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Delta

Delta

Deltas are complex depositional landforms that develop at the mouths of rivers . They are composed of sediment that is deposited as a river enters a standing body of water and loses forward momentum. Famous deltas include the Mississippi delta in Louisiana and the Nile delta in Egypt.

Every river flows, under the force of gravity , from its headwaters to its mouth. The mouth of a river is the location at which the river enters a standing body of water, such as a lake, sea, or the ocean. As the river enters standing water and the current is no longer confined to a channel, it spreads out, slows down, and eventually stops. The reduction in speed of the current causes the river to become unable to continue carrying suspended sediment. As sediment is deposited a series of smaller channels, called distributary channels, forms causing the shoreline to build out, or prograde. The landform created is the delta. In smaller rivers with weaker currents, forward momentum may cease almost immediately upon reaching the lake or ocean. This is especially true where the river empties into an area of strong wave action. In this case, no significant delta will be formed. Larger rivers, such as the Amazon, may be able to maintain some current for several miles out to sea, creating an extensive delta.

As a river reaches and enters a standing body of water, sediment is deposited according to grain size. The coarsest sediment, such as sand , is dropped first, closest to the mouth of the river. With progressive distance from the mouth, finer sediment including fine sand, silt, and clay is deposited. This results in a distinct sequence of layers, known as topsets, foresets, and bottomsets. The topsets, as the name implies, are the uppermost layer. They are comprised of the coarse sediment forming the area of the delta that is above sea level. The foresets include fine sand grading into silt and clay deposited in seaward sloping layers beyond the mouth. Bottomsets are made up of clay particles, carried furthest out to sea where they settle into horizontal layers. Although this sequence is

deposited laterally with increasing distance from land, as a delta progrades the bottomsets are covered by new foresets, which are then covered by topsets as sediment builds up, and so on. The resultant coarsening up sequence is a distinguishing feature of deltaic deposits.

The sequence of topsets, foresets, and bottomsets provides an accurate picture of a simple delta system. Large marine deltas are often more complex, depending on whether the river, wave action, or the tides play the most important role. In stream-dominated deltas, fluvial deposition processes remain strongest, and distributary channels build far out to sea. These deltas are known as bird's foot deltas because of the appearance of the collection of channels extending into the sea. The Mississippi delta is probably the most famous example of a bird's foot delta. In wave-dominated deltas, distributary channels are not maintained for any great distance out to sea; rather, wave action reforms their sediment into barrier islands oriented perpendicular to the direction of flow. This type of delta is more compact, and shaped like a triangle. The Nile delta in Egypt is an example of a wave-dominated delta. Lastly, tide-dominated deltas are also compact, but broad tidal channels and sand bars form parallel to the tide direction. The Mekong delta in Vietnam is an example of a tide-dominated delta.

See also Alluvial system; Estuary; Landforms; Sedimentation

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Delta

DELTA

Often called Lower Egypt, the land between the mouths of the Nile.

The delta is a triangular area (shaped like the Greek letter Δ) that has been built up by the silt carried within the waters of the Nile River. When the Nile approaches the Mediterranean, much of the solid wastes and organic matter picked up during its long trip to the sea is screened out at the marshy estuaries and left behind to build more delta land. Although in ancient Egypt the Nile delta had seven mouths, today it has twothe Damietta on the east and Rosetta on the westand many small channels. The broad coastal rim of the delta measures about 150 miles (240 km) from Alexandria in the west to Port Saʿid in the east. It is about 100 miles (160 km) from the Mediterranean coast south to Cairo, Egypt's capital.

The delta landscape is flat and mostly fertile, but the area nearest the coast is marshy, dominated by brackish inlets and lagoons. Since the construction of the Delta Barrages in the early nineteenth century, most of the farmland has been converted from basin to perennial irrigation, which supports two or three crops per year instead of one. Almost half the inhabitants are small landowners, sharecroppers, or peasants working for wages who live in villages surrounded by the lands they till. The others live in towns or cities. Fruits, vegetables, and cotton are the important delta crops. Delta Egyptians have generally had more contact with the outside world than have Upper Egyptians and are therefore more Westernized.


Bibliography


Metz, Helen Chapin, ed. Egypt: A Country Study, 5th edition. Washington, DC: U.S. Government Printing Office, 1991.

arthur goldschmidt

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delta

delta A discrete protuberance of sediment formed where a sediment-laden current enters an open body of water, at which point there is a reduction in the velocity of the current. This results in rapid deposition of the sediment, which forms a body, for example, at the mouth of a river where the river discharges into the sea or a lake. There is a characteristic coarsening upwards of sediments. A river provides the sediments to form a delta; but the shape and nature of a delta is controlled by a variety of factors including climate, water discharge, sediment load, rate of subsidence of the sea or lake floor, and the nature of the river-mouth processes (particularly tidal and wave energy). One classification of delta types, based on variations in transport patterns on the delta, subdivides deltas into three classes: (a)river-dominated (e.g. the Mississippi and Po);(b)wave-dominated (e.g. the Rhône and Nile); and(c)tide-dominated (e.g. the Ganges and Mekong).See also Gilbert-type delta.

