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The term estuary comes from the Latin aestus , the tide, and aesto, boil, from the boiling effect of the rising tide at river mouths where the river and ocean waters meet. The estuary is usually defined as that part of the lower river course that is affected by the mixing of salt water and fresh. The tidal rise and fall may extend considerably farther upstream; the Oxford English Dictionary gives this tidal limit definition as "rare in modern usage." The upper limit of watermixing varies with the state of the tide, the freshwater discharge and the season of the year, so that the brackish biotope is subject to extreme variations.

A river debouching into the sea may do so in a number of ways or associations, including:
  • Direct, as with a "youthful" type stream on a steep coast, there being no delta and no salt-water encroachment;
  • Delta- building , where the sediment discharge exceeds the net wave energy available for sediment dispersal (see Delta Dynamics);