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River Kilometers from Battery

Salinity cross-sections in the Hudson estuary during different discharge and tidal Upper panel: high discharge (2,000 m3/s) spring tide; middle panel: low discharge neap tide; lower panel: low discharge (100 m3/s), spring tide. (adapted from Geyer oceanographers as practical salinity units or psu), which renders salt water about 3 percent more dense than fresh water. This density contrast causes the fresh water to flow over the salt water and vice versa, leading to anestuarine "salt wedge" (Fig. 3.6). Salt wedges are most evident at the mouths of rivers with weak tidal currents relative to the river flow, such as the Mississippi. The Hudson estuary is notable in that it exhibits a salt wedge structure during neap tides, when velocities are at their fortnightly minimum, but it goes through a remarkable transition to almost well-mixed conditions during spring tides (Fig. 3.6). Other estuaries exhibit this spring-neap change in stratification - it was first noted by Haas (1977) in the Rappahannock Estuary in Chesapeake Bay. However, the Hudson exhibits a more extreme range of stratification between neap and spring tides than any estuary in which this phenomenon has been observed (Geyer, Trowbridge, andBowen, 2000).

the estuarine circulation

Although the vertical salinity gradient varies considerably between neap and spring tides, there is always a strong horizontal salinity gradient along the estuary. This salinity gradient causes a horizontal density gradient (due to the difference in density between fresh and salt water), which in turn induces a depth-varying, or "baroclinic" pressure gradient in the estuary. The baroclinic pressure gradient drives the deep water landward, and a compensating tilt of the water surface drives the surface water seaward (Fig. 3.7). This vertically varying motion is called the estuarine circulation (Pritchard, 1952). The estuarine circulation has the strange property that, at the bottom, is directed toward land against the direction of the river flow. This tendency is counterintuitive, particularly because the estuarine circulation owes its origin to the forcing by the freshwater outflow. The explanation for this is the forcing by the

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