Mhw Msl

MHW+1

Figure 20.2. Diagram of tidal zones and plant communities of the Hudson River tidal wetlands. (Drawn by Laura T. Heady for Biodiversity Assessment Manual for the Hudson River Estuary Corridor, E. Kiviat and G. Stevens, New York State Department of Environmental Conservation, copyright ©Hudsonia Ltd., 2001.)

(Kiviat, personal observation). Squires (1992) also estimated that 850 ha of marsh were created by the railroads and other changes.

The wetlands of Tivoli Bays were probably restricted to small areas in the mouths of tributaries and in the Cruger Island neck (Kiviat, 1974) amounting to <30 ha; construction of the east shore railroad circa 1850 caused more rapid deposition of sediments and expanded the wetlands to the approximately 240 hectares (600 acres) present today. Farther upriver, many wetlands and subtidal shallows were filled with dredge spoil and new wetlands formed on portions of the spoil. Most of the dredge spoil is sandy and many recently created wetlands on dredge spoil have large areas of soil lacking vascular vegetation, and low densities of benthic macroinvertebrates and fish (Mihocko et al., 2003). A few marshes such as Stockport Flats and the marsh at Hell Gate (Fig. 20.1) appear to have silty deposits over sand and are functionally intermediate between sandy fringe marshes and silty or mucky sheltered and enclosed marshes; Stockport and Hell Gate may represent the development of more "natural" marsh conditions on older, lower elevation spoil deposits (see Mihocko et al.,

2003). Because most currently existing wetlands are apparently different than pre-European wetlands in sediment type, vegetation, and other characteristics, the management and restoration of Hudson River wetlands are a challenge to science and policy.

Vegetation

Table 20.3 shows the dominant plants of typical tidal wetlands. Figure 20.2 shows idealized plant communities along the marsh elevation gradient. Tidal wetlandplant species distributions are delimited by substrate (sediment) elevation because different species tolerate different lengths of inundation and exposure. The intertidal zone is the vertical stratum between low tide level (MLW) and high tide level (Mean High Water or MHW). In the Hudson River, the tidal amplitude or height of the intertidal zone varies from about 0.75 to 1.8 m, lowest in the Hudson Highlands and highest in the southern and northern ends of the estuary. Tidal wetlands have substrate levels within the intertidal zone, and supratidal wetlands have substrates <1 m above MHW (Kiviat and Stevens, 2001).

Table 20.3. Predominant plant species of Hudson River tidal wetlands. IT = intertidal zone

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