Marine sources

The landward (i.e, up-estuary) transport of sediment seems common at the mouths of estuaries, and a marine supply can be substantial in some estuaries (Biggs, 1970; Bokuniewicz, Gebert, and Gordon, 1976; Meade, 1969; Hobbs et al., 1992; Turner, Millward, and Tyler, 1994). In Chesapeake Bay, for example, flood-dominated channels transport sand into the estuary mouth (Ludwick, 1974). This situation also seems to exist at the mouth of the Lower New York Bay (Swift and Ludwick, 1976). Large sandwaves have been found on the floor of the Ambrose Channel with asymmetry indicating landward transport. In the Hudson itself, the coarsening of sediments from the Hudson Highlands to the Battery has been attributed in part to the up-estuary transport of sandfrom the Coastal Plain (Coch, 1976).

Many estuaries are sinks for sediments (e.g., Nichols, 1977; Yarbo et al., 1983; Hobbs et al., 1992). The Hudson River Estuary also appears to be an effective trap for fine-grained sediment that is capable of absorbing not only fine-grained sediment supplied by its rivers but also a substantial ocean source (Bokuniewicz and Coch, 1986; Olsen et al., 1984; Ellsworth, 1986). Such behavior seems common, especially in partially mixed estuaries like the Hudson. It has been explained by the superposition of characteristic estuarine circulation on the suspended sediment distribution (Schubel and Carter, 1984) in conjunction with rapid particle settling speeds due to agglomeration. In general, the estuarine, density-driven circulation drives saline bottom water landward into the estuary while fresher surface water flows out. Higher concentrations of suspended sediment tend to be found near the estuary floor both because particles tend to sink to the bottom and because sediment on the seafloor can be resuspended by waves and tidal currents. Higher concentrations of suspended sediment in the bottom waters are, therefore, imported by the estuarine circulation.

The import of fine-grained, marine sediment into estuaries along the east coast has often been proposed. In the Hudson, the geochemical signature of silts and clays provided evidence that 30 percent of the fine-grained sediment being deposited in the estuary entered at its mouth (Olsen et al., 1984) and, an attempted sediment budget for the Hudson River estuary (Ellsworth, 1986) needed to invoke a marine source to balance the sources and sinks. At the Battery, the estimated input from the sea was between 139,000 and 734,000 MTy-1 (with a fluvial input at Troy estimated at 870,000 MT y-1; Ellsworth, 1986). In addition, grain size analysis of bottom sediments suggests that the bottom sediments in the lower estuary are composed of one component of sand from the ocean and another of particles in flocs (Gibbs,Jha, andChakrapani, 1994; Coch, 1976).

There is little information concerning the production of sediment particles (opal) in situ. Production rates have been estimated to correspond 135,000 MT y-1 over the entire area of the estuary (Ellsworth, 1986).

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