Deposition

Fine-grained sediment may ultimately be deposited in wetlands, in dredged channels or in un-dredged areas of the estuary floors. Bridge borings disclosed a layer of estuarine sediment as much as 61 m thick in the Hudson (Newman et al., 1969). If we assume that estuarine conditions were establishedby 12,000 years B.P., the long-term accumulation rate issomethinglessthan0.5cmy-1 Direct measurements of deposition rates using radiometric techniques vary from 1 to 5 cm y-1 in undredged areas south of the George Washington Bridge, 1 to 3 cm y-1 in marginal zones and 0.1 to 0.3 cmy-1 on the estuary floor north of the George Washington Bridge (Olsen, 1979).

The conventional wisdom is that marshes accumulate to keep pace with sea level rise. Indeed they must if they are going to maintain their position over thousands of years. At the Battery, sea level is rising at an average rate of about 3 mm y-1. Combining this with an estimate of marsh sediment composition, Ellsworth (1986) calculated a total deposition of 12,000 MTy-1 over the 22.8 km2 of marshland. In the lower Hudson, measured rates are more rapid. Measurements of sedimentation rates at five marshes (Piermont, Iona, Tivoli Bay North, Tivoli Bay South, and Stockport Flats) yielded rates from 2 mm y-1 to greater than 11 mmy-1 (Peller, 1985; Robideau, 1997). With an average rate of 6 mm y-1 the rates tended to be slightly higher in the north and slightly lower in the south. On the average, this would raise the total marshland deposition to 24,000 MTy-1.

The lower 18 kilometers of the estuary has been extensively dredged. Ninety-five percent of the total annual dredged sediment is removed from this area (Ellsworth, 1986). In the decade between 1966 and 1976, 705,990 m3 of sediment were removed (Conner et al., 1979). Deposition rates in dredged areas, therefore, mayaverage 14 cm y-1. The deposition rate is not uniform. Recently in the estuary along the Manhattan shore, observations show that 15 cm or more could be deposited in a single freshet (Woodruff, 1999).

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