Watershed sediment supply and potential impacts of dam removals for an estuary

Observations and modeling are used to assess potential impacts of sediment releases due to dam removals on the Hudson River estuary. Watershed sediment loads are calculated based on sediment-discharge regressions for gauges covering 80% of the watershed area. The annual average sediment load to the estuary is 1.2 Mt, of which about 0.6 Mt comes from tributaries entering below the head of tides. Sediment yield varies inversely with watershed area, with regional trends that are consistent with differences in substrate erodibility. Geophysical and sedimentological surveys in five subwatersheds of the Lower Hudson were conducted to characterize the mass and composition of sediment trapped behind dams. Impoundments were classified as 1) active sediment traps, 2) run-of-river sites not actively trapping, and 3) dammed natural lakes and spring-fed ponds. Based on this categorization and impoundment attributes from the dam inventory database, the total mass of impounded sediment in the Lower Hudson watershed is estimated as 3.1 Mt. Assuming that roughly half of the impounded sediment is typically released downstream with dam removal, then the potential inputs represent less than 2 years of annual watershed supply. Modeling of simulated dam removals shows that modest suspended sediment increases occur in the estuary within about a tidal excursion of the source tributary, primarily during discharge events. Transport in the estuary depends strongly on settling velocity, but fine particles, which are important for accretion in tidal wetlands, deposit broadly along the estuary rather than primarily near the source.