A landslide along the Chilcotin River roughly 54km south west of Williams Lake, B.C. is shown in this satellite image from August 1, 2024.Planet Labs PBC
Provincial modelling shows that a massive landslide currently blocking the Chilcotin River in B.C.’s Interior will likely be flushed down river without the need for major evacuations, the B.C. government said on Friday.
However, when the river will flow again and how the blockage will affect one of B.C.’s most important salmon runs remains uncertain.
Officials monitoring the dam now believe the 10-kilometre-long lake that has formed behind the landslide, located about 280 kilometres north of Vancouver, will gradually flow over the dam rather than breach it suddenly.
“Depending on the distribution and nature of the overtopping flow, the impacts could still be significant,” said Bowinn Ma, B.C.’s Minister of Emergency Management.
The hill slid into the Chilcotin early on Wednesday, forming a dam roughly half a kilometre long and 30 metres deep. The province has installed time-lapse cameras and a gauge on the impounded lake to monitor any sudden changes.
The Chilcotin flows into the Fraser River, providing about 10 per cent of the larger river’s volume as it snakes south toward the Lillooet, Lytton, Hope and, eventually, Vancouver. Ms. Ma said water from the dam would take between 12 and 24 hours to reach those major population areas, giving emergency officials time to alert people near the river.
Once the river flows over the dam, modelling at Simon Fraser University shows it could clear the silty debris within a day or two, according to environmental sciences professor Jeremy Venditti, who runs the school’s River Dynamics Lab.
“What will happen is eventually that lake will fill up, the water will start spilling over the top, and once that water starts going over the top, it’s just going to cut a slot through the landslide debris,” he said. “The river will start working through the sediment, and it’ll restore itself.”
The speed of that erosion stems from fine silt, sand and clay that make up the region’s soils. A smaller slide in 2004 disappeared in about four days, said Tl’etinqox Chief Joe Alphonse, who also represents five other local nations as tribal chairman of the Tsilhqot’in National Government.
“This one will take a little more time,” he said. “But once the dam opens a little, it’s anybody’s guess how fast it will release.”
Mr. Alphonse said the area’s shifting geography is embedded in the local language. In Tsilhqot’in, the area of the dam is called Nagwentled, which translates to “landslides across river.”
“It’s just something that has always happened and will always happen there,” he said. “Those river banks are silt and sand. You have the river cutting into the banks, along with natural spring water and forest fires destabilizing the soil.”
A wildfire engulfed the region in 2017 and officials have spotted charred material among the slide.
Mr. Alphonse said Nagwentled is also valued for dip-net fishing. Local fishermen would be taking Chinook right now, he said, and a prized sockeye run is expected to arrive around Aug. 10. “We rely on the salmon,” he said. “So this becomes very emotional for our people.”
Nathan Cullen, B.C. Minister of Water, Land and Resource Stewardship, said that historical data from 1964 and 2004 landslides indicate that salmon are likely to adjust to the current environmental factors.
However, a key concern is water temperatures, he said, noting that if water gets too warm salmon cannot survive.
“So we’re paying very, very close attention to not only extreme flow, but also temperatures, and working on plans with our partners at the DFO and First Nations to say, what can we do to mitigate and assist them and going to the system given any blockages or water temperature challenges that they face,” he said at briefing on Friday.
The SFU model provides some hope. Dr. Venditti said the debris should be cleared by the time the sockeye arrive. “It’s remarkable what salmon can get through,” he said. “They are adapted for this.”