Tidal exchange between the estuary and a wetland drives not only the sedimentation process but also many other processes within the wetland. Vertical tidal range varies from about 0.75 to 1.8 m along the river. The intercreek marsh ("high marsh") of most marshes is inundated by all but neap tides and at spring tides there may be 20-30 cm or more of standing water on the marsh surface. The position of the tidal wetland in the landscape (that is, in relation to both open water and uplands), and the presence of anthropogenic features (for example, railroad, road causeways, other areas of fill), affect the "hydrodynamic energy" level of the marsh which shapes its ecological structure and function.
Sediments must build up to the low tide level (Mean Low Water or MLW) to support wetland development. Wetlands occur in several types of sites in the Hudson River estuary: separated from the main river by a railroad, road, or sandbar ("enclosed" wetlands); not separated by such an obstruction but partly sheltered by an island, headland, fill, or other natural or artificial feature ("sheltered" wetlands); in the mouth of a tributary; narrow wetlands along exposed shorelines often of dredge spoil ("fringe" wetlands); broad wetlands exposed to the main river on one side; or occasionally in mid-river and exposed to deeper water on all sides (Green Flats and Upper Flats, Fig. 20.1). Physical shelter allows continued deposition of suspended sediment from estuarine waters and upland tributaries, and the surface elevation of a wetland continues to increase apparently until it reaches a steady state of deposition versus erosion (and decomposition) of sediment. Different wetland types support different ecological structure, biota, and function.
Sedimentation rates within the tidal marshes of the Hudson River National Estuarine Research Reserve (HRNERR) have been measured using radionuclide (210Pb and 137Cs) dating techniques (Peller andBopp, 1986; Stevenson, Armstrong, andSchell, 1986; Robideau, 1997; Benoit et al., 1999). Most of the cores that were dated were collected from shal-lowsubtidal areas (both tidal creeks and pools) with smaller numbers from intertidal marshes and tidal swamps. Depositionrates ranged from 0.053 to 2.92 cm yr-1 with the highest rates in the shallow sub-tidal and intertidal mudflats of Tivoli South Bay. Tables 20.1 and 20.2 summarize the rates in both the marshes and marsh habitat types.
Goldhammer and Findlay (1988) measured tidal fluxes of suspended inorganic materials in Tivoli South Bay over several tidal cycles and estimated a mean deposition rate of 1.2 cm • yr-1. Benoit et al. (1999), combining their data with the Goldhammer and Findlay (1988) data, concluded that the
Table 20.2. Sediment deposition rates (cm • yr-1) within vegetated tidal marsh habitats of the Hudson River as determined using radionuclide techniques (210Pb and 137Cs). IT = intertidal zone. Sample sizes are in parentheses
Shallow subtidal Lower IT Upper IT Tidal swamp
Robideau, 1997 0.12-1.16 (13) 0.31 (1) 0.21-0.53 (2)
measured tidal flux was not high enough to account for an average sediment accumulation rate of 1.18 cm • yr-1 .However, Benoit et al. (1999) noted that these flux studies were performed from May to November, which is typically a period of low freshwater discharge. Storms and higher flow periods in the spring could account for a greater import of sediment into Tivoli South Bay.
The Tivoli Bays contain areas of shrub or tree-dominated tidal swamp in the mouths of tributaries and in a 15 hectare neck between Cruger Island and the mainland. These areas of woody vegetation evidently represent pre-European (or at least pre-railroad) wetlands. The tributary mouth deposits are deltas where sediment suspended by swift currents in the tributary is deposited upon reaching the quiet waters of the bays. The shape of the Cruger Island neck and the sand underlying the organic sediments suggest the neck was a tombolo (a double sand spit swept out to the island by bidirectional currents along the mainland shore). In Tivoli North Bay, which was separated from the estuary by the railroad circa, 1850, the present form of the marsh was already recognisable by circa 1900 (ground photos) and 1936 (aerial photo; Roberts and Reynolds, 1938). Since then, the two large interior pools have progressively filled in whereas the two large pools just inside the railroad trestles have remained the same size. Apparently, the tidal currents rushing under the railroad trestles deposited sediments in the quiet waters of the bay, forming flood tidal deltas comparable to those at inlets through barrier beaches (Reinson, 1979). These deltas comprised shoals within the pool and a natural levee around the pool at a certain distance inside the trestle, with three to five primary tidal creeks radiating into the bay from the pool. Pools and primary creeks are relatively deep and hold water at low tide. Gradually the areas of the bay more distant from the trestles filled with sediment, forming progressively shrinking intertidal pools that do not hold water at low tide. Woody vegetation (especially tree and shrub willows, Salix spp., and false-indigo, Amorphafruticosa) is established on the relatively stable levees of the trestle pools, and shrubs are scattered along the banks of the primary tidal creeks. Following the primary creeks into the bays, there is less vegetational evidence of a natural levee, and a shift from woody vegetation or purple loosestrife to narrowleaf cattail on the creek banks. This spatial pattern of one or more trestle pools with natural levees and radiating tidal creeks is characteristic of many of the marshes. Because the Hudson River has such diverse topography and historic alteration, however, individual marshes vary greatly in landscape position, size, stage of development, and other features.
Although deposition is dominant in most Hudson River tidal wetlands, some areas are actively eroding where the main river or large tidal creeks scour wetland edges. Fringe marshes on dredge spoil are subject to relatively high levels of hydrodynamic energy, and are expected to be stable or eroding rather than depositing. In addition to currents, wind and boat waves, and ice, certain animals cause resuspension of fine sediments in tidal marshes. Snapping turtle (Kiviat, 1980b), muskrat (Ondatra zibethicus) (Kiviat, 1994; Connors et al., 2000), beaver (Castor canadensis), European carp, killifishes, and American eel treading, burrowing, and rooting, and human boating and treading all contribute to resuspension of sediments.
During pre-European time, the Hudson River had some large wetlands. For example, Piermont Marsh (now 114 ha) and Iona Island Marsh (64 ha) are several thousand years old (Newman et al., 1969). The distribution of pre-European wetlands was altered because railroads on both sides of the river, roads, disposal of dredged material (dredge spoil), historic industries such as brickworks and ice houses, and other development along the Hudson destroyed many wetlands but also created or enlarged certain wetlands. The railroads alone border 54 percent of the eastern shore and 63 percent of the western shore between the Troy Dam and Piermont (Squires, 1992). Based on old maps, Squires estimated that 121 ha (300 acres) of emergent marsh were lost to filling in the past 500 years but that there has been a net gain of 769 ha of wetlands. Squires also estimated that 2,713 ha of estuary overall were filled for spoil disposal, 810 ha for railroad construction, 729 ha for industrial development, and 445 ha for other purposes. We think that 121 ha of filled marsh is an underestimate because many maps do not show wetlands accurately; for example, old maps of Tivoli Bays variously show wetland or not in the Cruger Island neck between North Bay and South Bay
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