Lagoon
Shallow coastal water body separated from open sea by a barrier.
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A lagoon is a shallow body of water separated from a larger body of water by a narrow landform such as a reef, barrier island, barrier peninsula, or isthmus. Lagoons are commonly divided into coastal lagoons (or barrier lagoons) and atoll lagoons, and they occur on mixed-sand and gravel coastlines. They are common coastal features around many parts of the world, with some overlap between bodies of water classified as coastal lagoons and those classified as estuaries.
Etymology
The term originates from the Venetian word for the waters around Venice, the Venetian Lagoon, which itself derives from a word meaning “lake.” The word entered English by the early 1600s and was later anglicized. Early explorers such as William Dampier and Captain James Cook used the term to describe saltwater lakes and islands with central lagoons.
In various languages, the word for lagoon can be a type of lake, such as the Chinese term for a lagoon being a specific kind of lake, or the French use of “étang” for certain Mediterranean lagoons. Other languages have distinct words for coastal versus atoll lagoons, such as French differentiating “lagon” for atoll lagoons from “étang” for coastal ones. In New Zealand, Māori uses “waihora” for a coastal lagoon formed at the mouth of a braided river on mixed sand and gravel beaches, while “waituna” describes an ephemeral coastal waterbody that is not truly a lagoon, lake, or estuary.
Atoll lagoons
Atoll lagoons form when tropical coral reefs grow upward as the islands they surround subside, leaving only the reefs above sea level. These lagoons often contain deep portions, unlike those formed shoreward of fringing reefs.
Coastal lagoons
Coastal lagoons develop along gently sloping coasts where barrier islands or reefs can form offshore, and where sea level is rising relative to the land. They do not form along steep or rocky coasts, nor where tidal ranges exceed a certain threshold, and they are typically shallow due to the gentle coastal slope. Many well-known bodies of water are classified as lagoons despite not including “lagoon” in their names, such as various sounds in North Carolina, Great South Bay in New York, and Lake Illawarra in Australia.
Quick Facts
- Definition
- Shallow body of water separated from a larger body by a narrow landform
- Types
- Coastal lagoons and atoll lagoons
- Depth
- Coastal lagoons are shallow; atoll lagoons often contain deep portions (>20 m / 65 ft 7 in)
Facts from the source article.
Lore & Background
Lagoons are defined as shallow, often elongated bodies of water separated from a larger body by a shallow or exposed shoal, coral reef, or similar feature. Some authorities include fresh water bodies in the definition, while others restrict it to bodies with some degree of salinity. The distinction between lagoon and estuary varies; Richard A. Davis Jr. restricts lagoon to bodies with little or no freshwater inflow and little or no tidal flow, while Timothy M. Kusky characterizes lagoons as normally elongated parallel to the coast, whereas estuaries are drowned river valleys perpendicular to the coast. Many lagoons do not include 'lagoon' in their common names, such as Currituck, Albemarle, and Pamlico Sounds in North Carolina, Great South Bay in New York, and Banana River in Florida.
Reader's Guide
Lagoons are significant as dynamic coastal features that form along gently sloping coasts where barrier islands or reefs develop offshore and sea level is rising relative to the land. They do not form along steep or rocky coasts or where tidal range exceeds 4 meters. Coastal lagoons are young and may be short-lived in geological terms, accumulating sediments from rivers, runoff, and tidal inlets. They can be classified as leaky, restricted, or choked based on water exchange with the ocean.
Lagoons support diverse ecosystems and are influenced by mangroves, marsh plants, and benthic organisms. The term 'lagoon' is also used synonymously with 'back reef' in coral reef contexts. Their etymology traces to the Venetian łaguna, referring to the waters around Venice, and the word has been Anglicized since the 17th century.
Did You Know?
- Coastal lagoons occur along nearly 15 percent of the world's shorelines.
Frequently Asked Questions
What are the main types of lagoons?
Geologists generally split lagoons into two categories: coastal (or barrier) lagoons and atoll lagoons. Coastal varieties tend to be shallow, whereas atoll lagoons frequently contain deep sections exceeding 20 meters.
How do lagoons form?
Coastal lagoons develop along gently sloping shorelines where sea level rises or the land subsides, trapping water behind a newly established barrier. Atoll lagoons form through a different process: coral reefs build upward as the underlying island gradually sinks, enclosing a central basin.
What is the salinity like in lagoons?
Lagoon water can range from highly saline to entirely fresh, depending on inlet size, local rainfall, evaporation rates, and the volume of freshwater inflow. There is no single 'lagoon salinity'—it varies widely from one basin to the next.
How widespread are lagoons around the world?
Lagoons line roughly 15 percent of the planet's coastlines, making them a genuinely common geomorphological feature. In the United States alone, they occur along well over three-quarters of the Eastern and Gulf Coast shorelines.
More in Coastal & Marine Geology
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Lagoon (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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