Recent sediment accumulation within the Tappan Zee region appears to be limited to a few local sites of anthropogenic disturbance and to the western wall of the channel bend off of Hook Mountain. These sites of recent accumulation are readily identified in backscatter data as regions of very low backscatter and in the sub-bottom data as thin sediment layers that unconformably overlay older sediments. The sites of recent sedimentation include a linear band of very low backscatter up to 50 cm thick which extends 2 km into the river from Nyack and which appears to be the recent fill of a dredged channel, maintained in the late nineteenth and early twentieth centuries, to the coal-to-tar plant in Nyack. Low density sediments are also found in a dredged area off Tarrytown near the site of the former GM plant. Recent sediments are also foundin the western channelbendbetween Croton Point and Hook Mountain. This generally arcuate-shaped region of low backscatter stratigraphically overlays the channel axis and channel bank deposits, and is up to 25 cm thick. A small deposit from Nyack Creekimaged in the side-scan data also appears to be recent. These recent deposits contain the only occurrence of wedge clams (Rangia cuneata) recovered in the study. Sediment cores in regions of lowbackscatterreveallow-densitymuds, and radionuclide dating confirms the presence of sediments deposited since the 1950s within the areas identified as recent sediments on the basis of geophysical data.
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