Disaster Dodged in Chilcotin River Landslide

On Monday morning, cellphones, radios and televisions in the B.C. Interior finally rang with the two-tone emergency alert many in the area had been waiting for.

For nearly six days, water had been accumulating behind a 30-metre-high accumulation of rock, trees and other debris that was deposited by a massive landslide across the Chilcotin River late on the evening of July 30. The river — which had previously been running at about 200 cubic metres of water a second — was reduced to a trickle.

At 9 a.m. Monday, the reservoir that had been gradually forming behind the 600-metre-wide landslide reached the top of the barrier. It formed a 15-metre-wide channel through the debris and began spilling into the empty riverbed beyond.

There was a risk of flooding, the emergency alert informed people in the immediate area and downstream for about 100 kilometres.

They were urged to evacuate immediately.

“Water from the Chilcotin River that has breached the blockade has now reached the Fraser River and is expected to make its way down to Hope and arrive this afternoon,” Emergency Management Minister Bowinn Ma said Tuesday during a provincial update.

Ma reiterated the message provincial authorities have been repeating for the past week: Stay well back from the Chilcotin and Fraser rivers. While this was not the worst-case scenario provincial authorities had feared — something Ma said would have involved “injuries and possibly death” — she said “we are not in the clear yet.”

“A worst-case scenario would have been having people on the shores, on the water, being hit by debris, and not being aware of the potential for these risks coming their way,” Ma said, adding that residents had heeded the call to stay off the rivers and away from riverbanks. “We do ask people to continue to stay off of the Chilcotin and the Fraser River and its banks.”

According to Connie Chapman, executive director of water management in the Ministry of Water, Land and Resource Stewardship, the worst-case scenario modelled by the province was a rapid drain of the reservoir within a six-hour window.

Instead, what occurred was something closer to the gradual 24-hour draining authorities had been hoping for.

That slow release began Monday morning, with the reservoir falling by about 2.5 metres each hour. At its peak, it produced flows of around 3,000 cubic metres per second — about 10 times Chilcotin River levels during an average spring runoff.

By Monday afternoon, the flow had reached the Chilcotin’s confluence with the Fraser River, pushing a “large amount of woody debris,” Chapman said. By 6 p.m., the front edge of the surge had reached Big Bar, 70 kilometres south on the Fraser.

It peaked there at about 1 a.m. Tuesday, raising water levels by 3.5 metres.

By then, the river had begun to rise in Lillooet, 100 kilometres south of Big Bar.

As it reached Lillooet, the surge had begun to dissipate. Water rose in Lillooet by just under three metres, Chapman said — less than what would be seen during a typical freshet.

“We are seeing that increase in elevation slowly decline as we move our way down the Fraser River.”

As flows finally reached Lytton, 60 kilometres farther downstream, they were estimated to raise water levels by about two metres. By Tuesday evening, they arrived in Boston Bar, 45 kilometres south of Lytton, having taken 34 hours from the time they breached the landslide.

The peak was expected to hit Hope, another 65 kilometres to the south, around midnight.

“What we are really looking for is watching that pulse of water move through the system, looking for where debris is settling out, and understanding where the debris may have been caught up,” Chapman said.

While flows were below annual peaks in the Fraser, they came with their own challenges. “This water will behave very differently,” Ma cautioned, adding that the sudden surge of water and potential for debris would bring its own risks.

Ma added that slope stability risks remain near the original slide — along with the potential that the river could again become blocked.

“Now that the water is flowing past the natural dam, there is a risk of further landslides, both upstream and downstream,” she said. “When you have a significant amount of water flowing in the way that we see through the Chilcotin, in an area that is very silty and sandy, a lot of loose sediment, there is a risk of additional landslides.”

Some live action of the Farwell Canyon slide on the Chilcotin River on the afternoon of August 2. pic.twitter.com/1f4dVZVCCL

— Ryan Onthevedder (@RyanOnthevedder) August 4, 2024

Chapman added that as the released water undercuts riverbanks, they could slump into the river. She said that water levels will need to recede before the risk can be thoroughly assessed.

“When you look at this landscape on the bigger picture, it is evident that slides are part of its natural cycle,” she said.

When asked about the potential impacts on salmon, with sockeye expected to return to the area in a few weeks, officials said it’s too soon to comment. The current focus, they said, is on keeping people safe.

“We aren't quite clear of the woods, yet. We have still a few hours left for the water to drain from behind the dam and then we need to take the time to assess how the landscape fares following that event,” Ma said.

“I am extremely grateful that the worst-case scenario did not come to be.”

The province is encouraging anyone in the vicinity to stay informed through EmergencyInfoBC and B.C.’s drought preparation website.