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Typically an estuary is the stretch of a river mouth directly affected by sea tides. It can be a ria that formed when a river valley was submerged by rising sea level or a sinking coast.

A broader definition of estuary is "a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage" (Pritchard, 1967). This definition does not limit an estuary to a river mouth. The important characteristics of an estuary are that sea water mixes with fresh water, and there is an influence of the ocean tide creating a dynamic relationship between the two waters.

See also: Brackish water; List of waterways