Landslide north of Whitehorse caused 'tsunami' on ice-covered Takhini River, geologist says

A recent landslide on the Takhini River just north of Whitehorse sent a torrent of debris crashing through the ice and into the river, blocking part of the channel.   

That's unusual for this time of year, says one expert.

Derek Cronmiller, the head of surficial geology with Yukon Geological Survey, says the landslide moved about 20,000 cubic metres of soil, snow and trees, directly into the river. He says that's larger than all of the landslides seen on the Whitehorse escarpment along Robert Service Way in recent years. 

He compared the volume of it to almost 40,000 bathtubs worth of material. It happened a few kilometres west of the Takhini River bridge.

For Cronmiller, the most unique element of the landslide was that it caused what he referred to as a "tsunami." 

"It broke through the ice of the Takhini River and created a large displacement wave — and it broke a lot of the ice and it rafted large chunks of river ice up to 250 meters away ... which can be very destructive, damaged trees and things like that," Cronmiller said.

The landslide was in a relatively remote area and didn't cause any threat to public safety, he added. However, the debris has blocked "a little bit more than half" the channel, he said.

He says the area affected by the landslide-generated wave is much larger.

Cronmiller refrains from calling it a "disaster," mostly because it happened away from a populated area where it would have potentially had an higher impact.

Other landslides within the city in recent years have forced the closure of Robert Service Way for periods and have also prompted plans to reroute the road.  

Still, the Takhini River area is used by snowmobilers and dog-sledders in the winter. The Yukon government is advising people to avoid the area right now.

Cronmiller says the government won't be removing any of the debris from the Takhini River as nature is expected to be able to do that on its own. 

He also recommends that paddlers travelling on the river next summer skip that part of the river, as there might be a faster flow in the area. 

"The water needs to pick up the velocity, typically, to actually not flood further. So it's usually the easiest way for it to do is to just go faster — which does help with some of the removal of that material." 

A landslide in the winter?

Cronmiller also refrained from linking the landslide to climate change. 

"What we look at is ... trends in the number of slides, the frequency that they're occurring and sort of the magnitudes of the slides," he said. 

"In this case, we don't have like this obvious trigger."

He says the Tahkini River area is prone to landslides because of permafrost thaw affecting the clay cliffs, which are shaping one side of the riverbank. The most recent landslide in the area before this recent one was reported during the summer of 2017. 

Cronmiller says landslides are typically triggered by heavy rainfall, snowmelt, and change in groundwater levels. 

Those triggers, however, most often occur when the ground is not frozen. 

"People are always looking for a trigger, and triggers are important but they're not always the smoking gun," Cronmiller said. 

"It's just something that was on the way to happening through just slow fluvial processes and gravity, in this case."

Cronmiller says while it's always hard to link a single slide to climate change, his team has seen in the last few years an increase in snowfall in the region, as well as a slight increase in intense precipitation. 

"Some of these more classic landslide triggers are occurring with greater frequency. So we expect to see landslides occurring with greater frequency as a result," he said.