Large-scale dam removal on the Elwha River, Washington, USA: Erosion of reservoir sediment

摘要

Base-level lowering of reservoirs impounding upstream sediment supply triggers a series of channel evolution steps such as degradation, lateral erosion, and redeposition that can dramatically alter the reservoir landscape and decouple the relationship between stream power and sediment supply. Many case studies exist for small dam removals with a few years of sediment storage or dam breaches triggering instantaneous large sediment releases. However, quantitative information for a controlled drawdown initiating erosion of a large sediment deposit is rare. We investigate reservoir sediment response to the phased and concurrent drawdown of two reservoirs on the Elwha River, Washington, USA, during the largest dam removal in history by measuring changes in reservoir topography and channel morphology as a function of base-level lowering, river discharge, and cohesion.