The Hudson River is, in the parlance of estuarine scientists, a partially mixed estuary, which means that a distinct mixing occurs between the ocean and the freshwater river, leading to a layered structure. Higher salinity water is overlain by lower salinity water (Chapter 3) over a broad stretch of mixing between the river and the ocean. The estuary canbe divided into four salinity zones: polyhaline (18.530 psu), mesohaline (5-18), oligohaline (0.3-5), and limnetic (<0.3). The location of these zones varies seasonally as well as daily depending on tidal and freshwater inputs. In New York Harbor, between the Verrazano Narrows and approximately the George Washington Bridge, the distinct two-layer vertical structure occurs, with a saline bottom layer overlain by a freshwater surface lens. Wind and tide mix the two layers so there is a vertical salinity gradient from fresh to saline, as one goes from the surface to bottom waters. At some point upstream the Hudson is fresh water from surface to bottom.
Although this basic salinity structure is always present, it varies radically with season and weather. After the spring freshet, when snows melt in the upper part of the watershed and spring rains increase freshwater flow downstream, the surface freshwater lens in the mixing zone extends much further down into New York Harbor. Thus, the three-dimensional distribution of salt is complex and dynamic. Water circulation also differs from bank to deepest depth to opposite bank and this also affects the distribution of salt in the river.
North and above the Federal Dam at Troy, the Hudson has a conventional river flow, with a series of rivers and streams feeding from eastern and western uplands into the main course of the river. Below the dam the river is tidal. It must be upsetting for a stranger to see the river flowing opposite ways, depending upon the movement of the tide. The surface of the river at Albany is not much more than a meter higher than at the narrows, 240 km downstream. Thus tidal currents propagate a surprisingly large distance upstream.
Tides are caused by the gravitational interaction of the sun, moon, and the waters of the earth. In our region, the rotation of the earth, combined with gravitational attraction, results in approximately two high and two low tides per day at the coast. In the transitional zone in New York Harbor, where fresh or very low-salinity water becomes salty, the tide exerts a major vertical mixing effect. During spring tides this mixing is nearly complete and the salinity in New York Harbor is relatively homogeneous from surface to bottom. During neap tides flow is more reduced and one sees a more distinct lower salinity later overlying a higher salinity bottom layer.
As the tide rises, a tidal current moves northward up the Hudson. The time taken for the propagation of this current results in a significant delay between the time of high tide at the Verrazano Narrows and points upstream. High tide at West Point is a full three hours later than at the Narrows. The propagation is so drawn out that it can be high tide at one part of the length of the Hudson while being dead low at another. This makes for complex flow patterns. Flow is further complicated by the changing width and depth. As the width decreases a greater volume of water is forced through a narrower channel, which increases the flow velocity. Thus water flow through the narrow throat adjacent to the Hudson Highlands is far greater than in the broad width of Haverstraw Bay in the vicinity of Indian Point Power Plant.
On the shortest time scale, tidal flow is the controlling factor of water motion. But over longer seasonal time scales, the inevitable flow of fresh water from uplands to the oceans creates a net movement of fresh water downstream. This flowvaries strongly and is generally strongest during the late winter and spring. On the still longer decadal time scale, one encounters some years with far more freshwater flow than others, with an inevitable shifting of fresh water downstream into New York Harbor.
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