SEDIMENT SUPPLY IN NATURAL RIVERS: IMPACTS ON GRAIN-PROTRUSION AND THE THRESHOLD FOR MOTION?
To determine if τ*c varies systematically with sediment supply, we measured the normalized resisting force (a proxy for τ*c) and grain protrusion (height of a grain above the mean bed elevation) in 12 different rain-dominated streams in northwestern Washington State, USA, spanning a range of low, medium, and high sediment supply. Our initial results suggest that the relationship between normalized resisting force, sediment supply, and protrusion is quite complex, with protrusion and resisting force varying substantially between sites in the same sediment supply category. We find no statistically significant difference in grain protrusion or normalized resisting force between sediment supply categories, suggesting that sediment supply may not act as a systematic control on τ*c. However, normalized resisting force does vary systematically with protrusion, with higher protrusion leading to a lower resisting force, consistent with findings from recent studies. While τ*c does not systematically vary with sediment supply in this initial dataset, our findings support the idea that bed structure acts as an important control on the threshold for motion and should be accounted for in sediment transport models.