University of California Water Resources Center

Changes in sediment supply relative to transport capacity In .e,ravel-bed streams lead to adjustments in stream geomorphology. Dietrich et al. (1989) propose that when sediment supply is low relative to transport capacity, bed surface coarsening develops due to selective erosion of fines. They developed a dimensionless bedload transport rate (q*), which is the ratio between the sediment transport rate of the surface layer and the transport rate of material as fine as the subsurface or bedload. Flume experiments show that q* approaches unity when supply meets or exceeds capacity, and decreases in value when supply decreases. Longitudinal profiles and crosssection surveys on 7 km of the North Fork American River, CA show a downstream increase in supply relative to transport capacity. Q* values calculated from 15 sediment samples show corresponding increases downstream, indicating that the q* method may be useful for detecting trends in the supply-capacity relationship. Strong trends, however, were only seen when looking at the entire 7-km reach. On short reaches with limited data, absolute values of q* cannot be used for an accurate assessment of the supply-capacity relationship.