Permafrost Thaw Causes Lake Drainage and Flow of Debris, Peel Plateau, Northwest Territories, Canada
Permafrost thaw in the northwestern Northwest Territories has caused development of very large thaw slumps. Individual thaw slump disturbances can impact over 30 ha of terrain, displace millions of cubic metres of sediments to reconfigure slopes and impact downstream environments.
One particular thaw slump about 20 km northwest of Fort McPherson has been growing for about a decade. It has caused gradual hillside collapse, resulting in the rapid, partial drainage of a small upland lake about 1.5 ha in area.
The drainage event was captured by remote cameras. When the slump eroded to the edge of the lake, about half of the lake volume estimated to be about 30,000 m3 drained in about 2 hours by pouring over the slump headwall and forming a temporary waterfall 10 to 15 m high (12 seconds mark of video). Peak flows lasted less than 30 minutes and flow rates reached at least 10 m3/second. Water rushed over the slump debris and down a narrow valley before emptying into a larger lake about 5 km downstream in the Mackenzie Delta. The turbidity in the downstream lake was significantly elevated as a result of sediments transported by the drainage event. Two days following drainage, saturated debris flowed downslope at rates of up to about 50 m/hour for several days to enlarge a debris tongue deposit that has infilled over 1 km of valley.
In 2016, the Northwest Territories Geological Survey and its university, government and community partners will further investigate the impacts of slumps on landscapes, streams and lakes and explore the use of remote sensing to better monitor the landscape changes.