Salt wedge

A layer of salt water that lies beneath a layer of fresh water within the lower reaches of an estuary; the separation of the two layers persists as the fresh water is substantially less dense than the salt water and the water column is not sufficiently mixed by wave and tidal action; the salt water layer decreases in depth with increasing elevation of the channel bottom, with the upstream point defined by an intercept with the bottom; the salt wedge moves downstream and upstream within the distributary channels of a estuary with outgoing and incoming tides, respectively. (Hudson River National Estuarine Research Reserve, Hudson River Shoreline Restoration Alternatives Analysis, 2006)