A rather unusually timed landslide on the Takhini River bank on the edge of Whitehorse this winter generated a “small” tsunami that reminded a Yukon geologist of Dawson City's risk.
Derek Cronmiller, the Yukon Geological Survey’s head of surficial geology, said the equivalent of about 1,500 dump trucks of material like silt and clay slid down the steep escarpment located about 13 kilometres west of the confluence of the Takhini River and the Yukon River during the third week of December 2024.
He said a wave caused by the slide pushed the ice on the Takhini River across and down the river, up to about 200 to 250 metres away from where it originally was.
While the Takhini River landslide was small “in the grand scheme of things,” slides like this can create what Cronmiller called a “hazard cascade.”
That effect is a concern regarding the “Sunnydale slide” on the west side of the Yukon River in Dawson City, according to Cronmiller.
“That's an area that the geological survey is monitoring very closely, and the real risk to the public there is not so much from the slide material itself but is actually from a landslide-generated wave,” he said.
Cronmiller said one of the largest known waves to happen was the result of a very large landslide in Taan Fjord in Alaska.
“That produced a wave that was almost 200 metres in height and extremely destructive to the surrounding area,” he said.
The largest wave recorded occurred in Lituya Bay in Alaska due to a landslide.
Cronmiller noted the recent Takhini River slide took place in a relatively remote part of the river just south of the Eclipse Nordic Hot Springs.
No one was hurt and no infrastructure was damaged or put at risk due to the local slide, per Cronmiller.
Cronmiller advised paddlers on the Takhini River to be aware of the obstruction on the river from the debris and any hazard that it might present over the next few years.
Cronmiller said no major flooding is expected because of the slide, although there is potential for minor localized flooding in the next few months to years — until the river removes or transports the material that has fallen.
“It did block about 50 per cent of the channel, and so we expect there will be some back watering and slight diversion of the river in that location, but nothing major,” he said.
The slide’s cause remains unknown, according to Cronmiller.
“This location is not at all surprising, and there's not an obvious trigger associated with the slide,” he said.
Cronmiller mentioned there was no rainfall, no warming event, no major snowmelt and no earthquake – things his team would typically look for to determine what caused a slide.
Cronmiller noted it isn’t unusual for banks to give out in steeper areas along the Takhini and Yukon rivers like it did in this case last month. The river still hasn’t fully eroded the deposit from a slide that took place at the same site in 2017.
The earliest photo known to the News of the recent Takhini River landslide comes from Dec. 23, 2024, which Cronmiller said indicates the slide occurred earlier that day or within a few days of that photo being taken.
Cronmiller’s team was notified about the slide by Liam Ferguson, who Cronmiller described as a local collaborator involved with Northern Sonic Drilling and Consulting Inc.
Cronmiller said Ferguson was flying in a plane over the area and snapped a photo that was in turn provided to the News.
Cronmiller said it is unlikely any action needs to be taken given the low risk in the area. His team will visit the site on Jan. 10 with representatives from the Yukon’s water resources branch to get their opinion on what the impact might be.
“It’s something that like has happened to this location before, and will continue to happen,” he said.
“It’s just part of the normal process.”
Athina Vazeos, communications manager for the federal fisheries department, or DFO, told the News by email that the DFO is assessing the extent of the sediment released in the Takhini River to determine if it will obstruct fish movement or impact the fishing community.
“Aside from impeding fish movement or direct impacts from the slide event itself, this sediment release is highly unlikely to pose meaningful short or long term impact to chinook salmon," reads the DFO’s email.
“Currently, we are not concerned about an impact to salmon and other fish species. Bank instabilities are not uncommon and release of sediments into watercourses in Yukon do occur.”
Chinook salmon spawning in the Takhini River occurs upstream of the site, the relatively small release of sediment is expected to have minimal impact on fish, adult chinook migration and spawning in the Takhini River happened in early fall and the scope of the release isn’t enough to dampen this summer’s salmon run, according to DFO.
“Presently, either a physical or velocity barrier (when water is constricted through a smaller path) could impact juvenile chinook salmon’s ability to move through this site, however this is not expected given initial observations,” Vazeos wrote.
Contact Dana Hatherly at dana.hatherly@yukon-news.com
