Landslides and Salmon

Landslide Impact on Flow Dynamics, Fish Migration and Genetics of Fraser River Salmon was a 3-year project led out of Simon Fraser University (SFU) and funded by Fisheries and Oceans Canada, British Columbia Salmon Restoration and Innovation Fund.

On June 23, 2019 a rockslide was reported in the Fraser River near the small, remote community of Big Bar, British Columbia, Canada that blocked the river to upstream salmon migration. Salmon are iconic animals on the west coast of North America, a critical element of Indigenous culture, a food fishery for Indigenous people, and an important commercial fishery in British Columbia. The landslide initiated one of the largest and most expensive emergency river restoration projects in the history of Canada.

We investigated the landslide, its impact on the physical environment, and how those changes affected salmon migration to the upper Fraser Basin. We found that ~8.9×10⁵ m³ of rock failed into the Fraser River on November 1^st, 2018, creating a ~7.5 m high overfall that is similar to a waterfall, but without freefalling water into a plunge pool. Our observations of flow and salmon migration tracking show that migration success at the overfall was discharge dependent. At high water discharge, velocities at the overfall exceeded 10 m/s and well above the maximum swim speed of salmon. Migration success for early-timed salmon populations exposed to the highest flows was <1%. Survival improved to 50% and up to 80% for successive timed populations as flows decreased through the summer. Blasting of the canyon and slide debris in 2020 fundamentally changed the hydraulic and construction of a rock rampart along the eastern bank increased the salmon passage threshold discharge near the mean annual flow, preventing potential extirpation of the early timed salmon populations in the Northern Fraser Basin.

The Big Bar Landslide jeopardized salmon populations in the Upper Fraser Basin, but this landslide is not unique – the chance of future slides in the Fraser Canyon is real, and past landslides have likely shaped current salmon biodiversity. The likelihood and impacts of these events on salmon remain uncertain. To improve our ability to respond to this and future events we undertook an investigation of past, present, and future impacts of landslides on Fraser salmon.

To understand past landslides, we searched the Fraser Canyon Corridor for other past landslides to determine how often landslides occur and how big they are. We found evidence for >200 landslide events since the last glaciation (Queston 1). We searched the river for other places where the river may have been blocked by a landslide and found three that impacted the abundance and diversity of Fraser salmon (Question 2). We found that a major landslide blockage in the southern Fraser Canyon, below the confluence with the Thompson River, could be catastrophic to Fraser Salmon abundance and diversity, suggesting long-term monitoring of locations at risk a future landslide is necessary.

We undertook a Reconnaissance of the Fraser Canyon where we searched for locations where present-day flow velocities may affect salmon migration in the Fraser River and identified 22 that may be problematic and many more where the river morphology could produce problems for salmon migration at moderate to high flows (Question 3). We undertook detailed studies of river morphology, hydraulics, and salmon migration at prototypical morphologies where we thought salmon migration might be impeded or blocked at high flow and found that the speed of the river exceeded the maximum speed at which salmon can swim at high discharges (Question 4). We used our reconnaissance to map the topography and velocity distributions in all the bedrock canyons of the Fraser River (Canyon Velocity Maps Appendix). We also examined how complex river morphologies affect salmon migration rate and how salmon navigate complex flow fields (Question 5). We investigated whether the Big Bar Landslide affected the genetic diversity of Fraser salmon and fortunately found no evidence for impacts at this time (Question 6). We also investigated whether salmon who passed the Big Bar Landslide where genetically different than those who did not and found the landslide selected for migration timing (Question 7).

Our Reconnaissance of the Fraser Canyon allowed us to identify 13 Sites of Concern (SoC) where we think a future landslide would produce a blockage to salmon migration which is based on expert opinion and supported by a detailed analysis of the riverbanks and valley. We find the landslide risk to salmon increases from north to south along the Fraser Canyon as the valley walls get steeper and there are fewer places to store landslides in the valley (Question 8). There are two zones where landslide risk is heightened: from Yale and Boston Bar, and between Bridge River to White Canyon. We produced an interactive map showing the geospatial data we used in our analyses (https://arcg.is/1iqyi5) and a living document called ‘Fraser Canyon Sites of Concern for Future Landslides’ posted on our website (https://www.sfu.ca/landslides-salmon.html) that show SoC, salmon populations that would be affected, and where the SoC are in relation to traditional territories of the Indigenous Nations along the Fraser Canyon. The reconnaissance also provided us with the data necessary to consider what size of landslide debris could be moved by the Fraser River and how long a blockage would last (Question 9). We find that the volume of the landslide and the size it delivers to a river will control the persistence of a salmon blocking landslide. To communicate our findings to a broader audience we built an interactive map (https://arcg.is/1eau010) that allows users to see which salmon populations could be affected by a landslide at our sites of concern (Question 10).

Through the course of our work, we developed new relationships with Indigenous leaders, communities, and Nations, upon whose traditional territories we have been privileged to work (see Engagement Report). We endeavor to maintain and build those relationships as we disseminate results of our investigations and undertake new scientific studies.

The 2018 rockfall that blocked the Fraser River to upstream salmon migration in a remote location of British Columbia reminds us how interconnected we all are with the river. Many of the world’s rivers with critical infrastructure and ecological communities to protect have a monitoring centre tasked with generating the data required to make significant decisions about a river’s environmental management. In British Columbia this does not exist. Instead, we have a patchwork of government agencies, NGOs, consulting companies, and university researchers all struggling to put the pieces together, and all plagued by an absence of core information on the state of the river. The sum total of work reported here has produced a baseline dataset that is unparalleled in the world. We have more information about the Fraser Canyon than almost any other large bedrock river in the world (see Data Availability Report). Our dataset provides an opportunity to build a comprehensive baseline dataset of the Fraser Basin that can be used to understand the effects of climate change and other anthropogenic influences. We need a Fraser River Science Monitoring and Research Centre organized as a collaboration between First Nations, academia, and government, tasked with watching for the next landslide, building baseline datasets for other parts of the Fraser Basin, and studying the fragility of the mighty Fraser River.