Watershed Suspended Sediment Supply and Potential Impacts of Dam Removals for an Estuary

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 rating curves 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 side tributaries. Sediment yield varies inversely with watershed area, with regional trends that are consistent with substrate erodibility. Geophysical and sedimentological surveys in seven subwatersheds of the Lower Hudson were conducted to estimate 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 sediment, and (3) dammed natural lakes and spring-fed ponds. Based on this categorization and impoundment attributes from a dam inventory database, the total mass of impounded sediment in the Lower Hudson watershed is estimated as 4.9 ± 1.9 Mt. This represents about 4 years of annual watershed supply, which is small compared with some individual dam removals and is not practically available given current dam removal rates. More than half of dams impound drainage areas less than 1 km2, and play little role in downstream sediment supply. In modeling of a simulated dam removal, suspended sediment in the estuary increases modestly near the source during discharge events, but otherwise effects on suspended sediment are minimal. Fine-grained sediment deposits broadly along the estuary and coarser sediment deposits near the source, with transport distance inversely related to settling velocity.

Invasive water chestnut hinders tidal wetland development

“…Despite research showing that inundation time during the tidal cycle is the primary driver of sedimentation rates in freshwater tidal wetlands (Butzeck et al, 2015; Neubauer et al, 2002; Odum, 1988; Temmerman et al, 2003), our results suggest that terrestrial sediment inputs may be important enough to adjust wetland morphology in locations that already receive a robust sediment supply from the main tidal river. A previously published estimate for the entire Tivoli watershed was developed through scaling regional sediment yields by watershed area and estimated annual yield for Tivoli at 100 tons/km 2 /year, distinctly higher than our estimates (Ralston et al, 2021).…”

“…The Mohawk River and Upper Hudson (above Troy, NY) drain roughly 60% of the estuarine watershed, with a mean discharge of approximately 420 m 3 /s (Figure 1). Recent sediment discharge estimates determine that approximately 1.2 Mt/year is delivered to the estuary (Ralston et al, 2021). In New York Harbor, current rates of sea level rise stand around 3 mm/year, with 40–190 cm of total sea level rise expected over the next century (Horton et al, 2015; Kemp et al, 2017; Kopp, 2013).…”

Sources, Mechanisms, and Timescales of Sediment Delivery to a New England Salt Marsh

“…Here, we define the mesotidal New England coast as the northern shore of Cape Cod through Maine‐Canada border. Small salt marsh/estuarine complexes with relatively low fluvial sediment yields are common to this region (e.g., Meade, 1969; Millman & Farnsworth, 2011; Ralston et al., 2021; Weston, 2014), and frequent, moderate‐intensity extratropical cyclones are the primary cause of flooding (e.g., Baranes et al., 2020; Kirshen et al., 2008). The great diurnal tide range increases northward from ∼3 m in Massachusetts to ∼6 m at the Canadian border, exceeding even the most extreme storm surges for the region (∼1.3 m; Talke et al., 2018).…”

Spatiotemporal Variation in Distribution, Size, and Relative Abundance within a Salish Sea Nearshore Forage Fish Community

“…Moreover, nearshore regions are also susceptible to climate variability, extreme storm events, and rising sea levels (Flint 1985;Hay et al 2008). In addition to direct human-derived effects in marine waters and along shorelines, nearshore habitats can be affected by in-river changes that alter sediment dynamics or flow near river mouths through deposition or scour (Day et al 2000;Syvitski 2005;Slagel and Griggs 2008;Ralston et al 2021). Dams in the lower portions of rivers are one specific modification that can generate such effects by blocking sediments that help build a variety of habitats such as beaches, spits, and wetlands (Day et al 2000;Wilcox et al 2014;Warrick et al 2019); conversely, their removal can help rebuild beaches and estuarine habitats.…”

“…2000; Syvitski 2005; Slagel and Griggs 2008; Ralston et al. 2021). Dams in the lower portions of rivers are one specific modification that can generate such effects by blocking sediments that help build a variety of habitats such as beaches, spits, and wetlands (Day et al.…”