Large sections of the Hudson River Estuary have been dredged regularly by the United States Army Corps of Engineers over the last eighty years. This paper examines sediment cores and geophysical data taken from Haverstraw Bay and the Edgewater Weehawken Channel to investigate how this disturbance has changed the estuary’s natural sediment deposition patterns.
137Cesium measurements from two transects of Haverstraw Bay cores suggest that sedimentation rates within the dredged channel have been 1.8 to 4 cm per year since a 1960-61 dredging event. This is 4-7 times faster than rates in cores from less than 320 m outside the disturbed area. 137Cs in cores from inside the Edgewater Weehawken Channel last dredged in 1994 support very fast sedimentation of 12-18 cm per year in one core and 2.2 to 3 cm per year in another. No 37Cs is found in the top 2 cm of a third core taken 200 m east of the dredged channel. Both sets of results support the concept of an “equilibrium profile” as a framework for understanding the effect dredging on sedimentation. These findings are supported by two other sets of analyses.
Strong down core changes in density, water content, and organic carbon content in general correlate with the end of significant 137Cs activity in all core groups. In Haverstraw Bay, these characteristics, plus magnetic susceptibility peaks, are used to determine the dredge boundary for two transects for which there is no C137s data. Sedimentation rates from these transects fall within the ranges for both inside and outside the dredged channel defined by radionuclide dating.
In seismic profiles and sidescan data from Haverstraw Bay, the dredged channel closely overlaps with a hard impenetrable reflector. A hard reflector is also found in seismic profiles in the dredged areas of the Edgewater Weehawken reach and south to the Battery, as well as in the Ossining Channel. These reflectors are interpreted to be the result of saturation levels of methane gas in the sediment and porewaters resulting from increased rates of sedimentation and organic carbon burial in the dredged area. Preliminary loss on ignition tests show higher levels of organic carbon in cores from within the dredged channel than in cores from outside the channel.
Core from the Hudson River. The image highlights the recent sedimentation layer evidenced by a lighter coloring representing oxidation.
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Last updated: 8 January 2001, KAK.