Perspectives in the Mitigation of Rockslides affecting Fish Migration

Year: 2020
Presenter/s: Barry Chilibeck
Symposium Session: 2020 - 00 Invited Speaker
Topics covered: beavers, canada, fish passage, hydraulics, lessons learned, risk and resilience, sediment transport, and stream


Landslides are a physical process that had an undeniable effect on the landscapes within the Pacific Northwest. Recently, they have been experienced on several rivers that have affected Pacific salmon in profound ways. This talk is going to look at several recent landslides on rivers with Pacific Salmon and other fisheries resources, the effects on the communities and people that depend on fish, the work done to mitigate the physical and biological damage, and lasting influences and future risks.
The Seymour River is a small coastal watershed in the heart of Vancouver’s North Shore mountains. The upper watershed forms part of Metro Vancouver’s water supply while the lower areas provide recreational and park values. A 50,000 m3 rockslide in the lower Seymour River canyon in December 2014 resulted in a complete blockage of the river and the migration of Steelhead, Coho, Pink and Chinook salmon upstream – and downstream. This river’s story chronicles the community-led efforts and success in ensuring salmon prevail and restoring access to the river for fish for future generations. Technically, this mitigation work examines the trade-off analysis completed to assess the potential slide mitigation, and the issues that led to a passive, river-based approach that utilized the stream power of the slide to mobilize, reform and distribute the slide debris downstream, effectively re-grading the rockfall cascade. The process was not without risk due to the time required to seasonally work on the slide during the low flow period and then monitor and assess the slide during the interim months. The approach relied on determining the size of slide debris that could be mobilized, as well as the configuration of the slide debris as it shifted and moved during high flow events. By continually oversteepening the instream face of the rockslide by selective rock breaking, the slide was removed.

Biological programs and monitoring played a key part in ensuring that the salmon and steelhead stocks native to the Seymour were not extirpated during the 5 years of seasonal work. Without the efforts of community partners – notably the Seymour Salmonid Society – both the slide mitigation and support work would not have happened. They led the operation of the floating fish fence and fish collection; conservation hatchery restocking of the river; and continued assessment of downstream and upstream migrants. Shortly after completion of the 2019 work program on the slide, late summer rains brought some high flows into the watershed and in late August 2019, the first adult coho was observed above the slide. Follow-up radio tagging confirmed passage through the slide in September 2019 and there are ongoing efforts to monitor the movement and recovery of salmon and steelhead.

The Fraser River is one of North America’s foremost freshwater habitat for wild salmon encompassing largely an undammed, unregulated ecosystem from the Canadian Rockies to Vancouver, BC. While most people know about Hell’s Gate and the fishways that continues to mitigate that rockslide, they are largely unaware that the Fraser River continues to flow through hundreds of kilometers of canyons and bedrock formations that have inherent rock fall and slide risks. Roughly 100 years after Hell’s Gate Slide, the Big Bar Slide is proving that history repeats itself. On or about November 1st 2018, a massive slide event occurred near Big Bar on the Fraser River, 40 km North of Lillooet, BC. Unbeknownst for months, the slide was discovered in early spring 2019 before freshet by local ranchers and river rafters.

The chain of events and actions triggered by the impacts of the slide on one of the largest natural salmon-producing rivers is a continuing story. While the biological and cultural implications of this event are much better understood than Hell’s gate, the technical challenges are perhaps even more daunting given the location and local topography of the Big Bar slide. This presentation highlights the capcity and effort that can be harnessed when organizational barriers are dissolved and people come together for a common cause, the limits that still persist and challenges of what can be achieved to mitigate a slide to restore fish passage on a massive river in a remote isolated canyon.