ENCYCLOPEDIA BRITANNICA:

 Estuaries

Estuaries are places where rivers meet the sea and may be defined as areas where salt water is measurably diluted with fresh water. On average, estuaries are biologically more productive than either the adjacent river or the sea because they have a special kind of water circulation that traps plant nutrients and stimulates primary production. Fresh water, being lighter than salt water, tends to form a distinct layer that floats at the surface of the estuary. At the boundary between fresh and salt water, there is a certain amount of mixing caused by the flow of fresh water over salt and by the ebb and flow of tides. Additional mixing may be caused from time to time by strong winds and by internal waves that are propagated along the interface between fresh and salt water.

Salinity in the oceans is constant but is more variable along the coast where seawater is diluted with freshwater from runoff or from the emptying of rivers. This brackish water forms a barrier separating marine and freshwater organisms.