Abstract
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.