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From Wikipedia, the free encyclopedia

Estuary of Klamath River

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.[1]

Estuaries form a transition zone between river environments and ocean environments and are subject to both marine influences, such as tides, waves, and the influx of saline water; and riverine influences, such as flows of fresh water and sediment. The inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world [2]

Most modern-day estuaries were formed during the Holocene epoch by the flooding of river-eroded or glacially-scoured valleys when sea level began to rise about 10,000-12,000 years ago.[3] Estuaries are typically classified by their geomorphological features or by water circulation patterns and can be referred to by many different names, such as bays, harbors, lagoons, inlets, or sounds, although sometimes these water bodies do not necessarily meet the above criteria of an estuary and may be fully saline.

Estuaries are amongst the most heavily populated areas throughout the world, with about 60% of the world’s population living along estuaries and the coast. As a result, estuaries are suffering degradation by many factors, including sedimentation from soil erosion from deforestation; overgrazing and other poor farming practices; overfishing; drainage and filling of wetlands; eutrophication due to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, PCBs, radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.[3]

River Exe estuary
River Nith estuary
Estuary mouth located in Darwin, Northern Territory, Australia
Estuary mouth
Estuary mouth of the Yachats River in Yachats, Oregon
Amazon estuary


Definition of an estuary

The word “estuary” is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide. There have been many definitions proposed to describe an estuary. The most widely accepted definition is: “a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with freshwater derived from land drainage.” [1] However, this definition excludes a number of coastal water bodies such as coastal lagoons and brackish seas. A more thorough definition of an estuary would be “a semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff; however the freshwater inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible.” [3] This definition includes classical estuaries as well as fjords, lagoons, river mouths, and tidal creeks. Estuaries are a dynamic ecosystem with a connection with the open sea through which the seawater enters accordingly to the rhythm of the tides. The seawater entering the estuary is diluted by the freshwater flowing from rivers and streams. The pattern of dilution varies in different estuaries and is dependent on the volume of freshwater, tidal amplitude range, and the extent of evaporation from the water within the estuary. [2]

Classification of estuaries

Estuaries have been classified according to many different schemes. Most commonly, estuaries are classified according to their geomorphology or water circulation.


Classification based on geomorphology

Drowned river valleys

Many drowned river valley estuaries were formed between about 15,000 and 6000 years ago following the end of the Wisconsin (or 'Devensian') glaciation when a eustatic rise in sea level of 100 m to 130 m, flooded river valleys that were cut into the landscape when sea level was lower, creating the estuarine systems. Additionally, the general subsidence of coastal regions contributed to the development of drowned river valleys. Well developed drowned river valleys are generally found on coastlines with low, wide coastal plains. Their width-to-depth ratio is typically large, appearing wedge-shaped in the inner part and broadening and deepening seaward. Water depths rarely exceed 30 meters. Examples of this type of estuary include the Chesapeake Bay and Delaware Bay, along the U.S. mid-Atlantic coast, and along the U.S. Gulf coast, Galveston Bay and Tampa Bay[4].

Lagoon-type or bar-built estuaries

These estuaries are semi-isolated from ocean waters by barrier beaches (barrier islands and barrier spits). Formation of barrier beaches partially encloses the estuary with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than 4 meters. The barrier beaches that enclose bar-built estuaries have been developed in several ways: 1) upbuilding of offshore bars from wave action, in which sand from the seafloor is deposited in elongate bars parallel to the shoreline, 2) reworking of sediment discharge from rivers by wave, current, and wind action into beaches, overwash flats, and dunes, 3) engulfment of mainland beach ridges (ridges developed from the erosion of coastal plain sediments approximately 5,000 years ago) due to sea level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons, 4) elongation of barrier spits from the erosion of headlands, with the spit growth occurring in the direction of the littoral drift due to the action of longshore currents. Barrier beaches form in shallow water and are generally parallel to the shoreline, resulting in long, narrow estuaries. The average water depth is usually less than 5 m, and rarely exceed 10 m. Examples of bar-built estuaries include Barnegat Bay, New Jersey, Laguna Madre, Texas, and Pamlico Sound, North Carolina.

Fjord-type estuaries

Fjord type estuaries are formed in deeply eroded valleys formed by glaciers. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The shallowest area of the estuary occurs at the mouth, where terminal glacial deposits or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 meters. The width-to-depth ratio is generally small. When estuaries contain very shallow sills, tidal oscillations only affect near surface waters to sill depth, and waters below sill depth may remain stagnant for very long periods of time, resulting in only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted and a slow, but steady exchange of water from the estuary and the ocean occur. Fjord-type estuaries can be found along the coasts of Alaska, eastern Canada, Greenland, Iceland, New Zealand, and Norway.