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delta

delta A discrete protuberance of sediment formed where a sediment-laden current enters an open body of water, at which point there is a reduction in the velocity of the current. This results in rapid deposition of the sediment, which forms a body, for example at the mouth of a river where the river discharges into the sea or a lake. There is a characteristic coarsening upwards of sediments. A river provides the sediments to form a delta; but the shape and nature of a delta is controlled by a variety of factors including climate, water discharge, sediment load, rate of subsidence of the sea or lake floor, and the nature of the river-mouth processes (particularly tidal and wave energy). One classification of delta types, based on variations in transport patterns on the delta, subdivides deltas into three classes: (a) river-dominated, e.g. the Mississippi and Po; (b) wave-dominated, e.g. the Rhône and Nile; (c) tide-dominated, e.g. the Ganges and Mekong. See also GILBERT-TYPE DELTA.

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delta

del·ta1 / ˈdeltə/ • n. 1. the fourth letter of the Greek alphabet (Δ, δ), transliterated as “d.” ∎  [as adj.] the fourth in a series of items, categories, etc. ∎  (Delta) [followed by Latin genitive] Astron. the fourth (usually fourth-brightest) star in a constellation: Delta Cephei. 2. a code word representing the letter D, used in radio communication. • symb. ∎  (δ) Math. variation of a variable or function. ∎  (Δ) Math. a finite increment. ∎  (δ) Astron. declination. del·ta2 • n. a triangular tract of sediment deposited at the mouth of a river, typically where it diverges into several outlets. DERIVATIVES: del·ta·ic / delˈtāik/ adj.

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delta

delta [from triangular shape of the Nile delta, like the Greek letter delta], a deposit of clay, silt, and sand formed at the mouth of a river where the stream loses velocity and drops part of its sediment load. No delta is formed if the coast is sinking or if there is an ocean or tidal current strong enough to prevent sediment deposition. Coarse particles settle first, with fine clays last and found at the outer regions of the delta. The three main varieties of deltas are the arcuate (the Nile), the bird's-foot (the Mississippi), and the cuspate (the Tiber). The Nile, Mississippi, Niger, Rhine, Danube, Kuban, Volga, Amu Darya, Indus, Ganges-Brahmaputra, Ayeyarwady, Tigris and Euphrates, and Huang He (Yellow) rivers are among those that have formed large deltas, many of which are fertile lands that support dense agricultural populations.

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delta

delta the fourth letter of the Greek alphabet (Δ, δ), transliterated as ‘d’.

The word is used for a triangular tract of sediment deposited at the mouth of a river, typically where it diverges into several outlets. Originally (in the mid 16th century) the term was applied specifically as the Delta (of the River Nile), from the shape of the Greek letter.
Delta Force the name of an elite American military force whose main responsibilities are rescue operations and special forces work.

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delta

delta Fan-shaped body of alluvium deposited at the mouth of a river. A delta is formed when a river deposits sediment as its speed decreases while it enters the sea. Most deltas are extremely fertile areas, but are subject to frequent flooding.

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delta

delta triangular tract of alluvial land at the mouth of a river, orig. of the Nile. XVI. Name of the fourth letter of the Greek alphabet. Δ, derived from Phoenician daleth (Δ).

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delta

deltaabetter, begetter, better, bettor, biretta, bruschetta, carburettor (US carburetor), debtor, feta, fetter, forgetter, getter, go-getter, Greta, Henrietta, letter, Loretta, mantelletta, operetta, petter, Quetta, setter, sinfonietta, sweater, upsetter, Valletta, vendetta, whetter •bisector, collector, connector, convector, corrector, defector, deflector, detector, director, ejector, elector, erector, hector, injector, inspector, nectar, objector, perfecter, projector, prospector, protector, rector, reflector, rejector, respecter, sector, selector, Spector, spectre (US specter), vector •belter, delta, helter-skelter, melter, pelta, Shelta, shelter, swelter, welter •pre-emptor, tempter •assenter, cementer, centre (US center), concentre (US concenter), dissenter, enter, eventer, fermenter (US fermentor), fomenter, frequenter, inventor, lamenter, magenta, placenta, polenta, precentor, presenter, preventer, renter, repenter, tenter, tormentor •inceptor, preceptor, receptor, sceptre (US scepter) •arrester, Avesta, Chester, contester, ester, Esther, fester, fiesta, Hester, investor, jester, Leicester, Lester, molester, Nestor, pester, polyester, protester, quester, semester, sequester, siesta, sou'wester, suggester, tester, trimester, vesta, zester •Webster • dexter • Leinster •Dorchester • Poindexter • newsletter •genuflector • implementer •experimenter • trendsetter •epicentre (US epicenter) •typesetter • jobcentre • photosetter •Cirencester • interceptor • Sylvester

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