Holocene landslides in the Fraser Canyon Corridor and implications for ‘ecohazard’ assessment

Abstract Landslides are important natural hazards to infrastructure and humans, but they also pose a hazard to the biosphere and constitute an ‘ecohazard’. Recent and prehistoric landslides in southwest British Columbia have significantly impacted local fluvial processes and nearby ecosystems. For example, the 2018 Big Bar landslide caused a major hydraulic barrier in the Fraser River, which impeded upstream salmon migration for 2 years. To better understand the impact of landslides on the biosphere, defined here as ’ecohazard’ impacts, we assessed the distribution, size and character of landslides near the Fraser River over the past 12 to 15 ka. We created a landslide inventory using 2560 km 2 of airborne lidar along the Fraser Canyon Corridor, a 375‐km stretch of the Fraser River, which flows through varied topography and bedrock. We mapped 274 landslides with planform areas between 2 × 10 3 and 2 × 10 6 m 2 and estimated volumes that range from 600 to 1.05 × 10 8 m 3 . While the landslides with the largest area were flows/spreads in volcanic rocks, landslides in intrusive and metamorphic rocks had the highest median area. Area–volume relations were consistent across lithology and failure styles. Area–frequency and volume–frequency distributions have a positive skew, in agreement with other landslide inventories. Area–runout relations were also consistent across classifications, meaning the likelihood of a given landslide event reaching the Fraser River is primarily controlled by valley morphology rather than lithology or specific failure mechanics. We propose an ecohazard impact framework to classify the risk posed to salmon migration for the different classes of landslides documented in our inventory. We found that landslides of similar size and character to the Big Bar landslide are relatively common within the Fraser River Corridor; however, their location within the valley will determine how likely they are to significantly impact the ecosystem.