Turbidity Hysteresis in an Estuary and Tidal River Following an Extreme Discharge Event

Nonlinear 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. Plain Language Summary Turbidity is a widely accepted proxy for suspended sediment concentration and an important factor for contaminant transport and water quality. Here we show that turbidity depends on river discharge in long-term observations at multiple locations in an estuary. Such relationships are often used in rivers, but have not been commonly used in estuaries and tidal rivers, where tides and salinity also contribute to variability. Turbidity in the freshwater tidal region was more sensitive to discharge than in the saline estuary. Massive inputs of sediment due to extreme precipitation and flooding in 2011 resulted in increased sediment availability in the tidal river over multiple years. Turbidity throughout the tidal river was elevated for 2 years following the events, but changes were not apparent in the saline estuary. The observations provide guidance on recovery time scales for estuaries and tidal rivers to event-driven sediment inputs, which affects the delivery of material from the watershed to the coastal ocean as well as other impacts on water clarity.