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Water building behind B.C. slide appears to have doubled in size: First Nation chief

WILLIAMS LAKE, B.C. (CP) – The chief of Williams Lake First Nation says water building behind a landslide of debris that has dammed the Chilcotin River in British Columbia’s central Interior appears to have nearly doubled in size since Wednesday.

Chief Willie Sellars, who toured the slide site by helicopter Wednesday and Thursday, says the water is continuing to build on one side of the dam, while the riverbed on the other side had “completely dried up” for about two kilometres. 

Officials have said the water collecting behind the slide south of Williams Lake could reach a level where it will start flowing over the debris, or it will erode the material, setting off a release.

They have estimated a release could come Friday or Saturday, and could set off dozens of evacuation orders and alerts downstream.

Evacuation orders span 107 square kilometres along the Chilcotin, and officials saying the slide poses an “immediate danger to life and safety.”

The nearby Tsilhqot’in National Government has declared a state of local emergency and warned people to stay away from the river.

The state of the river has also prompted Interior Health to relocate 21 patients out of a hospital and long-term care home in Lillooet to alternative areas.

Sellars says his community is about 45 minutes away from the slide, so an evacuation of the community likely won’t be necessary.

However, he says the region is of “massive cultural significance” to First Nation communities in the region, noting the slide will also affect navigation channels for salmon.

“(There’s) a lot of history, former village sites, burial grounds, and we need to make sure that we’re doing everything we can to document and protect those,” he says.

Sellars says “it’s hard to describe in words how massive this slide is, and how devastating it is,” adding he hasn’t received updated projections yet on when the dam may break.

“We want to get more information out and you know, our team is on it, and working collaboratively with the (Cariboo Regional District), the province and the federal government and I’m feeling very confident that we’re going to have that information soon.”

Meanwhile, Margo Wagner, chair of the Cariboo Regional District, said Thursday that the water building up behind the slide south of the City of Williams Lake could reach a level where it will start flowing over the debris, or it will erode the material, setting off a release. 

“As the water continues to flow and builds up behind that landslide, the pressure increases, but we do not have a definitive time of when that will let go,” she told a media briefing. 

Wagner said the riverbed below the slide is dry, and officials aren’t certain about the potential downstream impacts when the dam is breached.

But she said it’s clear there would be “a mass of water” coming down the Chilcotin, which flows south into the Fraser River.

“We don’t know exactly when this landslide is going to give way. We are hearing that it’s expected between 24 to 48 hours,” she said during the briefing.

The slide is “massive,” Wagner added.

Minister of Emergency Management Bowinn Ma told a separate news conference Thursday that if the dam were to break it’s possible that dozens of evacuation orders and alerts would be issued along both rivers and “people need to be prepared.”

“Let me be clear, the risk to people and communities along the river is unknown at this time, but it has the potential to be significant,” she said. 

She said engineers, geologists and hydrologists are on site and in the air assessing the landslide and working to understand the potential impacts downstream.

The minister said people living along the Chilcotin and Fraser rivers for hundreds of kilometres, to Hope, at the eastern end of B.C.’s Fraser Valley, are encouraged to pack and prepare their home for possible flooding by moving equipment and other assets from low lying areas. 

Ma said anyone planning to be on a boat on the Fraser needs to rethink that, and people must stay away from the shoreline between Hope and the confluence of the Chilcotin, as a gush of water and debris can make the banks unstable. 

Wagner said the landslide happened in an area that was “burned out” during wildfires in 2017.

“There is a lot of area (where) the trees are dead. They are still standing, but their root system is totally done, and they are not absorbing water, which is an issue.”

Dwayne Tannant, a geotechnical engineering professor at UBC Okanagan, said the topography in the region is “prone to landslides” and while the 2017 fires are likely to have been a factor, he doesn’t believe it was the main cause.

“I’m thinking it’s more than the wildfire. It’s the geology, topography and total erosion that are probably the key drivers here,” he said, noting the slide was not a preventable event.

“It wouldn’t be an economical use of provincial resources to do anything, particularly given the scale of this slide and where it’s located,” he said when asked about mitigation.

Tannant said a river-blocking landslide is “relatively rare,” but officials should be concerned that slopes upstream of the blockage could become destabilized once the dam breaks, potentially creating a “domino effect,” of other landslides.

“I think that’s a remote possibility, but it should be given some consideration,” he said.

B.C.’s River Forecast Centre has issued a flood warning for the Chilcotin north of the slide, saying the debris is “creating a lake” that extends several kilometres upstream.

The forecast centre says the eventual breach of the landslide debris could also lead to an “outburst flood” with a surge of water rapidly flowing downstream.

In the event of a breach, the forecast centre says the surge could reach the Fraser River within hours, and while flooding south of the Fraser Canyon would likely be less severe, substantial increases in flow could extend to the mouth of the river in Metro Vancouver.

Ma said there are worries that a sudden release of debris could cause additional erosion as it moves down the river, as well as concerns about what impact the blocked water is having on salmon.

“There are also concerns that persistent low water levels might impact navigation channels for fish like the ongoing chinook run and the sockeye run coming up,” she said.

“Those can have significant impacts on the ecology of the river system.” 

Canadian Press

The Canadian Press is Canada’s independent national news agency.

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