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

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.After two years, the Elwha Dam was completely removed, and three-quarters of Glines Canyon Dam were removed. Reservoir drawdown increments of 3 to 5 m were sufficient to initiate channel degradation and delta progradation across the width of the receding reservoir, redistributing decades of accumulated delta sediment throughout the reservoir while the lake still remained. The first year of dam removal resulted in up to 5 m of incision through the Lake Aldwell delta down to the predam surface and in just over 20 m of incision through the Lake Mills delta. In contrast, delta progradation resulted in a few meters of deposition in Lake Aldwell and 2 to 10 m in Lake Mills on top of prodelta and lakebed deposits. In coarse, noncohesive sediment, a braided channel developed and widened up to tenfold across the entire width of the reservoir. The most extensive lateral erosion occurred in noncohesive deposits during multiweek hold periods coinciding with flows greater than the mean annual flow, but less than a 2-year flood peak. Channel widening in more cohesive fine sediments of the prodelta and lakebed was less than half of that in the coarse, noncohesive delta sediments. Dam removal resulted in the erosion and downstream release of 23% of the sediment in Lake Aldwell (1.12 ± 0.07 million m3) and 37% of the sediment in Lake Mills (5.95 ± 0.12 million m3), representing nearly four decades of sediment supply from the upstream watershed within a two-year time frame. A significant portion of the reservoir sediment is expected to remain as sediment terraces within the reservoir landscape, but additional erosion is expected after the remainder of the Glines Canyon Dam is removed and during future floods until the river reaches quasi-equilibrium. After phased dam removal, the reservoir landscape consists of a series of sediment terraces of varying heights composed of prograded coarse sediment overlying fine lakebed deposits. The predam surface is exposed along the river corridor, and abundant 1- to 3-m stumps from pre-removal forests create unique morphology where the river interacts with the predam landscape.