Sandwaves are large ripples or underwater dunes that indicate the movement of sand along the estuary floor by currents. They may be active, with particles being moved continuously by the river flow or alternatively by the tides, or they may be relic, inactive fractures formed by unusual past events such as floods or exceptionally strong tides. They are often asymmetric in cross-sectional form with a steeper face in the direction of net transport. As has alreadybeenmentioned, asymmetric sandwaves in the mouth of New York Harbor indicate a net transport of sand up-estuary.
In the Hudson River Estuary, patches of asymmetric sandwaves are found as, for example, near Saugerties (Fig. 4.4; Bell et al., 2000). These usually show evidence of down-estuary transport of sand under the influence of the freshwater discharge and the ebbing tide. Where the estuary is divided into two channels, however, by a median shoal or island, one channel will show evidence of up-estuary transport under flooding tides while its partner will be dominated by down-estuary transport.
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