Flow and suspended sediment transport through the gravel-sand transition in the Fraser River, British Columbia

The Fraser River, British Columbia is a large alluvial channel that features an abrupt gravel-sand transition that occurs due to a dramatic slope change and the ocean base-level control. There have been no previous observations of the sediment dynamics through transitions in rivers of this scale. I examine the spatial and temporal changes in flow and the suspended sediment transport regime through the transition using hydro-acoustics in an attempt to test the hypothesis that sand in the gravel-bedded reach is coming out of suspension to form the sand-bedded reach. The results indicate that during high flows a downstream shear stress gradient does not exist and the suspended sediment flux declines downstream until the sand-bedded reach where the flux increases. These results suggest that there is a sediment supply gradient and during low flows sand is stored in the gravel-bedded reach and only released to the sand-bedded reach during large floods.