Landscape controls on sediment supply and stream channel morpho-dynamics in the northern Rocky Mountains.

摘要

Quantifying landscape variations in sediment supply to streams and rivers is fundamental to our understanding of both denudational processes and stream channel morpho-dynamics. Previous studies have linked a variety of sediment supply proxies to climatic, topographic or geologic factors, but few have connected these directly to the characteristics of fluvial systems draining these landscapes. Using measurements of water and sediment fluxes for over 80 basins in the northern Rocky Mountains, USA, it is shown that the sediment supply signal is dominated by basin lithology, while exhibiting little correlation to factors such as relief, mean basin slope, and drainage density. Bankfull sediment concentrations (bed load and suspended load) increase as much 100-fold as basin lithology becomes dominated by softer sedimentary and volcanic rocks, at the expense of more resistant lithologies.;Downstream hydraulic geometry relations for single-thread reaches in these basins are remarkably similar and reasonably well predicted based on a channel forming Shields number; yet this simple model cannot capture the 2–3 order of magnitude range in sediment flux for a given discharge. In these streams the difference in the magnitude of bed load flux is modulated regionally by changes in bed armoring, resulting in a non-unique Shields number for a given channel configuration. As a result, single-thread reaches can absorb a wide range in sediment concentration, but at very high concentrations, bed surface, subsurface and bed load grain sizes converge and a transition from single-thread to to braided channel patterns is commonly observed. A physically-based sediment concentration braiding—single-thread discriminant function is derived and tested using the empirical data, appropriately classifying 50 of 53 pattern types. Flow modeling shows that 2-dimensional variability in flow properties in braided reaches may become equal to or dominate over changes in slope in response to high sediment supply. The resilience of single-thread channels to morphologic change thus reflects the degree to which textural changes can modulate variations in sediment supply, and a transition to a braided planform likely represents a dynamic equilibrium form in the face of high sediment supply.