THE ROLE OF STREAMBANK EROSION IN CHANNEL ADJUSTMENT AND SEDIMENT LOADINGS

Streambank erosion and channel widening are integral parts of channel adjustment in disturbed, alluvial streams by providing sediment for downstream aggradation and by reducing boundary shear stress and transport capacity. Sediment loadings by streambank erosion can additionally be the dominant source of sediment in adjusting alluvial-stream systems. In both regards, it is the absolute and relative resistance of the banks that determine the role of streambank erosion in channel adjustment and sediment loadings. Still, analysis of streambank processes are often neglected in analyses of channel response and watershed sediment loads. Analysis of adjusting stream systems from different physiographic provinces shows how streams of divergent character (Coastal Plain vs. Cascade Mountains) adjust their morphologies such that flow energy and boundary shear stress are reduced with time. This is manifest in both aggrading and degrading reaches, largely due to the reduction in the pressure head due to widening. The relative importance of streambank erosion and channel widening is directly related to the resistance of the bank sediments to mass failure where streambank erosion is far more important in the non-cohesive system. This is further substantiated by simulating adjustment of a sand-bedded stream where only the cohesion and composition (sand, silt, or clay) of the bank sediments is changed. Although each simulated stream adjusts to similar values of energy dissipation, equilibrium morphologies and boundary shear stress (for a specific flow) are different because of the relative resistances of the bed and banks. Studies of the relative contributions of streambank erosion in unstable systems were conducted by various techniques including empirical methods, iterative bank-stability modelling, both in combination with measured sediment-transport data. Additional studies were conducted using coupled numerical simulations of upland and channel processes. Contributions of streambank sediment to total suspended-sediment loads were generally between 40 and 80% for streams in the Mississippi Valley Loess Plains, Southeastern Plains, Sierra Nevada, Lake Agassiz Plain, Northern Glaciated Plains. Streams with highly cohesive banks or banks containing dense grass roots had contributrions between 15 and 25%.