FLUVIAL AND COASTAL MORPHODYNAMIC EVOLUTION FOLLOWING A MASSIVE SEDIMENT RELEASE FROM THE WORLD’S LARGEST DAM REMOVAL

Quantifying landscape response to increased sediment supply is one of the longest-standing problems in geomorphology, but the unanticipated nature of most sediment pulses (e.g., landslides, volcanic eruptions) rarely permits detailed measurement of associated landscape processes and evolution. The intentional removal of two large dams on the Elwha River, Washington, USA, represented a unique opportunity to quantify source-to-sink landscape response to a ~20 million tonne sediment release—a disturbance similar in scale to a moderate volcanic eruption. Here we provide a 5-year sediment budget and morphodynamic analysis of the Elwha River and its delta, evaluating the trajectory of this dynamic system toward quasi-equilibrium after disturbance. The Elwha River efficiently transported sediment from the former reservoirs to the coast, causing extensive seaward expansion of the delta. Despite depositing/storing only ~10% of the released sediment in the fluvial system, channel morphology and floodplain connectivity changed substantially from the restored fluvial supply of sand, gravel, and wood. While the initial sedimentary and geomorphic response to dam removal began to fade after ~2 years, the river and coast remain significantly more dynamic than prior to dam removal, and several metrics suggest that increased dynamicity is a function of the restored sediment and wood flux. These responses contrast with simple conceptual models of the geomorphic effects of dam removal that assume a return to a similar equilibrium following the initial impulse-response.