Estuary
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225574864
2008-07-14T11:21:19Z
RobertG
232051
Remove confusing link: is the reader to think that "estuary" is another word for an "actuary"?
{{dablink|For other meanings, see [[Estuary (disambiguation)]]}}
[[Image:Estuary-mouth.jpg|thumb|right]]
[[Image:Rio de la Plata BA 2.JPG|thumb|right|[[Río de la Plata]] estuary]]
[[Image:Estuary mouth.jpg|thumb|Estuary mouth located in [[Darwin, Northern Territory]], [[Australia]]]]
An '''estuary''' is a semi-enclosed [[coastal]] body of [[water]] with one or more [[river]]s or [[stream]]s flowing into it, and with a free connection to the open [[sea]].<ref>Pritchard, D. W. (1967) ''What is an estuary: physical viewpoint''. p. 3–5 ''in:'' G. H. Lauf (ed.) ''Estuaries'', A.A.A.S. Publ. No. 83, Washington, D.C.</ref> Estuaries are often associated with high rates of biological productivity.
An estuary is typically the tidal mouth of a [[river]] (''aestus'' is Latin for tide), and estuaries are often characterized by [[sedimentation]] or [[silt]] carried in from terrestrial runoff and, frequently, from offshore. They are made up of [[brackish water]]. Estuaries are more likely to occur on [[submergent coastline|submerged coasts]], where the sea level has risen in relation to the land; this process floods [[valley]]s to form [[ria]]s and [[fjord]]s. These can become estuaries if there is a stream or river flowing into them. Large estuaries, like [[Chesapeake Bay]] and [[Puget Sound]], often have many streams flowing into them and can have complex shapes. Estuaries are often given names like [[bay]], [[Sound (geography)|sound]], [[fjord]], etc. The terms are not mutually exclusive. Where an enormous volume of river water enters the sea (as, for example, from the Amazon into the South Atlantic) its estuary could be considered to extend well beyond the coast.
''Estuarine circulation'' is common in estuaries; this occurs when fresh or brackish water flows out near the surface, while denser saline water flows inward near the bottom. ''Anti-estuarine'' flow is its opposite, in which dense water flows out near the bottom and less dense water circulates inward at the surface. These two terms, however, have a broader [[oceanography|oceanographic]] application that extends beyond estuaries proper, such as in describing the circulation of nearly-closed ocean basins. Estuaries are marine environments, whose [[pH]], [[salinity]], and water level are varying, depending on the river that feeds the estuary and the ocean from which it derives its salinity (oceans and seas have different salinity levels). The time it takes an estuary to completely cycle is called flushing time. As ecosystems, the estuaries are under great threat from human activities. They are small, in demand, impacted by events far upstream or out at sea, and concentrate materials such as pollutants and sediments<ref>G.Branch, Estuarine vulnerability and ecological impacts, TREE vol. 14, no. 12 Dec. 1999</ref>.
==Classes of estuary==
; Salt wedge : River output greatly exceeds marine input; there is little mixing, and thus a sharp contrast between fresh surface water and saline bottom water.
; Highly stratified : River output and marine input are more even, with river flow still dominant; turbulence induces more mixing of salt water upward than the reverse.
;Slightly stratified : River output is less than the marine input. Here, turbulence causes mixing of the whole water column, such that salinity varies more longitudinally rather than vertically.
; Vertically mixed : River output is much less than marine input, such that the freshwater contribution is negligible; longitudinal salinity variation only.
; Inverse estuary : Located in regions with high evaporation, there is no freshwater input and in fact salinity increases inland; overall flow is ''inward'' at the surface, downwells at the inland terminus, and flows outward subsurface.
; Intermittent estuary : Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine [[embayment]] to any of the other estuary types.<ref>M. Tomczak, "[http://www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html Oceanography Notes Ch. 12: Estuaries]". Retrieved on [[30 November]] [[2006]].</ref>
Grouped by structure rather than circulation, there are other types of estuaries. ''Bar-built estuaries'' are effectively synonymous with [[barrier island]] [[lagoons]], such as [[Texas]]'s [[Laguna Madre]]. ''Tectonic estuaries'' form when the sea floods a [[geology|geologically]] [[subsidence|subsident]] region, ''coastal plain estuaries'' are flooded river valleys, and ''[[fjords]]'' are submerged [[glacier|glacier-eroded]] valleys.<ref>"[http://omp.gso.uri.edu/doee/science/descript/esttype1.htm Types of Estuaries: Based on Geology]". Retrieved on [[1 December]] [[2006]].</ref>
==See also==
{{commonscat|Estuaries}}
{{wiktionary|estuary}}
* [[Ria]]
* [[Bay mud]]
* [[Brackish water]]
* [[Firth]]
* [[Liman]]
* [[List of waterways]]
* [[River Delta]]
* [[Coastal and Estuarine Research Federation]]
[[Image:Moore River estuary and Guilderton panorama.jpg|center|thumb|700px|The [[Moore River (Western Australia)|Moore River]] estuary, a seasonally closed estuary in [[Western Australia]]]]
== References ==
{{reflist}}
{{fisheries and fishing}}
[[Category:Estuaries| ]]
[[Category:Geodesy]]
[[Category:Water]]
[[Category:Coastal geography]]
[[Category:Fisheries]]
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