Background

The Hudson and Mohawk Rivers transverse predominantly erosion-resistant uplands, although the valley itself occupies a terrain of erosion-susceptible shale. The lower Hudson crosses six geologic terrains. From Troy to Cornwall, the river runs through a valley in the Appalachian Ridge and Valley Province. This area is underlain by gently folded and tilted siltstones, shales, and carbonate rocks (Sanders, 1974). From Cornwall-on-Hudson southward to Peekskill, the river cuts through the Hudson Highlands, a band of resistant, Precam-brian crystalline rocks. Below Peekskill, the west bank of the river skirts the rocks of the Newark Basin. These are predominantly Triassic and Jurassic Period sandstones, shales, and volcanic rocks (Sanders, 1974) and include the Palisades escarpment. The east bank is formed of high-grade metamorphic rocks of the Manhattan Prong of the New England Uplands: Precambrian and Lower Paleozoic Era schists, marble, quartzite, and gneiss. The Hudson discharges into the Upper and Lower bays on New York Harbor spilling out across the unconsolidated sediments of the Coastal Plain to the Atlantic Ocean.

All these rocks constrain the river in a resistant foundation wherein sediment sources are largely confined to a veneer of glacial deposits. Glacial tills, drift, and outwash sands blanket the entire drainage area. Most valleys of the tributaries are lined with unconsolidated silts and clays originally deposited in glacial lakes during the retreat of the Wisconsin glaciation. Ground moraine tends to be relatively resistant to erosion but can supply a wide range of grain sizes to the river. Sand enters the system from local concentrations of glacial sand bodies, while silt and clay can be provided from the reworking of glaciolacustrine deposits. In the lower reaches of the river, these sources are supplemented by a supply of sediment up-estuary from the sea. Sand from the Coastal Plain can migrate into the river under tidal influences and finegrained, marine sediments are recycled into the Hudson by a characteristic, estuarine circulation.

Most of the lower Hudson drainage basin (57 percent) is forested, however anthropogenic influences permeate the entire estuary. Dredged areas comprise about 8 percent of the area of the estuary or some 23 km2 out of a total surface area of 282 square kilometers above the Battery (Ellsworth, 1986). The banks of the estuary have been extensively stabilized by bulkheads and "rip-rap," or railroad beds, which run up the shore on both sides of the estuary. About 2 percent of the west shore and 21 percent of the east shore is stabilized by the railroad (Ellsworth, 1986). Rocky shoreline or stabilized shoreline accounts for approximately 43 percent of the total shoreline. The main stem of the estu ary is dammed at Troy. Tributaries below Troy may be dammed or otherwise restricted by causeways supporting the railroads along the shores.

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