Above the threshold for motion: erosion rates recorded in the bedload transport capacity of West Coast channels?

Abstract:

Gravel-bedded river channels commonly evolve so that median sized grains on the bed surface are at, or near, the threshold for motion when the flow reaches bankfull stage. In addition, theory predicts the near- equivalence of the bankfull (bf) and critical (c) non-dimensional shear stresses (τ*_bf ~= τ*_c) for straight, cohesionless gravel channels. However, not all natural gravel channels conform to this simple relationship. To understand why some channels maintain bankfull stresses far in excess of what is needed to initiate sediment motion, we compile a dataset of the hydraulic geometry and median grain sizes of ~300 reaches from gravel-bedded rivers in North America. Notably, we find that the ratio of bankfull to critical Shields stresses is significantly higher in West Coast river reaches (2.6, n=77) than in river reaches in the rest of the continent (1.0, n=216). We explore the hypothesis that these reaches have adjusted to maintain a high excess shear stress at bankfull flows to accommodate elevated sediment supplies resulting from rapid erosion along the tectonically active margin of western North America. As a test of this hypothesis, we explore spatial patterns in calculated transport capacity and ¹⁰Be-derived erosion rates across North America. We find that an order of magnitude decrease in sediment transport capacity away from the Pacific plate boundary parallels an order of magnitude decrease in ¹⁰Be-derived erosion rates. These findings suggest that tectonically sustained high erosion rates on the West Coast may be recorded in the channel geometry and bed surface grain size of gravel-bedded channels.