DYNAMIC EQUILIBRIUM OF THE HUDSON ESTUARY REVEALED BY THE SEDIMENTARY RECORD

Sedimentary processes along 50 km of the Hudson Estuary extending from Tappan to Newburgh are revealed by geophysical and sediment data. The spatial and temporal variability of the sedimentary record is provided by side-scan sonar images, more than 100 cores, core x-rays and physical properties, radiocarbon dating, and heavy metal concentrations used as tracers to indicate modern sediments.

Results show that the riverbed has not accumulated a large supply of recent sediment. The most compelling information comes from radiocarbon ages that range from ~1600 to 3400 years BP (uncorrected 14C) cm’s beneath the surface. Evidence for very low or no accumulation rates comes from relics (coal, slag, magnetic particles) of the coal burning initiated in the 1850’s which was prevalent in the region until the 1960’s. Today these relics lay scattered throughout the riverbed forming lag deposits. These findings are consistent with radioisotope studies that show that upstream of New York harbor young sediment accumulates primarily in areas protected from currents (coves, marinas).

More energetic high-discharge events provide the space for sediment deposition by eroding the riverbed. Terrestrial sediments and coal-by-products cover the erosional surface that is easily recognized by its strong magnetic signal. A fine-grained sedimentary deposit above the scour surface contains high concentrations of Cu and Pb. In contrast, beneath the scour Cu and Pb concentrations are of pre-industrial values.

The working model is that the accommodation space of the Hudson Estuary was rapidly filled during the late Holocene and the system is in a state of dynamic equilibrium in which sediment bypasses a region unless space is made available. That space is provided by erosional events that are related to fluvial discharge (spring freshet, storm flooding) or tidal flooding, and by anthropogenic activities such as bridge footings and dredging. The space is filled by contaminated sediments and populated by organisms.