Riverbed sediment grain size distributions: why salmon care and we should too
The size distribution of sediment supplied to a reach of river influences nearly everything that happens there. For example, in gravel-bedded rivers, grain size affects hydraulic roughness and thus flow depth and flood risk for a given discharge. Grain size also affects the frequency and magnitude of sediment transport, and thus the pace and direction of morphologic change. Channel geometry and slope, in turn, along with bed material, influence the type and quality of habitat for aquatic organisms.
At the relatively short time scale of analysis for most river restoration projects, riverbed grain size is often treated as a function of the slope and discharge available to move bedload. However, at longer time scales, bed material is determined by the size distribution of sediments produced on hillslopes in the upstream catchment, and how those sizes are sorted and modified in transport through the river network.
This talk will explore river sediment grain size distributions by asking two questions, first the practical question of how sediment size influences spawning habitat for salmonids, and then the basic science question of what factors control the size of sediment supplied to rivers by hillslopes.
Leonard is a geologist and civil engineer, specializing in hillslope and fluvial geomorphology. He is currently an Adjunct Professor in the School of Environmental Science at Simon Fraser University in British Columbia, and previously was a Professor in the Department of Geography, Planning and Environment at Concordia University 2017-2021 and Professor in the Department of Earth and Climate Science at San Francisco State University 2003-2017.
Leonard’s current research projects include investigating: controls on the size of sediment particles produced on hillslopes and supplied to rivers; physical wear of sediment grains in transport; influences of sediment size on landscape evolution, debris flow hazards, and aquatic habitat; bedrock river channel erosion; and the influence of living organisms on river sediment transport and morphology.