Two massive permafrost landslides in Canada's Northwest Territories blocked rivers, created new lakes, and dumped sediment downstream, and scientists now say they represent a new type of slide that could spread to other thinly frozen slopes in Canada and Alaska. The study, published in Geophysical Research Letters, links the slides to severe wildfires in 1995 combined with decades of warming that let water reach the base of the frozen ground.

The researchers call them permafrost detachment slides because the entire frozen layer breaks loose from the slope beneath it. Lead author Joseph Young, who did the work as a doctoral student at the University of Alberta, said the slides may be unprecedented in the region over the past 10,000 years or more, according to Live Science. That assessment is the researchers' interpretation and has not been tested against a full long-term landslide record.

So far, the slides have not damaged roads, homes or other infrastructure because the area is remote. The concern is what happens when the same process reaches valleys near communities, winter roads or rivers that people rely on for travel, fishing and drinking water.

Two Slides, Two Blocked Rivers

The larger slide struck the Johnson River valley in 2017, moving about 18 million cubic meters (636 million cubic feet) of soil, rock and ice and leaving a scar nearly 3.2 kilometers (2 miles) long. The second hit the Redstone River valley in August 2018, moving about 4.5 million cubic meters (159 million cubic feet) and leaving a scar about 1.2 kilometers (0.75 mile) long and 350 meters (1,150 feet) wide, according to an American Geophysical Union summary and the study.

Both slides dammed their rivers, forming new lakes and sending pulses of sediment downstream. The Johnson River slide sits about 5 kilometers (3 miles) from that river's confluence with the Mackenzie River, between the communities of Wrigley and Tulita. Blocks of frozen ground up to 10 meters (33 feet) tall, about the height of a two-story house, were carried downslope intact.

Young and his colleagues say the two large slides are among more than 150 in the area. Most are smaller, but they still typically move more than 1 million cubic meters (35 million cubic feet) of ground each.

Fire, Warming and a Bottom-Up Failure

Most permafrost landslides thaw from the top down, turning frozen soil into a slurry that creeps downslope over years. These slides work the other way. Permafrost in the area has warmed by about 0.3 degrees Celsius (0.54 degrees Fahrenheit) per decade since monitoring began in 1980, and the 1995 fires 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 from below. That weakened the bond between the permafrost and the ground beneath until the whole slab broke free. Co-author Steve Kokelj of the Northwest Territories Geological Survey told CBC News, as carried by The Weather Network, that 15 to 20 years is about the time it takes "to permeate 20 metres into the ground."

That timeline matches what researchers observed — fire in 1995, then widespread failures about two decades later. The slides can move in days, whereas top-down thaw would take decades to shift.

Risk Beyond the Mackenzie Valley

The vulnerable setting is specific: slopes with relatively thin permafrost, about 20 to 40 meters (66 to 130 feet) deep, that have burned. Young said many places in Canada and Alaska fit that description, and that worsening northern wildfires raise the stakes. He described the slides as a "potential glimpse into the future" for those regions.

The most exposed are northern communities that depend on river travel, subsistence fishing, winter roads and remote highways — places where a single blocked channel can cut off a route. A sudden river blockage can flood land upstream as a new lake fills, while a later breach could send water and debris downstream. Heavy sediment loads can also cloud rivers and affect fish and water quality, concerns the Northwest Territories Geological Survey has tracked in its mapping of thaw-driven landslides.

The study does not predict where or when the next detachment slide will occur. It covers two well-documented events and a regional inventory, and the claim that such slides are new in 10,000 years rests on the absence of similar features in the landscape rather than a dated record.

Early Warning Signs on Burned Slopes

The researchers see one encouraging point: these slides do not come without notice. Young and Kokelj said burned slopes show slow downslope movement and cracking over roughly 20 years before failure. That gives land managers a window to map old burn scars, monitor slopes with satellites and ground surveys, and flag those above rivers, roads and pipelines.

For residents of permafrost regions in Alaska and northern Canada, the practical steps are to report new ground cracks, sudden muddy water, or unusual river blockages to territorial, state or tribal authorities, and to stay off unstable slopes and fresh debris. Travelers on rivers should heed local advisories, since a new landslide dam can change water levels quickly. Decisions about evacuation or closures belong to local emergency officials.

The next steps for scientists include mapping other burned, thin-permafrost slopes across the north and tracking how quickly they move. The bottom line is that wildfire and warming have already produced a faster, larger kind of landslide in northwest Canada; no homes have been hit yet, and the long warning period offers communities time to prepare if officials use it.

What Readers Want to Know

What is a permafrost detachment slide?

It is a landslide in which the whole frozen soil layer breaks free from the slope beneath it after thaw reaches its base, rather than melting from the top down.

Where did these slides happen?

In the Johnson River valley in 2017 and the Redstone River valley in 2018, both in Canada's Northwest Territories.

How big were they?

The Johnson River slide moved about 18 million cubic meters of ground, and the Redstone River slide about 4.5 million cubic meters.

What caused them?

Researchers link them to severe 1995 wildfires and long-term permafrost warming of about 0.3 degrees Celsius per decade, which let water thaw the frozen layer from below.

Could this happen in Alaska?

The researchers say many areas of Canada and Alaska with thin permafrost, about 20 to 40 meters deep, are vulnerable, especially where wildfires have burned.

Are there warning signs?

Yes. Burned slopes show slow movement and cracking for roughly 20 years before failure, which can be monitored.

What should residents do?

Report new cracks, sudden muddy water, or river blockages to local authorities, avoid unstable slopes, and follow local advisories.