Lagoon

This article is about natural lagoons. For other uses of lagoon, please see Lagoon (disambiguation).

Nearly half the area of Kiritimati is covered with lagoon, some being freshwater and some seawater.

A view of Lagoa dos Patos, Brazil

A lagoon is a body of comparatively shallow salt or brackish water separated from the deeper sea by a shallow or exposed sandbank, coral reef, or similar feature. Thus, the enclosed body of water behind a barrier reef or barrier islands or enclosed by an atoll reef is called a lagoon. This application of lagoon in English dates from 1769. It adapted and extended the sense of the Venetian laguna (cf Latin lacuna, 'empty space'), which specifically referred to Venice's shallow, island-studded stretch of saltwater, protected from the Adriatic by the barrier beaches of the Lido (see Venetian Lagoon). Lagoon refers to both coastal lagoons formed by the build-up of sandbanks or reefs along shallow coastal waters, and the lagoons in atolls, formed by the growth of coral reefs on slowly sinking central islands. Lagoons that are fed by freshwater streams are also called estuaries.

Coastal lagoons are usually found on coasts with relatively small tidal ranges. They constitute approximately 13 percent of all coastlines. They generally extend parallel to the coastland, separated from the sea by barrier islands, sand and shingle bars or coral reefs. Non-reef lagoon barriers are formed by wave-action or longshore currents piling up coarser sediments off shore of the beach. Once a lagoon barrier has formed, finer sediments can settle out in the relatively quiet water behind the barrier, including sediments brought into the lagoon by rivers. Coastal lagoons typically have only constricted openings to the sea. As a result, water conditions in the lagoon can differ significantly from the open water of the sea in temperature, salinity, dissolved oxygen and sediment load.

In many English-speaking countries, coastal lagoons are sometimes called sounds, bays, rivers, or lakes. Albemarle Sound in North Carolina, Great South Bay, between Long Island and the barrier beaches of Fire Island in New York, Banana River in Florida and Lake Illawarra in New South Wales are all lagoons. In the UK there are lagoons at Montrose, (Scotland) and Tywyn, (Wales), whilst the expanse of water inside Chesil Beach, England, known as The Fleet, could also be described as a lagoon. There is also one near the small town of Dingle in western Ireland.

In Mexico often the use of "laguna", which lagoon translates to, is used to describe a lake, such as Laguna Catemaco.