SEDIMENT SUPPLY CONTROLS RIVER BED SURFACE GRAIN SIZE AND THE AVAILABILITY OF SALMONID HABITAT (Invited Presentation)

The riverbed surface forms the physical structure on which river ecosystems grow. Predictive geomorphic models for bed surface grain size have the potential to inform habitat restoration and protection efforts.

A standard approach to predicting bed surface grain size is to assume a constant Shields stress, then use channel slope and estimated hydraulic geometry to solve for bed surface grain size. However, not all natural gravel channels conform to a constant Shields stress. We show that across North America, sediment supply to rivers varies by 3 orders of magnitude; this natural variation in sediment supply is a major driver of surface grain size in gravel bedded rivers. A database of >300 gravel-bedded rivers reveals trends in bankfull Shields stress (a function of grain size, slope, and hydraulic geometry) that mirror trends in both short- and long-term erosion rates. High sediment supply regions are correlated with high bankfull Shields stress. The ratio of bankfull to critical stress (τ*_bf/τ*_c) is significantly higher in West Coast river reaches (2.35, n= 96) than in river reaches across the rest of the continent (1.03, n = 245). In support of the hypothesis that sediment supply drives this pattern, we find a significant correlation between upstream erosion rate and local τ*_bf/τ*_c at sites where this comparison is possible.

Knowledge of this relationship between sediment supply and bed surface grain size is needed to inform our predictions of physical habitat characteristics. We have developed an approach to predicting grain size distributions from high resolution LiDAR (Light Detection and Ranging)-derived topographic data, which we test in Scott Creek, a 77 km² mountain watershed along the California Coast. The predictions require knowledge of bankfull Shields stress. We use the predicted grain size distributions to calculate the fraction of the bed area movable by spawning salmonids in each reach of the basin. Comparing these results to steelhead (Oncorhynchus mykiss) and coho (O. kistuch) spawning surveys, we find that redd density is highest in reaches of high ‘moveable fraction’. The predicted distribution of spawning gravel is very sensitive to bankfull Shields stress. Thus, the natural sediment supply conditions within a basin should shape our predictions of bed surface grain size and the distribution of lotic habitat.