Large-scale dam removal on the Elwha River, Washington, USA: Fluvial sediment load

Abstract

The Elwha River restoration project, in Washington State, includes the largest dam-removal project in United States history to date. Starting September 2011, two nearly century-old dams that collectively contained 21±3millionm3sediment were removed over the course of three years with a top-down deconstruction strategy designed to meter the release of a portion of the dam-trapped sediment. Gauging with sediment-surrogate technologies during the first two years downstream from the project measured 8,200,000±3,400,000tonnes of transported sediment, with 1,100,000 and 7,100,000t moving in years 1 and 2, respectively, representing 3 and 20 times the Elwha River annual sediment load of 340,000±80,000t/y. During the study period, the discharge in the Elwha River was greater than normal (107% in year 1 and 108% in year 2); however, the magnitudes of the peak-flow events during the study period were relatively benign with the largest discharge of 292m3/s (73% of the 2-year annual peak-flow event) early in the project when both extant reservoirs still retained sediment. Despite the muted peak flows, sediment transport was large, with measured suspended-sediment concentrations during the study period ranging from 44 to 16,300mg/L and gauged bedload transport as large as 24,700t/d. Five distinct sediment-release periods were identified when sediment loads were notably increased (when lateral erosion in the former reservoirs was active) or reduced (when reservoir retention or seasonal low flows and cessation of lateral erosion reduced sediment transport). Total suspended-sediment load was 930,000t in year 1 and 5,400,000t in year 2. Of the total 6,300,000±3,200,000t of suspended-sediment load, 3,400,000t consisted of silt and clay and 2,900,000t was sand. Gauged bedload on the lower Elwha River in year 2 of the project was 450,000±360,000t. Bedload was not quantified in year 1, but qualitative observations using bedload-surrogate instruments indicated detectable bedload starting just after full removal of the downstream dam. Using comparative studies from other sediment-laden rivers, the total ungauged fraction of <2-mm bedload was estimated to be on the order of 1.5Mt.