Salt Dispersion in the Hudson Estuary

Abstract

The seaward transport of salt by river discharge through an estuary is balanced under steady conditions by landward dispersion effected by various physical mixing processes. Observations of current and salinity in the lower Hudson estuary provide a basis for assessing the relative importance of these different dispersion processes. Wind effects were estimated from two moored current meter records of 1- and 2-months length. There was a significant but weak correlation between wind and currents. Three cross-sectional surveys each lasting 25 h provide estimates of salt dispersion by other processes. Current and salinity data from the surveys were decomposed into various temporal and spatial means and departures from these means. Covariances between the various quantities are interpreted in terms of physical dispersion processes. The largest contributor to salt dispersion in the Hudson is the steady shear of gravitational circulation. Steady shear dispersion varies by a factor of 5 between spring high-flow and summer low-flow conditions. Dispersion by tidally varying shear was much lower in magnitude. Correlation between sectionally averaged current and salinity yields a paradoxical negative dispersion which can be explained in terms of two documented estuary characteristics, the tendency of bottom tidal currents to lead upper layer currents in phase and the increased longitudinal salinity gradient near the surface.