An estuary is that portion of a coastal stream influenced by the tide of the marine waters into which it flows and within which the sea water is measurably diluted with freshwater derived from land drainage.

Zones of Ecological Transition
Estuaries are zones of ecological transition between fresh and saltwater. The coastal brackish water areas are rich in aquatic life, some species of which are important food organisms for anadromous fish species which use these areas for feeding, rearing, and migration. An estuarine area left untouched by man is rare since historically they have been the sites for major cities and port developments.

Because of their importance in the food production chain and their natural beauty, the limited estuarial areas require careful attention in the planning function. Close scrutiny should be given to all plans for development in estuaries which reduce the area of the estuary and interfere with water flow. Special attention should be given to plans for upstream projects which could deplete the freshwater supply of the estuary.