MODELING THE EFFECT OF SEDIMENT SUPPLY ON CHANNEL WIDTH CHANGE ALONG ELWHA RIVER, WASHINGTON

Even though width change is one of the most important responses of rivers to changes in governing conditions, many numerical models assume that banks are immovable. Understanding channel widening and narrowing is especially important on Elwha River where widening occured as part of the transition of its regime from sediment-starved to sediment-rich following removal of two nearly 100 year old dams. We adapted MAST-1D, a one-dimensional bed evolution model in which bank erosion and channel narrowing are decoupled processes. Widening of the channel occurs when the largest particles in the river are mobilized. Narrowing takes place during periods of low flow. MAST-1D was applied to reaches of Elwha River between the two dams over a 98-year period encompassing emplacement and removal of the upstream dam. A control reach upstream of the dam was also simulated to separate the effects of sediment supply on width change from that of the flow regime. The model successfully predicted rates of bank erosion in the study area between the two dams, but was unable to replicate observed rates in the control reach, where the sediment supply regime is less constrained. Simulations show that channel narrowing occurred within the first couple decades after dam emplacement, after which the channel became stable. Subsequent bank erosion rates were small but led to limited storage. Following dam removal, rates of bank erosion were highest in the alluvial reaches immediately downstream of the dam. While sediment supplied via bank erosion did not have a large impact on the overall sediment budget, locations with high widening rates experienced the most channel storage. Our results suggest that channel widening and narrowing are very sensitive to incoming bedload sediment. Incorporating channel width change into bed evolution models is imperative to predicting the response of alluvial rivers to dam removal. Improvements could be made to characterizing channel narrowing--especially vegetation encroachment--in decadal scale numerical modeling.