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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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Delta

Delta

Delta construction

Delta morphology

Delta abandonment

Delta destruction

Deltas and human activity

Resources

A delta is a low-lying, almost flat landform, composed of sediments deposited where a river flows into a lake or an ocean. Deltas form when the volume of sediment deposited at a river mouth is greater than what waves, currents, and tides can erode. Deltas extend the coastline outward, forming new land along the shore. However, even as the delta is constructed, waves, currents, or tidal activity may redistribute sediment. Although they form in lakes, the largest deltas develop along seashores. Deltas are perhaps the most complex of all sedimentary environments. The term delta comes from the resemblance between the outline of some deltas and the fourth letter in the Greek alpha-betdelta (Δ)which is shaped like a triangle.

Some areas of the delta are influenced more by river processes, while marine (or lake) activities control other parts. Deltas do not form if wave, current, or tide activity is too intense for sediment to accumulate. The degree of influence by river, wave, current and tide activity on delta form is often used to classify deltas. Among the many factors that determine the characteristics of a delta are the volume of river flow, sediment load and type, coastal topography and subsidencerate, amount and character of wave and current activity, tidal range, storm frequency and magnitude, water depth, sea level rise or fall, and climate.

Delta construction

Delta plain

As a river flows toward the sea or a lake, it occupies a single, large, relatively straight channel known as the main distributary channel. The main distributary may soon branch off, like the roots of a tree, into many separate smaller distributaries. The number of branches formed depends on many different factors

such as river flow, sediment load, and shoreline slope. Large sand-filled distributary channels occupy the delta plain, the nearly level, landward part of the delta, which is partly subaerial (above lake or sea level).

The natural levees that flank large distributary channels are another element of the delta plain. Natural levees are mounds of sand and silt that form when floodwaters flow over the banks of the distributary and deposit their sediment load immediately adjacent to the channel. Unlike natural levees on rivers, delta levees do not grow especially large. Therefore, they are easily broken through by floodwaters a process called avulsion. This forms small channels, or crevasses, that flow away from the distributaries, like the little rootlets from the larger roots of a tree. The fan-shaped deposits formed during breeching of the distributaries are called crevasse splays.

Between the distributary channels, a variety of shallow, quiet water environments form, including freshwater swamps and lakes, and saltwater marshes. It is in these wetland basins that large volumes of organic matter and finegrained sediment accumulate.

Delta front

When sedimentladen river water flows into the standing water at the mouth of a distributary, the river water slows and deposits its load. This forms a sediment body called a distributary mouth bar, or bar finger sandso named because the distributary channels look a bit like the fingers on a human hand. Distributary mouth bars form on the delta front, the gently seaward sloping, marine-dominated part of the delta that is all subaqueous, or below water level.

Subaqueous levees may extend out from the natural levees of the delta front onto the delta plain. These help confine the water flow seaward of the distributary mouth to a relatively narrow path, so that the delta continues growing outward at a rapid pace. The area of the delta front between the distributary mouth bars is called the interdistributary bay; the salinity here is brackish to marine.

Water velocity slows consistently outward from the distributary mouth; the river water consequently deposits finer and finer sediment as it flows seaward. Eventually, a point is reached where the average grain size decreases to clay-sized sediment with only minor silt. This is the prodelta area, where the bottom generally has a very low slope. On occasion, large blocks of sediment break free from the delta plain, slide down the steeper delta front, and become buried in prodelta muds.

Delta morphology

Vertical character

Ideally, if a delta is growing seaward, or prograding, as deltas typically do, a thick deposit with three stacked sediment sequences develops. The lower sequence, or bottomset beds, contains flatlying silt and clay layers of the prodelta. The middle sequence, or foreset beds, contains seaward-inclined layers of sand and silt produced by distributary mouth bar sedimentation. The upper sequence, or topset beds, consists of flatlying sand deposits formed in distributary channels and natural levees, interlayered, or interbedded, with fine-grained interdistributary bay deposits. A variety of factors, such as marine influence, changes in sediment supply or sea level, tend to complicate this picture; the actual sediment distribution in a deltaic sequence is typically very complex.

Surface character

The landward to seaward transectfrom dis-tributary channel sands to prodelta mudsoutlined above is typical of deltas, like the Mississippi River delta, which experience minimal marine influence. These lobate, or birds foot deltas, migrate farther and farther out into the ocean, because of multiple distributary channels and bar sands, each building seaward-extending lobes. On coastlines where waves or currents erode and redistribute much of the deltas sand, this lobate form becomes highly modified.

