Morphologically based model of bed load transport capacity in a headwater stream

Abstract

Field measurements of channel and bed material properties are used to develop a network‐based model of bed load transport capacity in Halfmoon Creek, a high‐gradient stream in the Rocky Mountains of Colorado. The formerly glaciated watershed contains self‐formed reaches that are generally disconnected from hillslopes and major sediment perturbations, strung together by a sequence of bedrock and cascade channels. Measurements of channel geometry, slope, and bed sediment texture (surface and substrate) were taken in 27 reaches with gradients ranging from 0.6 to 6% to examine interactions among key variables affecting bed load transport. The analysis shows that the grain size distribution of the substrate is relatively uniform throughout the basin; however, the distinction between surface and substrate texture varies systematically with channel gradient and shear stress. Reaches with high shear stress are characterized by coarse surface layers, with a high threshold for transport, limiting the mobility of the sediment supplied. In reaches with lower gradient and shear stress the difference between surface and substrate texture is less, increasing both the frequency of transport and the mobility of the bed load. While it might appear that the steeper reaches of Halfmoon Creek have a high capacity for carrying fine‐medium gravel, the relative mobility of these sizes is affected by the presence of very coarse sediment on the bed surface. As a result, modeled estimates of bed load transport rate indicate that average annual bed load sediment yields increase downstream nearly linearly with discharge, suggesting a simple scaling of bed load transport capacity with discharge and drainage area.