Linking fluvial dynamics to white sturgeon habitat in the Nechako River, BC.

Considerable effort has been dedicated to restoring sturgeon habitat within dammed rivers. However, sedimentation causes long-term failure because interstitial voids provide critical habitat during early life-stages. Based on the premise that a better understanding of geomorphic processes will improve restoration design, this study characterizes flow and sediment transport dynamics through a white sturgeon spawning reach on the Nechako River, BC. An extensive dataset was collected throughout the 2015 flood. Bedload transport was sampled on 36 days with flows ranging from 44 m³/s to 656 m³/s. During a high flow of 525 m³/s, channel bathymetry and water surface elevation were surveyed and velocity profiles were collected across 9 transects. Banklines, bars and island topography were later surveyed during low flow. Sediment transport into the reach was positively related with discharge. This relation was non-linear and transport rates increased rapidly once flows exceeded 400 m³/s. The relation weakened with downstream distance and sediment transport peaked progressively later throughout the year. No relation was observed at the downstream end of the reach, where transport rates remained low and constant relative to upstream. Sediment was primarily transported through secondary channels conveying a disproportionate amount of sediment compared to flow. Within the single-thread channel, the locations conveying the greatest amount of sediment remained spatially consistent over time. Hydrodynamic modelling indicates the Burrard Ave. Bridge causes backwatering once discharge exceed 225-275 m³/s. Velocity, shear stress and transport capacity at the downstream end of the reach do not increase with discharge because of the backwatering and the expansion in channel width through the island complex. The locations of maximum shear stress and transport capacity shift upstream with increasing discharge, but shear stress does not exceed 23 N/m² for flows up to 775 m³/s. The fluvial dynamics within the spawning reach create challenges and opportunities for habitat restoration. Backwatering is problematic because it causes mid-reach deposition during high flows and limits shear stress magnitude over the downstream spawning substrate. Meanwhile, the presence of sediment transport pathways through secondary channels and within the mainstem can be used to site restoration projects in areas apt to maintain suitable habitat.Arts, Faculty ofGeography, Department ofGraduat