In areas where wave power significantly modifies the delta, the sands of distributary mouth bars, and to a lesser degree, distributary channels and natural levees, are reworked into shore-parallel sand bodies known as barrier islands, or shore-attached beaches. These commonly form along coasts exposed to powerful waves and where water depth rapidly increases seaward. This allows waves to erode both the delta plain and delta front, and produces a delta with a smoother, more regular, convex shape. The Niger Delta located along the west coast of Africa is an example of a wave-dominated delta.

In locations where tidal rangethe difference between high and low tideis fairly high, strong tidal currents sweep across the delta front and up the channels of the delta plain. These reversing currents erode the delta and redistribute the deposits into large sand bodies oriented perpendicular to shore. This tends to make the coastline concave and, also, gives the delta a smoother, more regular shape. The Ganges-Brahmaputra Delta at the border between India and Bangladesh is an example of a tide-dominated delta.

Delta abandonment

As the river that formed a delta inevitably goes through changes upstream, a particular lobe may be abandoned. This usually occurs because a crevasse forms upstream by avulsion, and provides a more direct route or a steeper slope by which water can reach the sea or lake. As a result, the crevasse evolves into a new distributary channel and builds up a new delta lobe, a process called delta switching. The old distributary channel downstream is filled in by fine-grained sediment and, then, abandoned. Over the last 5, 000 years, the Mississippi River has abandoned at least six major lobes by avulsion.

An even larger-scale redirection of flow threatens to trigger abandonment of the entire Mississippi River delta. The Atchafalaya River, which follows an old course of the Mississippi, has a steeper slope than the modern Mississippi. At a point where the Atchafalaya River flows directly adjacent to the Mississippi, it is possible for the Atchafalaya to capture, or pirate, the flow of the Mississippi. This could permanently redirect the Mississippi away from New Orleans, Louisiana, and into Atchafalya Bay to the west. Since the 1950s, the U.S. Army Corps of Engineers has controlled the flow of the Mississippi in this area. In the 1980s, when the Mississippi River had several seasons of unusually high water levels, additional efforts were necessary to avert this disaster. What was called The Great Flood of 1993 once again threatened the surrounding areas of the Mississippi River, especially the area where it met with the Ohio River near Cairo, Illinois. No doubt, such flooding will threaten again in the future.

Delta destruction

Abandoned delta lobes experience rapid subsidence primarily due to sediment compaction. As water depth increases accordingly, enhanced wave and current attack contribute to rapid erosion of old delta depositsa process called delta retreatand the loss of vast tracts of ecologically important wetlands and barrier islands. Modern flood controls, such as channelization and levee construction, sharply reduce avulsion and delta switching. As a result, little or no new sediment is contributed to the delta plain outside of the large channels. Consequently, compaction, subsidence, and erosion continue unabated. This enhanced effect results in the loss of up to 15, 000 acres of wetlands per year in inactive areas of the Mississippi River delta plain. Global warming could accelerate this effect by triggering higher rates of global sea level rise and resulting in more rapid increases in water depth. Increased hurricane incidence in the 1990s, 2000s, and beyond also takes a toll. In August 2005, New Orleans experienced a gigantic natural disaster in the form of Hurricane Katrina, a category five hurricane, causing flooding in about 80% of the city. At the time of the hurricane, the U.S. Army Corps of Engineers said that the Mississippi River delta was receding faster than anyplace in the country, causing continuing problems for the residents of New Orleans.

Construction of dams upstream also impacts deltas. Dams not only trap water, they trap sediment as well. This sediment load would normally contribute to delta progradation, or at least help stabilize delta lobes. Construction of dams instead results in significant delta retreat. For instance, construction of Egypts Aswan High Dam on the Nile River in 1964 lead to rapid erosion of the delta plain with loss of both wetlands and agricultural lands.

Deltas and human activity

Deltas have been important centers of human activity throughout history, in part because of the fertility of the land and easy access to transportation. Many early human civilizations developed on deltas. For instance, the Nile River delta has hosted Egyptian cultures for over seven thousand years.

