ACOUSTIC MEASUREMENTS OF RAPID SEDIMENT ACCUMULATION IN THE HUDSON RIVER ESTUARY

Bottom mounted tripod observations of bed elevation, suspended sediment concentration, and water velocity taken during a nine-month period from October 2000 through June 2001 revealed that deposition takes place as series of transient large deposition and erosional events on individual tidal cycles which accumulate during a several spring tidal cycles to leave a seasonal deposit of the same magnitude as the individual tidal cycle events. During the fall of 2000 the region near the turbidity maximum was net erosional with losses of approximately 10 cm. During the spring of 2001 approximately 15 to 20 cm of deposition was observed at the tripod site one month after the seasonal freshet as the sediment that had been transported to the lower estuary during the freshet returned landward. This deposition occurred after pulses of mobile fluid mud were created on each ebb tide of the spring tides. This mobile fluid mud appears to be the product of frontal convergence between the landward near-bed estuarine circulation and seaward ebb tidal flows (Geyer et al. this session). At the slack tide after the ebb this mobile fluid mud settled out of suspension over a period of 1 to 1.5 hours into deposits of stationary fluid mud approximately 15 cm thick. These transient deposits were then eroded in the subsequent flood tide, however the erosion was generally smaller than the deposition leaving behind a net deposit of 2-4 cm per tidal cycle. This process was repeated over several spring tidal cycles to create the net 15 cm of deposition. Cores taken near the tripods provide stratigraphic evidence of this rapid depositional process (Sommerfield et al. this session) whereby gross deposition and erosion is far greater than net deposition.