SPATIAL VARIABILITY AND CONTROLS OF BANK INSTABILITY IN A SEMI-ARID DRAINAGE BASIN IN SOUTHEASTERN UTAH

Riverbank retreat presents serious problems to engineers, environmental managers, farmers, and those who reside near bank areas through loss of land, danger to riparian and floodplain structures, and increased downstream sedimentation. The objectives herein are to determine the spatial variability of bank instability within the upper South Cottonwood Creek drainage area, and to assess the dominant controls responsible for the erosive processes. Zones of significant bank instability and associated processes were recorded at individual units along the channel and a map of bank instability spatial variability was constructed. Cross-sections (herein called sites) were measured in both stable and unstable units, and estimates for geomorphic bankfull discharges were calculated in order to derive stream power values. Additionally, valley and floodplain dimensions, surrounding human disturbance information, and bank characteristics were recorded.

Results suggest unstable units have high unit stream power values with corresponding narrow valley widths, high discharge and slope values, significant channel curvature, lack of stabilizing vegetation, and poorly consolidated banks. Furthermore, longitudinal position in the drainage network proves to be an influential control on the types of bank retreat processes. The mountain segment is characterized by direct trampling and sub-aerial preparation processes, and bank characteristics are the best indicators of stability. The canyon segment is most influenced by fluvial erosion, a process determined by unit stream power. The wash segment is characterized by both direct fluvial entrainment and bank failure. Here, bank characteristics, especially bank angle and height, and unit stream power are the best indicators of stability. Additionally, human disturbance exerts strong control throughout each segment