A typology of hydraulic barriers to salmon migration in a bedrock river

Each year a variable portion of adult Pacific salmon in the Fraser River, British Columbia, Canada die trying to retrace and ascend the river network to their natal spawning grounds. A major factor in migration failure is the severe hydraulic conditions experienced in the Fraser Canyon where encounter velocities can exceed upstream swim speeds of adult salmon, creating a migration barrier. Hydraulic barriers are defined as reaches of river where upstream fish migration is delayed due to high water velocity. A few barriers have been identified along the river and have structures in place designed to help facilitate fish passage. We explore other locations in the Fraser River that are apt to be hydraulic barriers to fish migration based on measured centerline velocity. We classify the barriers as either 1) plunging flows in canyons where the channel is deep and the fastest velocities are observed deep in the water column, 2) rapids where flow is fast and shallow over one or more bedrock steps, or 3) overfalls where fast flow occurs over a step with a substantial drop in elevation. We used drone footage at various discharges and Large-Scale Particle Image Velocimetry (LSPIV) to examine flow structure at typical plunging flows, rapids and overfalls. Surface velocities for the discharges when salmon species are known to be migrating upstream were then compared with published salmon swimming capabilities to determine which locations are likely to create the greatest barriers to salmon migration. We find that there are twenty-two sites, sixteen measured and six suspected high velocity locations, that are potential hydraulic barriers. Overfalls present the greatest barrier to salmon migration, creating vertical barriers in addition to high velocity across the entire width of the channel in narrow laterally constricted reaches. Rapids have high velocity in the segments of the water column where salmon typically swim, but often have back eddies along the banks for fish to rest. Plunging flows in canyons have high depth-averaged velocities, and higher bank velocities as a result of turbulent upwelling along the walls, but typically lower surface velocities than the overfalls and rapids. Pacific salmon populations are already threatened by external factors – such as climate change, habitat degradation, fishing, and disease – and cannot afford to have these impacts amplified by additional barriers to migration. Our observations provide important information for salmon conservation and can be used to better understand salmon migration which in turn helps to inform future mitigation efforts to improve salmon survival rates.