SEDIMENT BUDGET AND ROUTING MODELS REVEAL HISTORICAL CHANGE AND VULNERABILITY OF PUGET SOUND RIVER-DELTAS

Over the last 100 years, levees, dams, channelization, and deforestation have significantly altered sediment transport processes and the geomorphology of Puget Sound river-deltas, resulting in impacts to important estuary ecosystem functions, wild salmon recovery, and valued infrastructure. By integrating analyses of new high-resolution mapping, historical bathymetric change, sediment cores, and models of sediment transport and projected climate change impact pathways we characterize human impacts and assess future vulnerabilities of six large river deltas in Puget Sound. The re-routing of rivers through extensive channelized levee systems has led to a ~10x increase in delta progradation rates over average Holocene rates where fluvial sediments have been focused and experience chronic sediment bypassing of the delta nearshore. Vast areas of the tidal flats have been transformed from mud-dominated to sand-dominated substrates commonly 1-2 m thick. The chronic sediment bypassing disrupts nearshore ecosystem functions essential to salmon, forage fish, and shellfish while the progradation has abruptly reduced the coastal hydraulic gradient which promotes stream channel aggradation, reduced flood conveyance, and reduced surface and groundwater drainage affecting coastal communities and agricultural productivity. Along shorelines where sediment has been diverted away, ~1m of subsidence and up to 1km of coastal erosion/retreat has occurred leaving coastal lands more vulnerable to storm surge inundation. New hydrodynamic models show that despite high sediment inputs, river connectivity to and sediment retention within recently restored tidal marshes varies widely because of complex estuarine mixing patterns and the effects of dikes and levees. Linked downscaled climate-hydrology models indicate that these legacy effects are likely to be exacerbated by climate change which is projected to produce more sediment and aggradation as peak stream flows increase with a shift from snow to rain and the continued rise in snowlines expose more basin area to rapid runoff. Models of sea-level rise suggest greater sediment trapping higher in the watershed and a change in the capacity for tidal marsh vegetation to attenuate wave impacts and sediment alongshore.