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. See William S. Reeburgh, D. L. Nebert, "The birth and death of Russell Lake", Alaska Science Forum 832 (3 August 1987) at http://www.gi.alaska.edu/ScienceForum/ASF8/832.html : an image on that page shows the record of the tide gauge behind the dam superimposed on an image of the lake, showing the buildup and then the release of all impounded water within 24 hours. 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 just too high, then the giant bridge collapses in what has become a major touristic attraction. This sequence occurred last on March 13, 2006, preceding the previous which took place only two years before, on March 12, 2004 [http://www.geolsoc.org.uk/template.cfm?name=Patagonia] . 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 2000 feet 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, such Columbia River floods happened at least 59 times, carving Dry Falls below Grand Coulee.

Caused by seasonal ice

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. 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 [http://www.boston.com/news/local/articles/2007/03/12/vermonts_capital_braces_for_possible_river_flooding/] .

On roofs of buildings

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 and ventilation in the attic. Warmth coming up through the roof melts the snow. 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. This can result in:

* Leaking roof (height of leak depends on extent of ice dam).
* Wet, ineffective insulation.
* Stained or cracked plaster or drywall.
* Rotting timber.
* Stained, blistered or peeling paint.

Under extreme conditions, with heavy snow and severe cold, almost any house can have an ice dam, whereas a house that is poorly insulated and ventilated will have ice dams during normal winter weather. Giant icicles hanging from the eave are one indication of a poorly insulated, poorly ventilated attic.

References

External links

* [http://www.nps.gov/iceagefloods/a.htm United States National Park Service]
* [http://www.williamcalvin.com/BrainForAllSeasons/76N.htm More images]
* [http://en.wikipedia.org/wiki/
]
* [https://dev.crrel.usace.army.mil/ierd/ijdb/index.html CRREL's Ice Jam Database]