In 1923, the geologist J Harlen Bretz began theorizing about how the landscape in Montana, Washington, Oregon and Idaho was shaped, and found a radical answer. He propose that a catastrophic water flood formed the coulees around the Pacific Northwest. Bretz used the existence of some structures - as dry waterfalls, potholes, huge coulees - as evidence to explain his theory, but he couldn't find how the explain the massive amount of water.
At his time, the scientists believe that the geologic features was made by time action. He just had some acknowledgement when Joseph Tomas Pardee find evidences for a huge glacial ice dam, that may could held a monstrous lake. This finding helps Bertz to explain the flood's source of water.
The movie NOVA: Mystery of the Megaflood, recreate what might happened when the dam itself melted, releasing millions of gallons of water and dragging anything on it's way. Evidences also shows that more then one megaflood probably happens to sculpt the amazing landscape that we know at present day.
Image 1: Glacial Lake Missoula's ice dam
Source: www.pbs.org
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Image 2: Purcell Lobe of the Cordilleran ice sheet
Source: www.pbs.org
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