago. U.S. Geological Survey hydrologist Jim O'Connor and Spanish Center of Environmental Studies scientist Gerard Benito have found evidence of at least twenty-five massive floods, the largest discharging ≈10 cubic kilometers per hour (2.7 million m³/s, 13 times the Amazon River). Alternate estimates for the peak flow rate of the largest flood include 17 cubic kilometers per hour and range up to 60 cubic kilometers per hour. The maximum flow speed approached 36 meters/second (130 km/h or 80 mph).

• published: 13 Nov 2014
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