The Warning Is Written in the Scars of Two Valleys

There is a particular kind of patience required to understand a landscape that fails not from the top down, but from the bottom up. For decades, scientists have watched permafrost thaw in the Arctic in a predictable way: warming air softens the surface, the ground turns to slurry, and the land creeps downhill over years. But two massive landslides in Canada's Northwest Territories did something different. They broke free from the base of the frozen ground, sliding in days rather than decades, and the warning they carry extends far beyond the remote valleys where they occurred.

The slides struck the Johnson River valley in 2017 and the Redstone River valley in 2018. The Johnson River slide moved roughly 18 million cubic meters of soil, rock, and ice—enough to fill about 7,200 Olympic swimming pools—and left a scar nearly two miles long. The Redstone slide moved about 4.5 million cubic meters . Both dammed their rivers, forming new lakes and sending pulses of sediment downstream. Blocks of frozen ground up to ten meters tall were carried downslope intact, according to a study published in Geophysical Research Letters .

What makes these slides distinct is the mechanism. Typically, permafrost fails from the top down, as warming air and precipitation thaw the surface layer. These detachment slides failed from the bottom up. Permafrost in the area has warmed by about 0.3 degrees Celsius per decade since monitoring began in 1980, and a severe wildfire in 1995 stripped away the insulating layer of moss and organic soil . Small movements opened cracks near the tops of slopes, letting water trickle down and thaw the base of the frozen layer. After about fifteen to twenty years, the bond between the permafrost and the ground beneath weakened until the entire slab broke free .

The timeline matters. Fire in 1995, then widespread failures roughly two decades later. Steve Kokelj of the Northwest Territories Geological Survey explained that fifteen to twenty years is about the time it takes for water to permeate twenty meters into the ground . The slides themselves moved in days, a rapid and abrupt failure that standard permafrost thaw would take decades to achieve. "It looked like the land all of a sudden got stretch marks," said Eric McLeod, a pilot who observed the aftermath. "It wasn't like some of the other ones that I've seen that look like they just melt away" .

The immediate damage was limited because the area is remote. No roads, homes, or infrastructure were destroyed. But the researchers are clear about what the slides portend. The vulnerable setting—slopes with relatively thin permafrost, twenty to forty meters deep, that have burned—exists in many places across Canada and Alaska. Lead author Joseph Young described the slides as a "potential glimpse into the future" for those regions . Northern communities that depend on river travel, winter roads, and subsistence fishing are the most exposed. A single blocked channel can cut off a route; a later breach can send water and debris downstream; heavy sediment loads can cloud rivers and affect fish and water quality.

There is, in the researchers' view, a silver lining. These slides do not happen overnight. They develop progressively over about twenty years after a fire, with slow downslope movement that can be detected. That window offers the possibility of early warning, of monitoring the slopes that are most at risk before they fail. The challenge is whether the monitoring will reach the places that need it most—the valleys near communities, the winter roads, the rivers that people rely on for travel and drinking water.

For now, the Johnson and Redstone rivers have resumed their courses, and the new lakes have settled into the landscape. But the study does not predict where or when the next detachment slide will occur. It covers two well-documented events and a regional inventory. The claim that such slides are unprecedented in the region over the past ten thousand years rests on the absence of similar features, not a dated record . What the researchers can say with confidence is that the conditions that produced these slides are not unique, and that a warming climate and worsening wildfires are raising the stakes.

Image note: The visuals accompanying this article were generated by artificial intelligence and do not depict the actual landslide sites or the researchers involved.

Sources: Nature World News, The Weather Network, CBC News, Geophysical Research Letters