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This small unnamed lake, shown in this undated University of Alberta handout photo, in the Northwest Territories is expected to burst through the rapidly shrinking land holding it in over the next few months and plunge 200 metres into the Mackenzie Valley. It's just one of a new wave of giant-sized permafrost slumps that are changing the territory's landscape on a scale not seen since the last Ice Age. (Scott Zolkos/THE CANADIAN PRESS)
This small unnamed lake, shown in this undated University of Alberta handout photo, in the Northwest Territories is expected to burst through the rapidly shrinking land holding it in over the next few months and plunge 200 metres into the Mackenzie Valley. It's just one of a new wave of giant-sized permafrost slumps that are changing the territory's landscape on a scale not seen since the last Ice Age. (Scott Zolkos/THE CANADIAN PRESS)

A lake in the Northwest Territories is about to fall off a cliff Add to ...

Some time in the next few months, a small northern lake will burst through the shrinking earthen rampart holding it back and fall off a cliff.

“It’s got a ways to travel,” says Steve Kokelj of the Northwest Territories Geological Survey. “This lake happens to be perched about 600 feet above the Mackenzie Valley.”

It will be spectacular, but it won’t be unique. Melting permafrost caused by climate change is causing changes in the northern landscape on a scale not seen since the end of the last ice age, says Kokelj.

“It’s changing the form of the landscape in ways that have not impacted this environment in the last several hundreds of thousands of years.”

The doomed lake, which has no name and sits in the northern corner of the territory near the community of Fort McPherson, is a victim of the region’s geology and changing climate.

Permafrost in this part of the N.W.T. contains a high percentage of ice in headwalls, which can be up to 30 metres thick. That ice has been there since the retreat of the Laurentide Ice Sheet 20,000 years ago.

Trouble starts when parts of the headwalls are exposed by erosion from wind or rain. The ice melts, which causes the soil and rock on top to collapse. That exposes more ice, which also melts and extends the collapse, and the cycle keeps repeating.

“It thaws in the summertime and will continue to work its way back upslope until you run out of ice or the headwall gets covered by sediment,” Kokelj says. “The slumps chew their way upslope.”

The slumps have been getting bigger and bigger as rainfall in the area increases and temperatures warm — the summers of 2010 and 2012 were the wettest on record and average temperatures have increased several degrees since the 1970s. There are slumps in the N.W.T. more than a kilometre long that have washed loose millions of cubic metres of rubble.

In a 2014 scientific journal, Kokelj estimated that the amount of land scarred by slumping and the area covered by debris have more than doubled since the late 1980s. Some slumps are as large as 40 hectares.

“In the last 30 years the slumps are much bigger than they were in the past.”

The slump that will eventually send the lake plummeting valleyward has been at work for most of a decade. It has eaten its way through much of the plateau on which the lake sits. Now just a few metres of land and unfrozen permafrost are holding the water back.

Although the lake is only a couple of hectares in size and a few metres deep, it will send tens of thousands of cubic metres of water crashing down when the last bit of soil collapses.

No homes or communities lie in the anticipated flood path, although the N.W.T. has issued a warning to steer clear.

Cameras have been installed, Kolelj says.

“We’re just hoping the cameras don’t get obliterated by the release of water.”

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