Flooding events in Canada: Yukon
The most common cause of flooding in the Yukon is the spring snowmelt freshet or a combination of snowmelt and rainfall. The annual freshet generally occurs in late-May or early June. Ice jam flooding also occurs primarily in May, during spring breakup, and in winter during freeze-up.
Flooding of the lakes in the upper reaches of the Yukon River sometimes happens as a result of a heavy winter snowpack followed by above-normal rainfall. It is most likely to take place during a year following the carryover of snow from the previous year. Maximum levels in the upper Yukon River are reached in August or September, coinciding with maximum glacial inputs from high elevations.
Intense summer rainfall events occur annually throughout the Territory. Their most notable impact is on highway stream crossings, occasionally necessitating road closures along the Alaska and South Canol highways.
Glacier outburst floods, or jökulhlaups, have happened in the recent past within the Alsek and Donjek River basins. Several active glacier-dammed lakes exist today in the St. Elias Mountains, some of which produce floods. Lake Alsek is one of the larger glacier-dammed lakes to have existed in the world since the Ice Ages.
Ice Jam Floods in the Yukon
In the Yukon, ice jam floods occur primarily during spring breakup, usually in May. They generally happen as a result of a backwater created by a downstream ice jam. The most severe events, however, are thought to be caused by surges moving downstream owing to the failure of one or more upstream ice jams which accumulate moving downstream. Dawson City on the Yukon River has historically been the community most susceptible to ice jam flooding.
Winter flooding also occurs in the Territory because of ice jamming during freeze-up. The most notable example of this happening is at Whitehorse, which experiences at least minor flooding annually, usually in December.
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