Continuous and Real-Time Sedimentation Monitoring Within Spawning Substrates On The Nechako River, Bc

Substrate change has been identified as a primary mechanism for the failure of white sturgeon recruitment in the Nechako River, however implementing substrate restoration within the active channel of the Nechako River presents multiple uncertainties. Two key uncertainties include how quickly spawning substrate infills and whether certain flows can flush infilled sands, thereby increasing the interstitial space between the gravel particles. Interstitial spaces provide voids for eggs and larvae hiding during the incubation period; if interstitial spaces are filled with sand, the potential for egg and larvae survival is greatly reduced. To improve our understanding of infilling processes in the Nechako River we used thermal conductance to monitor infilling. Heaters and temperature sensors deployed on the riverbed enabled near-continuous monitoring of sediment infilling and flushing in real time. The heaters are turned on for 30 minutes and the rise in temperature reflects the degree to which the interstitial spaces within the gravel are filled by fine sediment. In total, three sensor systems were installed across the width of the river, which showed very different rates of sediment accumulation. The sensor nearest to the right bank showed that fine sediment began infilling the gravel substrate within two days of deployment, while the middle sensor took 12 days and the left bank sensor took 27 days before infilling started. An increase in river flow from 75 to 500 m3/s (approximately a five-year return interval flood) resulted in additional increases to sediment infiltration. The data show no indication that high flows removed sediment from within the interstices of the gravel. The results from this study show that placing clean substrate into the river immediately before the spawning season may be of limited value as the substrate can infill within the duration of time required for incubation of eggs and larvae. Furthermore, the data suggest that high flows have not removed sediment from the interstitial substrate. Additional information will be collected during the fall of 2022 when the sites will be visually inspected and an underwater camera will be cleaned and redeployed to enable continuous monitoring of bed surface conditions. Results from this additional monitoring will help improve the collective understanding of substrate mobility at the surface of spawning gravels during lower flow conditions.