Ecohazard posed by a rockslide that blocked salmon migration in the Fraser River, British Columbia

On November 1^st, 2018 the Big Bar Landslide temporarily blocked Fraser River, the most productive salmon-bearing watershed in Canada, presenting a barrier to upstream salmon migration in 2019 and 2020.  The landslide is an example of an ecohazard, similar to a natural hazard, but with immediate and direct impacts on the biosphere, rather than on people and infrastructure.  Like natural hazards, ecohazards have cascading effects, where a geophysical process triggers additional events, often with dramatic consequences.  The Big Bar Landslide originated from collapse of a steep bedrock wall and deposited 89,000 m³ of rock into one of the narrowest sections of the Fraser River, damming the channel for over 7 hours, and impounding 650,000 m³ of water.  The rockfall debris formed a bank to bank step with an ‘overfall’ that was ~4 m at low flow and ~7 m at high flow.  The overfall resembled a waterfall, but without a freefall into a plunge pool.  A backwater formed upstream that extends ~750 m upstream at low flow, but several kilometers at high flow trapping incoming sediment.  The overfall generated a hydraulic barrier to upstream salmon passage, and significantly impeded salmon migration to the Upper Fraser Basin in 2019 and 2020, but rock work has partially ameliorated the impact.  Fish passage monitoring indicates success in passing the landslide in 2019 was species and discharge dependent with population-specific estimates ranging from <1% to over 80% success.  Passage success was particularly low for early timed populations exposed to the highest flow in 2019; so few fish successfully migrated to the spawning grounds that there was a risk of functional extinction of those runs.  There is an ongoing risk to all salmon populations above the landslide until hydraulic conditions at the slide stabilize.  The event will have cascading effects on upstream ecosystems, Indigenous peoples who rely on the salmon fishery throughout the Fraser River Basin, and commercial ocean fisheries, but the extent of the cascading effects is not yet known.  Collapse of bedrock canyon walls, like the one that started the ecohazard cascade in the Fraser River, are geologically commonplace, but the risk they pose to migratory fish populations in mountainous river systems is largely unknown.