Sediment and Salmon: The Role of Spawning Sockeye Salmon in Annual Bed Load Transport Characteristics in Small, Interior Streams of British Columbia

Abstract

The redd construction process of sockeye salmon Oncorhynchus nerka may be an important and unique component of bed load transport in some northern, interior British Columbia streams. Using pit trap samplers in four streams over 6 years, we examined bed load accumulation and size composition after three distinct periods: spring flood (high flow), salmon spawning (low flow), and winter (low flow). Bed load movement increased with stream velocity and more accumulated during the spring floods than during the winter. However, bed load accumulations were greater during the spawning period than during the winter at similar thalweg velocities. There was a significant influence of spawning fish density on the amount of bed load accumulated. The variability in bed load size composition was most attributable to the tendency for spawners to move proportionally more large material than was moved during the flood and winter periods, with a grain-size distribution that was comparable to the source material in all but one of the creeks. Mobilization of intermediate-size material distinguished the spring flood period from the low-flow winter period but accounted for considerably less of the overall variation. Variability among years was minor. Relative to hydraulic factors, salmon spawning activities have a density-dependent and unique influence on the mobilization of bed material in interior watersheds. We speculate that there is a positive feedback mechanism by which healthy salmonid stocks contribute to the maintenance of the small stream ecosystems on which they depend.