Sediment supply as a driver of river bed surface grain size and mobility, with consequences for aquatic habitat

Bed surface grain size and mobility are fundamental characteristics of a river. The size of coarse sediment on the bed of a river controls channel slope, shapes channel plan form morphology, and provides the architecture for aquatic habitat. Despite its importance to river morphology and habitat, the controls on bed sur- face grain size are imperfectly understood. Varying sediment supply, armoring of the bed surface, roughness effects, and patchiness complicate predictions. Because of its importance, understanding the controls on bed surface grain size is essential despite these complications.The goal of this dissertation is to further understanding of the controls on the size and mobility of surface grains in gravel bed rivers. The first study, Chapter 2, focuses on predicting reach-averaged grain size at the drainage basin scale. I build on previously proposed methods for grain size prediction, and demonstrate the effectiveness of a new method for predicting grain size in Scott Creek, a small gravel-bedded drainage in the Santa Cruz Mountains, California. This work is then applied to salmon spawning habitat prediction. As a part of that project, I show that grain size is much smaller in Scott Creek than the grain size predicted based on threshold channel theory. This finding motivates the second study in my dissertation. In Chapter 3, I show that there are significant regional trends in the ratio of bankfull Shields stress to critical Shields stress (τbf / τc) across North America. These trends are explained by differences in sediment supply. In Chapter 4, I consider the implications of these regional trends on the mobility of bed surface grains, and the potential implications for benthic habitat