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Estuary

Estuary

Estuaries are unique and complex environments located between oceans and river mouths. As freshwater flows into the sea from land, it dilutes the salty water in a small area around the shore. This relatively small space is the site of sediment build-up resulting from fluvial (stream or river) erosion along the riverbanks. The organic sediments and brackish (slightly salty, but undrinkable) water make a unique environment that supports a diverse community of plants and animals. The sediments themselves also form characteristic types of deposits and bed forms (the appearance of the horizontal layers of sand ) that are easily seen in cross-section.

It is well known that rivers often carry tremendous amounts of sediment which, when emptied into the oceans, construct distinctive patterns in the underlying sediments. In an estuary, the deposition of sediments is greatly influenced by tidal currents and ocean waves. Even climate is a factor in how the sands and muds settle into distinct patterns. During seasonal storms, erosion is increased and the waters become heavily laden with a wide variety of sediments. Unlike deltas, in which the finer sediments are often carried far out to sea, the estuary is bordered on the deeper ocean side by heavy sands while clays and muds are dropped at river mouths. As tidal forces work the sediments by tumbling and rolling them, the lighter and finer particles are left near the river mouth. The build-up of coarse-grained (larger particle size) sands at the estuary edge often makes a barrier at the outer edge of the estuary that contains the bulk of the fine sediment and diluted water. The sediment structures in these ridges are defined as longitudinal or oblique bars. The structures in the upper reaches of the estuary are described as asymmetric and longitudinal bars become point bars similar to those observed in rivers. A dendritic (tree-shaped) pattern of channels occurs in these finer, flat lying sediments.

The greatest force at work shaping and changing the estuary comes from moving water. Daily tidal fluctuation brings saltier water into the estuary and pushes medium-grained sands into the main body of the tidal flats. On the ocean side of the estuary, the sand bars are penetrated by channels through which the flow of water into and out of the estuary is restrained. These containment structures close the general water and sediment circulation paths around the main body of the estuary. Water and sediment flow is greatly restricted and additional build-up of medium and fine-grained sediments occurs. Water is forced to leave the estuary by these well-defined channels.

The tidal or exit channels of the estuary can be dangerous places for some life forms. As low tide occurs, the ebb of general sea level reverses the oceanic flow into the estuary. Water laden sediments release their burden and contribute to the general volume of water leaving the flats. The force of water exiting through the channels becomes great. The velocity of the water can reach dangerously high levels. In well-established estuaries, large animals can be swept to sea. However, for many marine animals this is a benefit.

The estuaries are safe places for many creatures such as crabs and other crustaceans to lay their eggs. The hatched larvae live in the estuary until they are ready to join the zoo-plankton community of the larger sea. The swift release of water from the tidal flats helps the floating larvae to jet far out into the ocean where they will spend the next phase of their lives. If not for the tidal currents, the larvae would have to live dangerously exposed to shore birds and other marine carnivores. The estuarine water channels help them grow and gain a slight advantage for survival that would not be found in typical shore lines.

When high tide returns, the encroaching water brings oxygen to the anaerobic (without oxygen) sediments. The water also brings marine organic food for mud dwelling inhabitants. The sediments are refreshed with salt water and another cycle of replenishment occurs. During stormy seasons , this effect can be exaggerated and actually quite harmful to inhabitants as sediments are churned and redeposited. However, this continuous recycling of sediments and resources keeps the estuary healthy and flourishing.

Because the water plays such a physical and active role near the outer borders of the estuary, the inner regions of the estuary are more protected. By containing the general flow of water to the channels, the finer sediments, such as silt and clay , are left relatively undisturbed near the river mouths. They build up into areas of fine muds and contribute to the distinctive tidal flats. Organic debris is carried along by the rivers as they carve through valleys and plains of the terrestrial environment. This lightweight material comes to rest in the tidal flat as the velocity of the water is drastically reduced in the tidal flats. The decay and spreading of organic material throughout the flats makes them rich in nutrients. Subsequently, clams and other burrowing animals thrive in the rich sediments of the upper estuary. In turn, birds are lured to this feast where they are able to rear young on the nearby shore. These life forms are relatively protected because the muds make it difficult for heavier predators to walk out into the estuary with any stealth. There are even places where the muds act as a sort of quicksand and can be very dangerous.