Tectonically produced estuaries

These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with faulting, volcanoes, and landslides. Inundation from eustatic sea level rise during the Holocene Epoch has also contributed to the formation of these estuaries. There are only a small number of tectonically produced estuaries; one example is the San Francisco Bay, which was formed by the crustal movements of the San Andreas fault system causing the inundation of the lower reaches of the Sacramento and San Joaquin rivers.[5]

Classification based on Hydrography

Salt wedge estuaries

In this type of estuary, river output greatly exceeds marine input and tidal effects have a minor importance. Fresh water floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land. As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example of a salt wedge estuary is the Mississippi River. [5]

Partially mixed estuaries

As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition. Examples include the Chesapeake Bay and Narragansett Bay. [5]

Vertically homogenous estuaries

Tidal mixing forces exceed river output, resulting in a well mixed water column and the disappearance of the vertical salinity gradient. The freshwater-seawater boundary is eliminated due to the intense turbulent mixing and eddy effects. The lower reaches of the Delaware Bay and the Raritan River in New Jersey are examples of vertically homogenous estuaries. [5]

Inverse estuary

Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of fresh water. A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone.[6] This water is pushed downward and spreads along the bottom in both the seaward and landward direction. [3] An example of an inverse estuary is Spencer Gulf, South Australia.

Intermittent estuaries

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. [7] [8]

(See also Estuarine water circulation)

Implications for marine life

Estuaries provide habitats for a large number of organisms and support very high productivity. Estuaries provide habitats for many fish nurseries, depending upon their locations in the world, such as salmon and sea trout[9]. Also, migratory bird populations, such as the black-tailed godwit, Limosa limosa islandica[10] make essential use of estuaries.

Two of the main challenges of estuarine life are the variability in salinity and sedimentation. Many species of fish and invertebrates have various methods to control or conform to the shifts in salt concentrations and are termed osmoconformers and osmoregulators. Many animals also burrow to avoid predation and to live in the more stable sedimental environment. However, large numbers of bacteria are found within the sediment which have a very high oxygen demand. This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux.

Plankton are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the tides. Their productivity is largely dependant upon the turbidity of the water. The main plankton present are diatoms and dinoflagellates which are abundant in the sediment.

It is important to remember that a primary source of food for many organisms on estuaries, including bacteria, is detritus from the settlement of the sedimentation.

Human impacts

Of the 32 largest cities in the world, 22 are located on estuaries.[11] For example, New York City is located at the orifice of the Hudson River estuary.[12]

As ecosystems, estuaries are under threat from human activities such as pollution and overfishing. They are also threatened by sewage, coastal settlement, land clearance and much more. Estuaries are impacted by events far upstream, and concentrate materials such as pollutants and sediments[13]. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into estuaries. Contaminants can be introduced which do not disintegrate rapidly in the marine environment, such as plastics, pesticides, furans, dioxins, phenols and heavy metals.

Such toxins can accumulate in the tissues of many species of aquatic life in a process called bioaccumulation. They also accumulate in benthic environments, such as estuaries and bay muds: a geological record of human activities of the last century.

For example, Chinese and Russian industrial pollution, such as phenols and heavy metals, in the Amur River have devastated fish stocks and damaged its estuary soil.[14]

Estuaries tend to be naturally eutrophic because land runoff discharges nutrients into estuaries. With human activities, land run-off also now includes the many chemicals used as fertilizers in agriculture as well as waste from livestock and humans. Excess oxygen depleting chemicals in the water can lead to hypoxia and the creation of dead zones.[15] It can result in reductions in water quality, fish, and other animal populations.

Overfishing also occurs. Chesapeake Bay once had a flourishing oyster population which has been almost wiped out by overfishing. Historically the oysters filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process takes almost a year,[16] and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless.