Deltas contain large expanses of wetlands where organic matter rapidly accumulates. Consequently,

KEY TERMS

Delta front The seaward, gently sloping part of a delta, which is below water level.

Delta plain The landward, nearly level part of a delta, some of which is below sea or lake level and some above.

Delta retreat Landward migration of a delta due to erosion of older delta deposits.

Distributary channel A large channel within a delta, which delivers water and sediment into an ocean or a lake.

Grain size The size of a sediment particle; for example, gravel (greater than 2mm), sand (2mm to 1/16 mm), silt (1/16 mm to 1/256 mm) and clay (less than 1/256 mm).

Sediment load The amount of sediment transported by wind, water, or ice.

Sedimentary environment An area on the Earths surface, such as a lake or stream, where large volumes of sediment accumulate.

Tidal range Vertical distance between high tide and low tide during a single tidal cycle.

delta muds are very rich in organic materials and make good hydrocarbon source rocks when buried to appropriate depths. Not surprisingly, deltaic deposits contain extensive supplies of coal, oil, and gas. Deltaic sand bodies are also excellent reservoir rocks for mobile hydrocarbons. This combination of factors makes deltas perhaps the most important hydrocarbon-bearing environment on Earth. Due to this economic bonanza, modern and ancient deltas have probably been more thoroughly studied than any other sedimentary environment.

Deltas are very low relief; most areas are rarely more than a few feet above sea level. Therefore, they contain freshwater, brackish, and saltwater basins with correspondingly diverse, complex ecologies. Minor changes in the elevation of the delta surface can flood areas with water of much higher or lower salinity, so delta ecology is easily impacted by human activities. As indicated above, humans have significantly altered deltas and will continue to do so in hopes of curbing flooding. As a result, accelerated delta retreat will continue, as well as wetlands destruction, unless humans develop new flood control technologies or new methods for wetlands protection.

Resources

BOOKS

Editors of Prentice Hall Science Explorer series. Earths Changing Surface. Needham, MA: Pearson Prentice Hall, 2005.

Giosan, Liviu, and Janok P. Bhattacharya, eds. River Deltas: Concepts, Models, and Examples. Tulsa, OK: Society for Sedimentary Geology, 2005.

Hsu, Kenneth Jinghwa. Physics of Sedimentology: Textbook and Reference. Berlin, Germany, and New York: Springer, 2004.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. Hoboken, NJ: John Wiley & Sons, 2004.

Thurman, Harold V., and Alan P. Trujillo. The Dynamic Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Pearson Prentice Hall, 2005.

Clay Harris

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Delta

Delta

A delta is a low-lying, almost flat landform , composed of sediments deposited where a river flows into a lake or an ocean . Deltas form when the volume of sediment deposited at a river mouth is greater than what waves, currents , and tides can erode. Deltas extend the coastline outward, forming new land along the shore. However, even as the delta is constructed, waves, currents, or tidal activity may redistribute sediment. Although they form in lakes, the largest deltas develop along seashores. Deltas are perhaps the most complex of all sedimentary environments. The term delta comes from the resemblance between the outline of some deltas and the fourth letter in the Greek alphabet—delta—which is shaped like a triangle.

Some areas of the delta are influenced more by river processes, while marine (or lake) activities control other parts. Deltas do not form if wave, current, or tide activity is too intense for sediment to accumulate. The degree of influence by river, wave, current and tide activity on delta form is often used to classify deltas. Among the many factors that determine the characteristics of a delta are the volume of river flow, sediment load and type, coastal topography and subsidence rate , amount and character of wave and current activity, tidal range, storm frequency and magnitude, water depth, sea level rise or fall, and climate.

Delta construction

Delta plain

As a river flows toward the sea or a lake, it occupies a single, large, relatively straight channel known as the main distributary channel. The main distributary may soon branch off, like the roots of a tree, into many separate smaller distributaries. The number of branches formed depends on many different factors such as river flow, sediment load, and shoreline slope. Large sand-filled distributary channels occupy the delta plain, the nearly level, landward part of the delta, which is partly subaerial (above lake or sea level).

The natural levees that flank large distributary channels are another element of the delta plain. Natural levees are mounds of sand and silt that form when flood waters flow over the banks of the distributary and deposit their sediment load immediately adjacent to the channel. Unlike natural levees on rivers , delta levees do not grow especially large. Therefore, they are easily broken through by flood waters—a process called avulsion. This forms small channels, or crevasses, that flow away from the distributaries, like the little rootlets from the larger roots of a tree. The fan-shaped deposits formed during breeching of the distributaries are called crevasse splays.

