Ice dam
An ice dam occurs when water builds up behind a blockage of ice. Ice dams form either when glacier blocks a river and forms a lake or when ice chunks in a river are blocked by something and build up to form a dam, often called an ice jam. Glacial ice dams have historically resulted in massive outburst floods. River ice jams can cause flooding upstream during the jam, flooding downstream when the jam releases, and damage from the ice itself on structures and ships in or near the river. Ice jams on a lake or ocean occur during the spring break-up if wind driven ice piles up along a shoreline.
The movement of a glacier may flow down a valley to a confluence where the other branch carries an unfrozen river. The glacier blocks the river, which backs up into a proglacial lake, which eventually overflows or undermines the ice dam, suddenly releasing the impounded water in a glacial lake outburst flood also known by its Icelandic name a jökulhlaup. Some of the largest glacial floods in North American history were from Lake Agassiz. 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. Roughly every four years the glacier forms an ice dam against the rocky coast, causing the waters of the Lago Argentino 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 4, 2012, the previous having taken place four years before, in July 2008.
About 13,000 years ago in North America, 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 (600 m) deep and containing more than 500 cubic miles (2,000 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, these Missoula Floods happened at least 59 times, carving Dry Falls below Grand Coulee.
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