Notable examples

The Moore River estuary, a seasonally closed estuary in Western Australia

See also


  1. ^ a b 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.
  2. ^ a b McLusky, D.S. and Elliott, M. (2004) "The Estuarine Ecosystem: ecology, threats and management." New York: Oxford University Press Inc. ISBN 0-19-852508-7
  3. ^ a b c d Wolanski, E. (2007) "Estuarine Ecohydrology." Amsterdam, The Netherlands: Elsevier. ISBN 978-0-444-53066-0
  4. ^ Kunneke, J.T., and T.F. Palik, 1984. "Tampa Bay environmental atlas", U.S. Fish Wildl. Serv. Biol. Rep. 85(15), page 3. Retrieved January 12, 2010.
  5. ^ a b c d Kennish, M.J. (1986) "Ecology of Estuaries. Volume I: Physical and Chemical Aspects." Boca Raton, FL: CRC Press, Inc. ISBN: 0-8493-5892-2
  6. ^ Wolanski, E. (1986). "An evaporation-driven salinity maximum zone in Australian tropical estuaries" Estuarine, Coastal, and Shelf Science 22, 415-424.
  7. ^ Tomczak, M (2000) "Oceanography Notes Ch. 12: Estuaries. Retrieved 30 November 2006.
  8. ^ Day, J.H. (1981) "Estuarine Ecology." Rotterdam, The Netherlands: A.A. Balkema. ISBN: 90-6191-205-9.
  9. ^ Bronwyn M. Gillanders, Evidence of connectivity between juvenile and adult habitats for mobile marine fauna: an important component of nurseries. 2003. Marine Ecology Progress Series
  10. ^ Jennifer A. Gill, The buffer effect and large-scale population regulation in migratory birds. 2001. Nature 412, 436-438
  11. ^ Ross, D A (1995) Introduction to Oceanography. New York: Harper Collins College Publishers. ISBN 978-0-673-46938-0
  12. ^ NOAA Estuaries tutorial Revised March 25, 2008
  13. ^ G.Branch, Estuarine vulnerability and ecological impacts, TREE vol. 14, no. 12 Dec. 1999
  14. ^ "Indigenous Peoples of the Russian North, Siberia and Far East: Nivkh" by Arctic Network for the Support of the Indigenous Peoples of the Russian Arctic]
  15. ^ Gerlach: Marine Pollution, Springer, Berlin (1975)
  16. ^ "Oyster Reefs: Ecological importance". US National Oceanic and Atmospheric Administration. Retrieved 2008-01-16. 

External links

Study guide

Up to date as of January 14, 2010

From Wikiversity

An estuary is a body of water where a river and the ocean connect and blend.

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ESTUARY (from the Lat. aestuarium, a place reached by aestus, the tide), an arm of the sea narrowing inwards at the mouth of a river where sea and fresh water meet and are mixed, i.e. the tidal portion of a river's mouth. Structurally the estuary may represent the long-continued action of river erosion and tidal erosion confined to a narrow channel, most effective where most concentrated, or an estuary may be the drowned portion of the lower part of a river-valley. In a map of Britain showing sea-depths it will be observed that under the Severn estuary the sea deepens in a number of steps descending by concentric V's that become blunter towards deep water until the last is a mere indentation pointing towards the long narrow termination of the present estuary. In this and in similar cases the progress of the estuary is indicated upon what is now the continental shelf. The chief interest in estuarine conditions is the mingling of sea and fresh water. Where, as in the Severn and the Thames, the fresh water meets the sea gradually the water is mixed, and there is very little change in salinity at high tide. The fresh water flows over the salt water and there is a continuous rapid change in salinity towards the sea, for the currents sweeping in and out mix the water constantly. Where the river brings down a great quantity of fresh water in a narrow channel, the change of salinity at high and low water is very marked. "When, however, the inlet is very large compared with the river, and there is no, bar at the opening, the estuarine character is only shown at the upper end. In the Firth of Forth, for example, the landward half is an estuary, but in the seaward half the water has become more thoroughly mixed, the salinity is almost uniform from surface to bottom, and increases very gradually towards the sea. The river-water meets the sea diffused uniformly through a deep mass of water scarcely fresher than the sea itself, so that the two mix uniformly, and the sea becomes slightly freshened.

throughout its whole depth for many miles from land" (H. R. Mill, Realm of Nature, 1897).

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Simple English

An estuary is a place near the ocean where saltwater mixes with freshwater. An estuary is often found where a river flows into an ocean. The river suddenly becomes wider and wider and flows slowly to the sea.

Estuaries come in all sizes and shapes, each according to its location and climate. They are found mostly where rivers meet the sea and fresh water mixes with salt water. Bays, marshes, swamps, and inlets are all examples of estuaries. A view of an estuary from the air is usually an interesting sight: rivers curve and bend to find their way back to the sea. Estuaries are usually filled with shallow waters and sunlight reaches all levels of the water. Marsh grasses, algae, and oher kinds of plants live in estuaries and provide food for a variety of fish, crabs, oysters, and shrimp.[1]

Some estuaries are very large. They may be large ocean bays that have more than one river flowing into them. For example, Chesapeake Bay is a very large estuary, and several different rivers meet the Atlantic Ocean there.

A good habitat and "nursery"

Estuaries are especially important since they play the role of "nurseries" for many different types of young fish and other animals before they head out toward the open ocean. [1]Many sea birds also nest in estuaries.

The United States government has a program to study and protect the natural environment in many different estuaries, called the National Estuarine Research Reserve System. One reason that the natural environment in so many estuaries is in danger is because they are also good places for people to live and build cities. Unfortunately, many of world's largest cities are located at estuaries.


  1. 1.0 1.1 Pous, Dinorah (2003). Blue Planet. McGrawHill. 


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