After a Massive Landslide, the Threat of Flash Floods

The province says it is closely monitoring a “dynamic” situation after a massive landslide blocked the Chilcotin River, raising the possibility of catastrophic downstream impacts if the dam created by the landslide suddenly gives way.

The slide occurred late Tuesday evening. Its size is estimated at between 600 and 800 metres long and 30 metres deep. It has almost entirely blocked water flows in the Chilcotin, a major tributary of the Fraser River, and has caused water to back up “several kilometres” upstream, Emergency Management and Climate Readiness Minister Bowinn Ma said at a provincial briefing held yesterday.

About 200 cubic metres of water — the equivalent of roughly 2,500 full bathtubs — would normally be passing through the canyon every second, according to the province.

“We must be prepared for the worst-case scenario right now,” said Ma. She urged people to avoid boating on the water or being near shorelines along the extent of the Chilcotin River and from its confluence with the Fraser River all the way to Hope — a distance of about 400 kilometres.

If the dam gives way, it could raise water levels all the way to the Vancouver, Ma confirmed.

According to flood warnings issued Tuesday afternoon, “substantial increases in flow may extend all the way to the mouth of the river through the Lower Mainland.” A high streamflow advisory has been issued for the Fraser River west of Hope.

Higher water levels would take at least a day to affect the Vancouver area, the province said. The flood surge, if it occurs, could reach the Chilcotin’s confluence with the Fraser River “within hours” and take up to 24 hours to reach Hope.

The River Forecast Centre issued a flood warning for the Chilcotin River upstream of the landslide and a flood watch for below the blockage all the way to Hope.

“This is a dynamic and developing situation,” Ma said. “If this dam were to break, it is possible that dozens of evacuation orders and alerts will be issued along those rivers and people need to be prepared.”

The Cariboo Regional District has issued two evacuation orders, affecting 60 parcels and 10,750 hectares, for the area around the landslide. The province said it is not aware of any primary residential properties being affected.

The BC Wildfire Service dispatched helicopters to search for anyone in need of assistance with evacuating.

The Canadian Press reported that one man was injured in the slide and subsequently rescued. The province said it couldn’t provide an update on his status.

“No further people have been reported missing or impacted,” Ian Cunnings, director of regional operations for the Emergency Management Ministry, said at yesterday’s briefing.

Ma added that engineers, geologists, hydrologists, biologists and other technical experts are currently assessing the site. The extent of the risk is currently unknown, she said.

While the landslide originally appeared to create a “total and complete blockage” of the Chilcotin River near Farwell Canyon, about 80 kilometres southwest of Williams Lake, by Thursday there was evidence of “water seepage through the dam,” Ma said.

She did not directly respond to The Tyee’s question about how many municipalities, regional districts and First Nations the Emergency Management Ministry is currently in communication with about potential downstream flooding, saying only that the slide is “creating a number of concerns.”

“There are also concerns that persistent low water levels might impact navigation channels [for] fish, like the ongoing chinook run and the sockeye runs coming up, and that can have significant impact on the ecology of the river system,” she said.

The Chilcotin River has seen a precipitous decline in steelhead numbers in recent decades. In 2018, the Committee on the Status of Endangered Wildlife in Canada determined that steelhead in the river were at imminent risk of extinction.

B.C.’s water, land and resource stewardship minister, Nathan Cullen, echoed Ma’s concerns about the long-term impacts to fish in the river.

“We also know that the Chilcotin is a critical spawning habitat for salmon and other fish and, very critically, the sockeye moving up the system right now are meant to arrive in a couple of weeks, so we are initiating early plans as to what we can do to make sure that those stocks remain intact,” Cullen said.

But Ma said the immediate concern is for everyone’s safety.

“We are working to actively understand the infrastructure and the properties that may be at risk downstream,” she said. “There are already a number of road bridge and ferry closures in place.”