Between the distributary channels, a variety of shallow, quiet water environments form, including freshwater swamps and lakes, and saltwater marshes. It is in these wetland basins that large volumes of organic matter and fine-grained sediment accumulate.


Delta front

When sediment-laden river water flows into the standing water at the mouth of a distributary, the river water slows and deposits its load. This forms a sediment body called a distributary mouth bar, or bar finger sand—so named because the distributary channels look a bit like the fingers on your hand. Distributary mouth bars form on the delta front, the gently seaward-sloping, marine-dominated part of the delta that is all subaqueous, or below water level.

Subaqueous levees may extend out from the natural levees of the delta front onto the delta plain. These help confine the water flow seaward of the distributary mouth to a relatively narrow path, so that the delta continues growing outward at a rapid pace. The area of the delta front between the distributary mouth bars is called the interdistributary bay; the salinity here is brackish to marine.

Water velocity slows consistently outward from the distributary mouth; the river water consequently deposits finer and finer sediment as it flows seaward. Eventually, a point is reached where the average grain size decreases to clay-sized sediment with only minor silt. This is the prodelta area, where the bottom generally has a very low slope. On occasion, large blocks of sediment break free from the delta plain, slide down the steeper delta front, and become buried in prodelta muds.

Delta morphology

Vertical character

Ideally, if a delta is growing seaward, or prograding, as deltas typically do, a thick deposit with three stacked sediment sequences develops. The lower sequence, or bottomset beds, contain flat-lying silt and clay layers of the prodelta. The middle sequence, or foreset beds, contain seaward-inclined layers of sand and silt produced by distributary mouth bar sedimentation. The upper sequence, or topset beds, consists of flat-lying sand deposits formed in distributary channels and natural levees, interlayered, or interbedded, with fine-grained interdistributary bay deposits. A variety of factors, such as marine influence, changes in sediment supply or sea level, tend to complicate this picture; the actual sediment distribution in a deltaic sequence is typically very complex.


Surface character

The landward to seaward transect—from distributary channel sands to prodelta muds—outlined above is typical of deltas, like the Mississippi River delta, which experience minimal marine influence. These lobate, or "bird's foot" deltas, migrate farther and farther out into the ocean, as a result of multiple distributary channels and bar sands, each building seaward-extending lobes. On coastlines where waves or currents erode and redistribute much of the delta's sand, this lobate form becomes highly modified.

In areas where wave power significantly modifies the delta, the sands of distributary mouth bars, and to a lesser degree, distributary channels and natural levees, are reworked into shore-parallel sand bodies known as barrier islands , or shore-attached beaches. These commonly form along coasts exposed to powerful waves and where water depth rapidly increases seaward. This allows waves to erode both the delta plain and delta front, and produces a delta with a smoother, more regular, convex shape. The Niger Delta located along the west coast of Africa is an example of a wave-dominated delta.

In locations where tidal range—the difference between high and low tide—is fairly high, strong tidal currents sweep across the delta front and up the channels of the delta plain. These reversing currents erode the delta and redistribute the deposits into large sand bodies oriented perpendicular to shore. This tends to make the coastline concave and also gives the delta a smoother, more regular shape. The Ganges-Brahmaputra Delta at the border between India and Bangladesh is an example of a tide-dominated delta.


Delta abandonment

As the river that formed a delta inevitably goes through changes upstream, a particular lobe may be abandoned. This usually occurs because a crevasse forms upstream by avulsion, and provides a more direct route or a steeper slope by which water can reach the sea or lake. As a result, the crevasse evolves into a new distributary channel and builds up a new delta lobe, a process called delta switching. The old distributary channel downstream is filled in by fine-grained sediment and abandoned. Over the last 5,000 years, the Mississippi River has abandoned at least six major lobes by avulsion.

An even larger-scale redirection of flow threatens to trigger abandonment of the entire Mississippi River delta. The Atchafalaya River, which follows an old course of the Mississippi, has a steeper slope than the modern Mississippi. At a point where the Atchafalaya River flows directly adjacent to the Mississippi, it is possible for the Atchafalaya to capture, or pirate, the flow of the Mississippi. This could permanently redirect the Mississippi away from New Orleans, and into Atchafalya Bay to the west. Since the 1950s, the U.S. Army Corps of Engineers has controlled the flow of the Mississippi in this area. In the 1980s, when the Mississippi River had several seasons of unusually high water levels, additional efforts were necessary to avert this disaster, but it may threaten again in the future.

