Mueller, Erich R.听1听;听Pitlick, John听2
1听Department of Geography, University of Colorado
2听Department of Geography, University of Colorado
Stream channel morphology and sediment textures are a product of the basin-wide delivery of sediment and water. Differences in sediment supply, particularly bed load, between watersheds should thus be reflected by differences in channel and bed-material properties. In order to address this directly, field measurements of channel morphology, substrate lithology, and bed sediment textures were made at over 50 sites distributed across three watersheds in the northern Rocky Mountains of Idaho and Wyoming (Fig. 1). Between these sites there exists a large range in sediment supply, primarily dictated by the underlying geology of the basins. In two directly adjacent Idaho basins, the Big Wood and North Fork Big Lost Rivers, differing proportions of soft (volcanic and sedimentary) and hard (granitic) rocks results in a 5-fold range in bed load sediment yield. In the Sunlight Creek basin in northwest Wyoming sediment yields are as much as 15 times greater than the Idaho basins (Fig. 2) owing to the very erodible volcaniclastic rocks of the Absaroka Mountains. This high sediment delivery drives Sunlight Creek to become braided, relatively rare for mountain streams in the interior western U.S. As a result, these three hydrologically and topographically similar sites provide a unique field-based experiment on the nature of channel adjustment to variations in sediment supply.
The results show that width and depth are remarkably similar between the two Idaho basins and for single-thread reaches in the Sunlight Creek basin (Fig. 2), with changes in stream bed armoring being the primary difference. For example, the Big Wood River has the lowest sediment supply and most armored stream bed as coarse granitic rocks come to dominate the substrate. A simple modeling approach using an excess shear stress-based bed load transport equation and observed channel geometry shows that subtle changes in sediment texture can indeed result in the marked difference in sediment yield between basins. Nevertheless, in the face of very high sediment supply armoring must reach a lower limit, thereby leading to significant morphologic change (e.g. braiding). This sediment supply threshold for the transition to a braided channel planform is supported by bed load sampling and measurements of channel morphology in the Sunlight Creek basin. Here braided reaches appear dynamically stable, whereby deviations about the mean state in flow and transport fields likely drive enhanced transport. The results from these three basins suggest that despite significant spatial variability in sediment supply, channel geometry may be relatively insensitive to change via textural-based modifications of transport intensity. Yet beyond some sediment supply threshold, morphologic change is necessitated, and standard hydraulic geometry relations break down in favor of increased channel complexity.