Hudson River Fishes and their Environment

<em>Abstract.</em>—The Hudson River Estuary can be classified as a drowned river valley, partially mixed, tidally dominated estuary. Originally, it had a fjord-like morphology as a result of glacial scour which filled in over the past 3,000 years with river sediments. The hydrodynamics of the estuary are best described by the drivers of circulation, including the upstream river inflows, the oceanographic conditions at the downstream end, and meteorological conditions at the water surface and the response of the waters to these drivers in terms of tides and surges, currents, temperature, and salinity. Freshwater inflow is predominantly from the Mohawk and Upper Hudson rivers at Troy (average flow = 400 m³/s, highest in April, lowest in August). At the downstream end at the Battery the dominant tidal constituent is the semidiurnal, principal lunar constituent (the M₂ tide), with an evident spring/neap cycle. The amplitude of the tide is highest at the Battery (67 cm), lower at West Point (38 cm), and higher again at Albany (69 cm), a function of friction, geometry, and wave reflection. Meteorological events can also raise the water surface elevation at the downstream end and propagate into the estuary. Freshwater pulses can raise the water level at the upstream end and propagate downstream. Tidal flows are typically about an order of magnitude greater than net flows. The typical tidal excursion in the Hudson River Estuary is 5–10 km, but it can be as high as 20 km. Temperature varies seasonally in response to atmospheric heating and cooling with a typical August maximum of 25°C and January-February minimum of 1°C. Power plants cause local heating. The salinity intrusion varies with the tide and amount of upstream freshwater input. The location of the salt front is between Yonkers and Tappan Zee in the spring and just south of Poughkeepsie in the summer. Vertical salinity stratification exists in the area of salt intrusion setting up an estuarine circulation. The effect of wind is limited due to a prevailing wind direction perpendicular to the main axis and the presence of cliffs and hills. Dispersive processes include shear dispersion and tidal trapping, resulting in an overall longitudinal dispersion coefficient of 3–270 m²/s. The residence or flushing time in the freshwater reach is less than 40 d in the spring and about 200 d in the summer. In the area of salt intrusion, it is about 8 d.