estuary, partially enclosed coastal body of water, having an open connection with the ocean, where freshwater from inland is mixed with saltwater from the sea. One type of estuary, called a drowned river valley, can be caused by crustal subsidence or a rise in sea level. Chesapeake Bay is one of the largest estuaries of this type in the United States and was formed during the melting of the Pleistocene ice sheets (see Pleistocene epoch). Fjords, or drowned glacial troughs, form similar types of estuaries, particularly in Norway, Alaska, New Zealand, and other glaciated, mountainous coastal regions. Salt marshes and lagoons found behind barrier beaches, such as along the south shore of Long Island, and down faulted sections of the earth's crust, such as San Francisco Bay, are additional types of estuaries. The shape of an estuary affects the height of the tide; some estuaries (such as the Severn and the Bay of Fundy) are characterized by a wavelike tidal bore. Estuaries represent one of the most sensitive and ecologically important habitats on earth. They provide sanctuary for many species of waterfowl, store nutrients for larval and juvenile marine life, and serve as breeding grounds for many desirable species of ocean fish. Since estuaries commonly provide excellent harbors, most of the large ports in the United States (New York, Philadelphia, Baltimore, Mobile, Galveston, Seattle, and San Francisco) are located in estuaries. However, the development of high-density population centers causes deleterious effects that can destroy the very properties of the estuary that made development of the region possible. Human impact on estuaries includes reclamation of tidal land by filling; pollution from sewage, solid waste, industrial effluent, and hot water; increased sedimentation filling the estuary; and alteration of the salinity of estuarine waters by withdrawal or increased influx of freshwater. Increasingly, federal and state governments are passing legislation to protect estuarine environments.

Partly enclosed coastal body of water in which river water is mixed with seawater. An estuary is thus defined by salinity rather than geography. Many coastal features designated by other names are in fact estuaries (e.g., Chesapeake Bay). Some of the oldest continuous civilizations have flourished in estuarine environments (e.g., the land between the Tigris and Euphrates rivers, the Nile delta, the Ganges delta, and the lower Huang He valley). Cities such as London (River Thames), New York (Hudson River), and Montreal (St. Lawrence River) developed on estuaries and became important commercial centres.

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Estuary on the Bay of Biscay, southwestern France. Formed by the confluence of the Garonne and Dordogne rivers, it extends for about 45 mi (72 km) inland. It is navigable for oceangoing vessels, although it has sandbanks and strong tides.

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An estuary is a semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries are often associated with high levels of biological diversity.

Estuaries are typically the tidal mouths of rivers (aestus is Latin for tide), and they 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 often given names like bay, sound, fjord, etc. The terms are not mutually exclusive.

As ecosystems, estuaries are under threat from human activities such as pollution and overfishing.

Estuary circulation

Estuaries are marine environments whose pH, salinity, and water levels vary, 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 its flushing time.

• 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 oceanographic application that extends beyond estuaries proper, such as in describing the circulation of nearly-closed ocean basins.

Estuaries are more likely to occur on submerged coasts, where the sea level has risen in relation to the land; this process floods valleys to form rias and fjords. 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. 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.

Types of estuary

Estuaries can be grouped by circulation.

• 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.

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 geologically subsident region.
• Coastal plain estuaries are flooded river valleys, and fjords are submerged glacier-eroded valleys.

Human impacts

Of the 32 largest cities in the world, 22 are located on estuaries. For example, New York is located at the mouth of the Hudson River estuary.

As ecosystems, estuaries are under threat from human activities such as pollution and overfishing. Estuaries are impacted by events far upstream, and concentrate materials such as pollutants and sediments. 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.

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. It can result in reductions in water quality, fish, and other animal populations.

Over fishing also occurs. Chesapeake Bay, North America's largest estuary, 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, 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.

See also

• Ria
• Bay mud
• Brackish water
• Firth
• Liman
• List of waterways
• River delta
• Coastal and Estuarine Research Federation
• National Estuarine Research Reserve

Notes

References

• Ross, D A (1995) Introduction to Oceanography. New York: Harper Collins College Publishers. ISBN 978-0673469380