Sediment input

The amount of sediment delivered to the Hudson estuary is an important, but elusive number. The most direct measurements are made by periodically sampling the river water, determining the amount of suspended sediment per liter of water, and multiplying that by the discharge around the time of sampling. It can only be done easily above the tidal influence. Because it is an engaging task, it is not done all the time nor has it been done on every tributary. In addition, the sediment delivery is discontinuous; almost all the sediment supplied in a given year maybe introduced over afew days during floods, exactly the time when measurements are most difficult to make. The sediment delivery can also vary widely from year to year. In the absence of direct measurements, sediment input may be calculated from estimates of the loss of soil from the land surface, but this isn't any easier or more certain.

As a result of such difficulties, estimates for the fluvially derived sediment input to the Lower Hudson Basin are scarce. Dole and Stabler (1909) put the total sediment discharge at Troy as 365,000 metric tonsperyear, (MTy-1) whilePanuzio (1965) places it at 750,000 MT y-1 at kilometer 120. For 1977, 1.02 million MT were supplied to the lower Hudson at Troy (Olsen, 1979), and 920,000 MTy-1, on average, over thirty years (1947-77). Additional sediment is supplied by the tributaries entering the tidal portion of the river below the dam at Troy; these values must be added to the sediment load entering at Troy. Based on the relative areas of the drainage basin (Olsen, 1979), the river-borne sediment input from the lower Hudson provides 310,000 MT y-1 for 1977, and 280,000 MT y-1 for the thirty-year average. A different estimate can be made using the data from the United States Department of Agriculture, Soil Conservation Service to obtain a delivery ratio. In this way, the suspended sediment yield for each square kilometer was estimated to be between 25 MT km-2 y-1 and 32 MT km-2 y-1 (Ellsworth, 1986). Correspondingly, the calculation for the entire lower Hudson drainage basin, which has an area of 1.2 million hectares, is between 300,000 MT y-1 and 390,000 MT y-1. A third estimate (Howarth, Fruci, andSherman, 1991) was calculated by applying a generalized watershed loading model to the Hudson River drainage basin. The model result gave a three-year average (198386) fluvial sediment input for the lower Hudson of 260,000 MTy-1.

Yet another model, the Hydrologic Simulation Program Fortran, was used for the quantification of the terrestrial source of sediment from the tributaries below Troy (Lodge, 1997). Twenty tributaries comprising the lower Hudson drainage basin were found to supply 80,000 and 100,000 MTy-1, for 1992 and 1993, respectively. The combined discharge of the Catskill, Kinderhook, Normans Kill, and Wallkill creeks alone contributed 60 percent of the sediment load. New material was calculated to have a residence time of 22 days in the estuary (Lodge, 1997).

The sources of fine-grained sediment are diverse and distributed over 34,000 square kilometers. Little is supplied directly by erosion of the river banks (Ellsworth, 1986). The abundance of sand in the Hudson north of Kingston (Coch, 1986) is supplied by the local tributaries and, in part, by scouring of the channel floor.

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