The first impression one gets of the Hudson is its turbidity and the abundance of soft sediment. As a Secchi disk (the aquatic scientist's traditional indicator of turbidity) is dropped below the surface, one is impressed at how the disk quickly fades from view, usually only a half meter or so below the surface. As a large barge moves upriver, one immediately sees silt and mud stirred from the bottom adjacent to the shore. Both these phenomena combined indicate that the Hudson is dominated by suspended particles and is very dynamic as a sedimentary environment. It is not unusual to find sites where 10-20 cm or more of sediment may be deposited in a matter of days, and where similar amounts can be eroded away.
The Hudson's load of suspended sediment derives principally from clays eroded from surficial glacial lake and glacial outwash deposits (Chapter 4). Otherwise, the main stem of the Hudson flows through relatively resistant rocky terrane, which does not supply a great deal of sedimentary material. While turbid along its entire main channel, peaks of suspended sedimentary particles are found at the northerly extent of saline water and in the mid-Hudson. Because of the two-layered flow of the Hudson in the New York Harbor region, up-current flow in the deep layer transports sediment from the ocean to within the estuary.
The strong tidal mixing throughout the estuary results in strong current conditions at the bottom. Recently, a bottom survey using multibeam scanning sonar has detected a truly fantastic system of sand ripples with wave lengths of many meters and ripple amplitudes of a few meters (Chapter 5). The New York-New Jersey Harbor region is also a dynamic sedimentary environment and water movement and sediment deposition is complicated by extensive dredge channels that are maintained for shipping traffic.
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