Non-linear turbidity-discharge relationships are explored in the context of sediment sourcing and event-driven hysteresis using long-term (≥12 year) turbidity observations from the tidal freshwater and saline estuary of the Hudson River. At four locations spanning 175 km, turbidity generally increased with discharge but did not follow a constant log-log dependence, in part due to event-driven adjustments in sediment availability. Following major sediment inputs from extreme precipitation and discharge events in 2011, turbidity in the tidal river increased by 20-50% for a given discharge. The coherent shifts in the turbidity-discharge relationship along the tidal river over the subsequent 2 years suggest that the 2011 events increased sediment availability for resuspension. In the saline estuary, changes in the sediment-discharge relationship were less apparent after the high discharge events, indicating that greater background turbidity due to internal sources make event-driven inputs less important in the saline estuary at interannual time scales.

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.

In field observations from a sinuous estuary, the drag coefficient C, much greater than expected from bottom friction alone. C are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of CD, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with CD during flood tides. While CD generally increased with water depth, CD decreased for the highest water levels that corresponded with overbank flow. The decrease in CD may be due to inhibition of flow separation with flow over the vegetated marsh. The dependence of CD during ebbs on discharge corresponds with inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple-scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.

Flow separation has been observed and studied in sinuous laboratory channels and natural meanders, but the effects of flow separation on along-channel drag are not well understood. Motivated by observations of large drag coefficients from a shallow, sinuous estuary, we found in idealized numerical models representative of that system that flow separation in tidal channels with curvature can create form drag that increases the total drag to more than twice that from bottom friction alone. In the momentum budget, the pressure gradient is balanced by the combined effects of bottom friction and form drag, which is calculated directly. The effective increase in total drag coefficient depends on two geometric parameters: dimensionless water depth and bend sharpness, or the bend radius of curvature to channel width ratio. We introduce a theoretical boundary layer separation model to explain this parameter dependence and to predict flow separation and the increased drag. The drag coefficient can increase by a factor of 2 - 7 in “sharp” and “deep” sinuous channels where flow separation is most likely. Flow separation also enhances energy dissipation due to increased velocities, resulting in greater loss of tidal energy and weakened stratification.Flow separation and the associated drag increase are expected to be more common in meanders of tidal channels than rivers, where point bars that inhibit flow separation are more commonly found. The increased drag due to flow separation affects the tidal amplitude and phasing along the estuary and creates potential morphological feedbacks.