Estuaries are fragile environments that are becoming increasingly threatened. They are being geologically altered as sediments are trapped upstream by dams. Diversion of water and sediments by agriculture is also reducing flow to the estuary. As a direct result, the life forms that rely on the dynamics of the estuary are decreasing in numbers. Many people are realizing the importance of estuarine environments and the important role they play in both marine and terrestrial ecosystems.

See also Oceanography; Sedimentation; Tides

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estuary

estuary (ĕs´chŏŏĕr´ē), 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.

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estuary

estuary Semi-enclosed coastal body of water which has a free connection with the open sea and where fresh water, derived from land drainage, is mixed with sea water. Estuaries are often subject to tidal action and where tidal activity is large, ebb and flood tidal currents tend to avoid each other, forming separate channels. In estuaries where tidal activity is small, the invading dense sea water may flow under the lighter fresh water forming a salt wedge. A positive estuary is one in which surface salinities are lower within the estuary than in the open sea due to freshwater inflow exceeding outflow caused by evaporation. A negative estuary is one in which evaporation exceeds freshwater inflow and therefore hypersaline conditions exist in the estuary. Normally an estuary is the result of valley drowning by the post-glacial rise in sea level. The action of tidal currents on the large amount of available sediment may give rise to a range of mobile bottom forms, including ebb and flood channels, sandbanks, and sand waves.

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"estuary." A Dictionary of Earth Sciences. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

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estuary

estuary A coastal body of water which has a free connection with the open sea and where fresh water, derived from land drainage, is mixed with sea water. Estuaries are often subject to tidal action and, where tidal activity is large, ebb and flood tidal currents tend to avoid each other, forming separate channels. In estuaries where tidal activity is small, the invading dense sea water may flow under the lighter fresh water, forming a salt wedge. A positive estuary is one in which surface salinities are lower within the estuary than in the open sea, owing to freshwater inflow exceeding outflow caused by evaporation. A negative estuary is one in which evaporation exceeds freshwater inflow and therefore hypersaline conditions exist in the estuary. Normally an estuary is the result of valley drowning by the postglacial rise in sea level. The action of tidal currents on the large amount of available sediment may give rise to a range of mobile bottom forms, including ebb and flood channels, sandbanks, and sand waves.

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"estuary." A Dictionary of Ecology. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

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estuary

es·tu·ar·y / ˈeschoōˌerē/ • n. (pl. -ar·ies) the tidal mouth of a large river, where the tide meets the stream. DERIVATIVES: es·tu·ar·i·al / ˌeschoōˈe(ə)rēəl/ adj. es·tu·a·rine / ˈeschoōəˌrīn; -əˌrēn/ adj.

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"estuary." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

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estuary

estuary Coastal region where a river mouth opens into the ocean and freshwater from the land mixes with saltwater from the sea. Estuaries usually provide good harbours and breeding grounds for many kinds of marine life.

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estuary

estuary XVI. — L. æstuārium tidal part of a shore, tidal channel, f. æstus swell, surge, tide; see -ARY.

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"estuary." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

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"estuary." The Concise Oxford Dictionary of English Etymology. . Retrieved June 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/estuary-1

estuary

estuaryFlorrie, Laurie, lorry, Macquarie, quarry, sorry, whare •Rhodri • Godfrey • hostelry •Coventry • quixotry •cacciatore, Corey, dory, Florey, flory, furore, glory, gory, hoary, hunky-dory, lory, Maury, monsignori, Montessori, multistorey, Pori, Rory, satori, saury, storey, story, Tory, vainglory •Aubrey • aumbry •Audrey, bawdry, tawdry •laundry •gallimaufry, orphrey •palfrey • paltry • outlawry • centaury •clerestory (US clearstory) •understorey •cowrie, kauri, Lowry, Maori •Cowdrey • foundry • Rowntree •ochry (US ochery) • poultry •coxcombry • matsuri • Kirkcudbright •shoetree •Hurri, potpourri •kukri • century • penury • estuary •residuary • augury • mercury

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"estuary." Oxford Dictionary of Rhymes. . Retrieved June 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/estuary