Role of Geomorphic Thresholds in Determining Alluvial Channel Morphology

摘要

Results of a combined laboratory and field investigation indicate that threshold conditions are associated with major changes in alluvial channel morphology. Thirty-one different channels were generated in the laboratory under a range of slope, discharge, and bedload transport conditions. At low values of these variables, straight channels were formed. As the values of these variables were progressively increased, threshold conditions were exceeded and meandering-thalweg channels resulted. The degree of meandering, as measured by thalweg sinuosity, increased with commensurate increases in the independent variables until critical conditions were again encountered. At this point, meandering tendencies abruptly decreased, and only braided channels were produced. When the sinuosity of these channels was plotted against slope, stream power, and other key variables, the presence and effect of thresholds became apparent. To test these observations under field conditions, geomorphic and hydrologic data were collected on 45 streams of widely ranging sizes in Indiana and Illinois. Variations analogous to those in the laboratory were observed, although there was comparatively more variation in the field relationships. From these and related results from other studies, it is clear that thresholds exist within the range of certain controlling variables and that when these thresholds are exceeded through natural processes or human influence, dramatic changes in channel morphology can occur. 38 references, 4 figures. (ERA citation 09:005431)