The Hudson River is one of the major watercourses of the United States East Coast. It originates on the slopes of Mt. Marcy in the Adirondack Mountains, extending nearly 600 km to New York City. Over this distance the Hudson's character changes dramatically, starting as a mountain stream, descending to become a lowland river, and then turning into a peculiar tidal river, slowly increasing in salinity to become an estuary, and finally joining the ocean as a complex network of tidal channels and bays bisecting the New York metropolitan area. These different environments are shaped by the interplay of a variety of physical processes with one element in common: the river flow. Runoff from the hillslopes coalesces to form the lakes and streams in the Adirondack highlands. The action of gravity on the accumulating water provides the driving force for this flow through the upper Hudson valley. South of Albany, the motion of the river becomes complicated by the influence of tides, which can be witnessed a remarkable 250 km from the sea. Although the tidal river flows both north and south, the net southerly river flow persists and provides the freshwater input that creates the Hudson estuary. This freshwater source is a dominant contributor to the physical regime of the estuary and harbor, as it controls the salinity structure, the vertical stratification, and the exchange of properties between the estuary, the ocean, and the atmosphere.
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