Delta destruction

Abandoned delta lobes experience rapid subsidence primarily due to sediment compaction. As water depth increases accordingly, enhanced wave and current attack contribute to rapid erosion of old delta deposits—a process called delta retreat—and the loss of vast tracts of ecologically important wetlands and barrier islands. Modern flood controls, such as channelization and levee construction, sharply reduce avulsion and delta switching. As a result, little or no new sediment is contributed to the delta plain outside of the large channels. Consequently, compaction, subsidence and erosion continue unabated. This enhanced effect results in the loss of up to 15,000 acres of wetlands per year in inactive areas of the Mississippi River delta plain. Global warming could accelerate this effect by triggering higher rates of global sea level rise and resulting in more rapid increases in water depth. Increased hurricane incidence in the 1990s and beyond also takes a toll.

Construction of dams upstream impacts deltas too. Dams not only trap water, they trap sediment as well. This sediment load would normally contribute to delta progradation, or at least help stabilize delta lobes. Construction of dams instead results in significant delta retreat. For example, construction of Egypt's Aswan High Dam on the Nile River in 1964 lead to rapid erosion of the delta plain with loss of both wetlands and agricultural lands.


Deltas and human activity

Deltas have been important centers of human activity throughout history, in part because of the fertility of the land and easy access to transportation. Many early human civilizations developed on deltas. For example, the Nile River delta has hosted Egyptian cultures for over seven thousand years.

Deltas contain large expanses of wetlands where organic matter rapidly accumulates. Consequently, delta muds are very rich organic in organic materials and make good hydrocarbon source rocks when buried to appropriate depths. Not surprisingly, deltaic deposits contain extensive supplies of coal , oil, and gas. Deltaic sand bodies are also excellent reservoir rocks for mobile hydrocarbons. This combination of factors makes deltas perhaps the most important hydrocarbon-bearing environment on Earth . Due to this economic bonanza, modern and ancient deltas have probably been more throughly studied than any other sedimentary environment .

Deltas are very low relief; most areas are rarely more than a few feet above sea level. Therefore, they contain freshwater, brackish, and saltwater basins with correspondingly diverse, complex ecologies. Minor changes in the elevation of the delta surface can flood areas with water of much higher or lower salinity, so delta ecology is easily impacted by human activities. As indicated above, humans have significantly altered deltas and will continue to do so in hopes of curbing flooding . As a result, we will continue to see accelerated delta retreat, and wetlands destruction, unless humans develop new flood control technologies or new methods for wetlands protection.


Resources

books

Leeder, Mike. Sedimentology and Sedimentary Basins: FromTurbulence to Tectonics. London: Blackwell Science. 1999.

Selby, M.J. Earth's Changing Surface. London: Oxford University Press.1985.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth:An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Thurman, Harold V., and Alan P. Trujillo. Essentials ofOceanography. 7th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.


Clay Harris

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Delta front

—The seaward, gently sloping part of a delta, which is below water level.

Delta plain

—The landward, nearly level part of a delta, some of which is below sea or lake level and some above.

Delta retreat

—Landward migration of a delta due to erosion of older delta deposits.

Distributary channel

—A large channel within a delta, which delivers water and sediment into an ocean or a lake.

Grain size

—The size of a sediment particle; for example, gravel (greater than 2mm), sand (2–1/16 mm), silt (1/16–1/256 mm) and clay (less than 1/256 mm).

Sediment load

—The amount of sediment transported by wind, water, or ice.

Sedimentary environment

—An area on the earth's surface, such as a lake or stream, where large volumes of sediment accumulate.

Tidal range

—Vertical distance between high tide and low tide during a single tidal cycle.

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"Delta." The Gale Encyclopedia of Science. . Encyclopedia.com. 12 Dec. 2018 <https://www.encyclopedia.com>.

"Delta." The Gale Encyclopedia of Science. . Encyclopedia.com. (December 12, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/delta-1

"Delta." The Gale Encyclopedia of Science. . Retrieved December 12, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/delta-1

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Notes:
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