Caused By A Glacier
Sometimes a glacier flows down a valley to a confluence where the other branch carries an unfrozen river. The glacier blocks the river, which backs up into a lake, which eventually overflows or undermines the ice dam, suddenly releasing the impounded water.
In modern times, the Hubbard Glacier regularly blocks the mouth of Russell Fjord at 60° north on the coast of Alaska.
A similar event takes place after irregular periods in the Perito Moreno Glacier, located in Patagonia. Every four years the glacier forms an ice dam against the rocky coast, causing the waters of the Argentine Lake to rise. When the water pressure is too high, then the giant bridge collapses in what has become a major tourist attraction. This sequence occurred last on March 13, 2006, preceding the previous which took place only two years before, on March 12, 2004.
About 13,000 years ago, the Cordilleran ice sheet crept southward into the Idaho Panhandle, forming a large ice dam that blocked the mouth of the Clark Fork River, creating a massive lake 2,000 feet (610 m) deep and containing more than 500 cubic miles (2,100 km3) of water. Finally this Glacial Lake Missoula burst through the ice dam and exploded downstream, flowing at a rate 10 times the combined flow of all the rivers of the world. Because such ice dams can re-form, such Columbia River floods happened at least 59 times, carving Dry Falls below Grand Coulee.
Read more about this topic: Ice Dam
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