TETLIT'ZHEH/FORT MCPHERSON
Sometime later this year, the NWT will most likely lose a body of water.

This permafrost thaw slump is working its way uphill toward a small pond about 18 km northwest of Fort McPherson. - photo courtesy of Scott Zolkos, University of Alberta

A small, unnamed lake on the Peel Plateau appears almost ready to burst from its shores because of a growing permafrost thaw on the hillside below it.

The thawing has created a land slump that is working its way up the hill. The water is now held back by what looks like a natural dam.

Steve Kokelj, a permafrost scientist with the Northwest Territories Geological Survey, said a rapid release of water is expected from the roughly two-hectare pond, located about 18 km northwest of Fort McPherson.

"Once it starts to go, I think the whole lake will probably drain certainly within a day, probably less," said Kokelj.

The Geological Survey has issued a hazard advisory about the situation, warning of a "catastrophic lake drainage" and potential debris flow.

The advisory states a potential flash flood would head down a small creek valley west of Husky Lake in the Gwich'in Settlement Area.

"A flash flood could happen at any time during the summer or fall of 2015," the advisory warns.

Wilbert Firth, president of the Tetlit Gwich'in Council in Fort McPherson, said there is a lot of interest among community residents about the unique land slump below the pond.

"They're Googling it now," he said with a laugh.

Firth said land slumps are common in the region, but it is unusual to see one eating its way uphill and into a pond.

There is another large slump just off the Dempster Highway that is moving toward a lake, he said. "If it keeps going, it's going to get there."

Still, the hazard advisory is telling people to avoid camping or travelling in the area.

Kokelj explained the water flow is expected to travel about five km down the valley and then go into Husky Lake or another nearby water body.

"Those are both big lakes, so they'll just dissipate the flow," he said.

There is a travellers' cabin about five or six km down slope from the pond and the structure has been listed in the hazard advisory as a potential impact area.

Kokelj said it is not expected that the flow of water will hit the cabin but it has been listed in the hazard advisory to err on the side of caution.

Firth also doesn't expect anyone to be in the area, including at the cabin.

"But it's not used usually during the summer, just during the winter," he said.

Firth said everyone, especially researchers, have been advised to stay away from the area, not just because of the potential flash flood but because the permafrost thaw itself is a hazard.

"Because it's like quicksand," he said. "You get into it and you have a hell of a time to get back out."

In fact, he said moose have become stuck and died in such areas. The permafrost thaw and slump is about 10 hectares in size.

"That one has been there for at least 10 years," said Kokelj.

The scientist said all land users in the region are familiar with such permafrost thaws.

"Unlike the southern part of the NWT, up north, particularly in the Western Arctic, there's lots of ice in the permafrost, and when that ice is exposed on slopes it starts to thaw and then you get these collapses that progressively grow upslope," he explained. "And they kind of live for 10 years or 20 years, and continue to enlarge. So it's a pretty common phenomenon in the delta region."

Kokelj says sometimes the slumps, which have been impacting the landscape for thousands of years, can heal themselves as moving earth accumulates over time and covers the exposed ice.

"What we've noticed is that, in really heavy rainstorms, all that material just slides away, exposes the ice again and the slump can keep moving upslope," he said, adding some disturbances on the Peel Plateau are 30 to 40 hectares in size.

The intensity of summer rainfall has increased in the last decade, he added.

"Intuitively, people kind of think that thawing permafrost is due to warmer temperatures. Well, it is in part, but in this case rainfall is a big part of the story. It removes the slump sediments and it allows the slump to continue to grow upslope as opposed to it, so to speak, healing itself."

While permafrost thawing and land slumps have caused lakes to drain elsewhere in the region, the situation northwest of Fort McPherson is unusual.

"I think what's unique about this situation is that the lake is perched pretty high up on a hilltop," said Kokelj.

Other ponds have drained on flat land because of coastal erosion and sometimes permafrost thaw.

Trail cameras will monitor the site to record the release of water, and the area will be resurveyed in the fall.

The monitoring is part of an ongoing permafrost research project by the Northwest Territories Geological Survey.

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