Chapter 23: Estuaries and Lagoons

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Abstract
An estuary is the wide mouth of a river or arm of the sea where the tide meets the river currents, or flows and ebbs (L. aestuarium, fr. aestuo to boil or foam, aestus, heat, tide). Or, “an estuary may be defined as a body of water in which the river water mixes with and measurably dilutes sea water.” (Ketchum, 1951b). A lagoon is defined as a shallow lake or sheet of water connected with the sea or a river (L. lacuna, fr. lacus, a lake). These dictionary definitions are not mutually exclusive, since a lagoon connected with the sea may also be affected by the tide. Some ecologists prefer to describe the environment in terms of the salinity (as saline, brackish, or fresh), but saline water is not restricted to marginal marine areas, and such a description of the environment does not consider the most characteristic aspect of the estuarine environment—that it is a region of steep and variable gradients in environmental conditions (Fig. 1). Physiographically, estuaries are bodies of water bordered by and partly cut off from the ocean by land masses that were originally shaped by nonmarine agencies. They are usually perpendicular to the coast line, and most of them occupy the drowned mouths of stream valleys and are, therefore, usually considered as evidence of submergence. Lagoons are bodies of water separated in most cases from the ocean by offshore bars or islands of marine origin and are usually parallel to the coast line. They have...

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