TETLIT'ZHEH/FORT MCPHERSON
A nameless lake outside Fort McPherson is now only half full after it drained into the surrounding valley on July 15.

Permafrost thaw caused more than 30,000 cubic metres of water to drain from a lake outside of Fort McPherson on July 15 when its banks finally gave way. - photo courtesy of Mike Pisaric, Brock University

An estimated 30,000 cubic metres of water drained in two hours when the earth gave way at the edge of the lake, located about 20 km northwest of Fort McPherson, said Steve Kokelj, a permafrost scientist with the Northwest Territories Geological Survey.

"For about a little bit less than a half an hour, there was a 10 to 15 metre waterfall that was flowing at about 10 cubic metres a second," he said.

The event was the result of permafrost thaw, Kokelj said. Over the past decade, thawing permafrost created a thaw slump on the slope beside the lake. A thaw slump occurs when the top layer of ice-rich permafrost is exposed and begins to thaw, causing the soil and rocks to collapse. Over time, the slump eroded the hill slope to the edge of the lake where the water finally broke through and partially drained, Kokelj said.

"These slumps can grow for decades and continue to enlarge and consume parts of hills," he said.

Having reached unfrozen ground beneath the lake, the slump is expected to stop its progression - for now.

"If there is no ice in the ground, the slump will basically run itself out and stabilize," Kokelj said. "There is a possibility that it will just remain that way. But again, it's a bit of a wait-and-see."

While the lake has stabilized, the GNWT is still advising people to be careful in the area, just in case.

The causes and effects of the event will continue to be studied, Kokelj added. Water that drained from the lake pushed debris down the valley and into streams and lakes along the way, which will impact water quality.

"Obviously the absence of water in the lake has an impact on the biota in the lake," Kokelj said. "The lake drainage was just one consequence of the increase in this kind of disturbance. Regionally, the biggest impact has been in the increase in sediment in the streams."

While thaw slumps are a natural process, especially in the Peel Plateau which contains more ice than other areas, they have become more common and much larger over the past 30 years, Kokelj said.

"The disturbances we see presently are extremely large," he said.

An NWT Geological Survey time-lapse camera captured video as the lake drained, first showing the waterfall as the water breaks through the earth on the side of the lake then the subsequent flow of mud and debris as it slowly moves down the valley a few days later.

"It just oozed down this valley and filled in this valley for the length of about a kilometre and a half," Kokelj said.

Kokelj said the event is an example of how a warming climate will impact areas of the North.

"It provides us with an idea on how certain landscapes are likely to respond if climate continues to change," he said.

The video is available on the NWT Geological Survey website.

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