Ice dam

An ice dam (or ice jam) occurs when water builds up behind a blockage of ice. Ice dams can occur in various ways.

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.

On rivers

The town of Eagle, Alaska, is inundated by flood water and ice floes after an ice jam formed on the Yukon River downstream.

If the upstream part of a river thaws first (possibly because it flows away from the equator), and the ice gets carried downstream into the still-frozen part, the ice can form an ice dam and flood the areas upstream of the jam. This occurred during the 2009 Red River Flood and the 2009 Alaska floods. After the ice dam breaks apart, the sudden surge of water that breaks through the dam can then flood areas downstream of the jam. While this usually occurs in spring, it can happen as winter sets in when the downstream part becomes frozen first. Where floods threaten human habitation, the blockage may be artificially cleared. Ice blasting using dynamite, or other mechanical means may be used.

On roofs of buildings

Ice dam forming on slate roof

An ice dam, on a smaller scale, is a problem of house and building maintenance in cold climates. An ice dam can occur when snow accumulates on the slanted roof of a house with inadequate insulation. Heat conducted through the insufficient insulation and warm air from the attic bypasses warms the roof and melts the snow on those areas of the roof that are above living spaces, but does not melt the snow on roof overhangs. Meltwater flows down the roof, under the blanket of snow, onto the eave and into the gutter, where colder conditions on the overhang cause it to freeze. Eventually, ice accumulates along the eave and in the gutter. Snow that melts later cannot drain properly through the ice on the eave and in the gutter, resulting in leaks to the roof space resulting in damaged ceilings, walls, roof structure and insulation. The ice that builds up on the roof can be removed by trained professionals that use special steam equipment to ensure quick and safe removal without causing damage to the roof.

References

• Allen, John Eliot; Burns, Majorie; and Sargent, Sam C. (1986). Cataclysms on the Columbia. Portland: Timber Press. ISBN 0-88192-215-3. 

External links

• United States National Park Service
• More images
• CRREL's Ice Jam Database
• RoofIce Dam Images
• http://bct.eco.umass.edu/publications/by-title/preventing-ice-dams
• RoofIce Dam Melt Socks
• Ice Dam Removal