Pushed by climate change: Lake in Northwest Territories falls off cliff

In a dramatic example of how climate change is altering the Arctic landscape, a small northern lake has fallen off a cliff after bursting through the melting earthen rampart that restrained it.

A video released Wednesday by the Government of the Northwest Territories shows how the lake, undermined by melting permafrost, collapsed into a valley below and created a large temporary waterfall and an oozing tongue of mud and debris.

"It drained quickly," said Steve Kokelj of the N.W.T. Geological Survey.

The lake, which has no name and sits in the territory's northern corner 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 last ice age.

Trouble starts when the headwall tops are exposed by wind or rain. The ice melts, causing the soil and rock on top to collapse. That exposes more ice, which then melts and extends the collapse, and the cycle keeps repeating.

On July 15, the narrow rib of land that had kept the 1.5-hectare lake from plummeting into the valley below gave way.

Within two hours, 30,000 cubic metres of water — the equivalent of a dozen Olympic-sized swimming pools — gushed over the edge in a waterfall up to five storeys high.

Mud and debris filled more than a kilometre of the valley below and flowed for two days at the rate of 50 metres an hour.

"It was one of those things that you can get out of the way of but you can't stop," said Kokelj.

Such slumps have been getting bigger as rainfall 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 and as large as 40 hectares that have washed loose millions of cubic metres of rubble. Kokelj estimates that the amount of land affected has more than doubled since the late 1980s.

The slump that sent the lake plummeting valleyward had been at work for most of a decade.

Not all the water drained. Kokelj said unfrozen sediments underneath the lake blocked further erosion and stabilized the banks. The territorial government is advising people to stay away from the area, however, because the rest of the lake might still collapse.

The melting will continue around the lake, which will ultimately leave it isolated and elevated on a small plateau, Kokelji said.

Similar landforms, with hilltop lake sediments, are found in Wisconsin. The difference is those features formed 13,000 years ago.

"There's an analogue to what we're seeing today," said Kokelj.