The provincial and federal governments, along with affected First Nations, are assessing how to move salmon runs above the massive July 31 landslide that blocked the Chilcotin River a few kilometers upstream from where it meets the Fraser River, near Williams Lake, B.C.
Authorities are worried debris blockage may impede the ability of salmon and trout to reach their spawning grounds. They are also uncertain about damage done to fish that were in the river when the slide occurred, as well as the condition of the downstream river-bottom because of high velocity flows and debris scour.
The Chilcotin River is a major tributary of the Fraser River flowing eastward 241 kilometers from its headwaters in the Coast Mountains. Its watershed encompasses 19,200 square kilometers of mixed terrain over a significant elevation drop – it is not a small river, with a mean discharge of 102 cubic metres per second.
The river holds important salmon and trout populations which have sustained the Tsilhqot’in First Nation people for thousands of years. The slide raised immediate concerns for both Sockeye and Chinook salmon whose runs are beginning to enter the Chilcotin River and will peak over the coming weeks.
These salmon, particularly the Chilko Sockeye, are already a major conservation concern to the Tsilhqot’in, on whose lands the slide occurred. The nation, in coordination with the Canadian Department of Fisheries and Oceans (DFO), agreed to stop fishing for them in 2021.
Chilcotin Steelhead trout are also at perilously low escapement levels. These runs were impacted by the Big Bar landslide on the Fraser River four years ago near Lillooet, B.C.
“Chilcotin Steelhead runs are currently seeing historic lows in their population. This slide and its impacts on those fish are extremely concerning,” said Brian Braidwood, president of the Steelhead Society of BC.
Unstable aftermath remains a ‘dynamic situation,’
The slide was immediately classed as a “dynamic situation” by B.C.’s Minister for Emergency Management and Climate Readiness, Bowinn Ma. The dam, consisting of highly unstable materials, completely blocked the river’s flow creating a 10-12 kilometer long lake. This precipitated immediate evacuation orders for the Chilcotin River both up and downstream of the slide.
Speculation is that a significant 2017 fire was the catalyst leading to the slide because the loss of the trees and their root system de-stabilized the terrain. It’s now a near certainty that a recent significant rainfall further weakened the ground triggering the huge slide.
Initial government reporting put the size of the total slide at 30 metres deep, 400 to 600 metres wide and 600 to 800 metres long. Of this, as much as 30 per cent, by unofficial visual estimates, is blocking the Chilcotin River. Other unconfirmed estimates put the total volume of materials in the slide at 9.3 million cubic metres, spread over 0.22 of a square km.
Preliminary assessments by government of the site predicted the river would likely overflow the debris dam, cut a new channel, and drain the lake slowly. If the blockage let go, technical experts estimated it would take 12 to 24 hours for the debris-filled surge to reach Hope.
Within a few days, the Tlishqot’in First Nation reported the river had begun to flow over the dam as predicted, but not as slow as hoped. Water levels on the Chilcotin River below the blockage rose dramatically, and banks eroded, causing significant loss of riverbank property as the fast-moving current, filled with thousands of cubic metres of debris, scoured the riverbed from the slide site to the confluence with the Fraser River.
Even though half the floating debris, roughly 30,000 cubic metres, was estimated to have been captured in a Shxw’owhamel First Nation’s-managed debris trap near Agassiz, the danger level remains high.
Worst case scenario is that water saturation of the debris pile will weaken it leading to rapid dam failure and significant downstream damage caused by fast moving water loaded with debris from the landslide.
No-boating advisories continue along the entire length of the Fraser.
Tsilhqot’in warn more slides may occur
“DFO is working internally and with partners to ensure the safety of people and infrastructure in an around the impacted area,” a Department of Fisheries and Oceans spokesperson said.
Long term concerns are focused on mitigating and preventing future landslide events caused by human activity and/or climate change in vulnerable B.C. watersheds.
Tsilhqot’in leadership warn additional slides could occur in the short term given the appearance of new cracks along the watershed slopes, and they expressed concern about potential loss of culturally significant sites.
The Farwell Canyon region has a history of slides due to a mix of sandy soils and clay, and some Tsilhqot’in members report having experienced two or three slides in their lifetimes. However, none of the previous slides approach the magnitude of this event.