Around 15 000 residents of the city of Fort McMurray in northeast Alberta, Canada, have been forced to evacuate after the lower part of the town began to flood on Sunday 26 April 2020. The flooding has been caused by ice jams that have formed on the Athabasca and Clearwater rivers, creating a blockage almost 25 km long, resulting in water piling up behind the obstruction to form a dam lake, which has begun to invade the town.
Flooding in Fort McMurry, Alberta, caused by ice jams on the Athabasca and Clearwater rivers. McMurray Aviation.
Ice jams can be a problem in areas where there is a strong seasonal freeze and thaw cycle. They are generally associated with a rapid spring thaw, which can cause many large chunks of ice to be deposited into a river at the same time. When these ice blocks reach an obstruction on the river, such as a narrow or shallow stretch they can become jammed together, forming a blockage. These blockages cause flooding in two ways; firstly by creating a dam lake behind them, and secondly (and more dangerously) by suddenly giving way, allowing all the water piled up behind them to escape at once, and causing a flash flood downstream. Although generally associated with the spring thaw, ice jams can also occur at the onset of the winter freeze, if an unseasonal warm spell causes a sudden thawing.
An ice jam on the Athabasca River in Alberta, Canada, on 26 April 2020. Vincent McDermott/Fort McMurray Today/Postmedia Network.
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