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River delta

A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water.[1][2] This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the Greek letter Delta. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment.[3][4] The size, geometry, and location of the receiving basin also plays an important role in delta evolution.

The Ebro River delta at the Mediterranean Sea.
Sacramento–San Joaquin (California) Delta at flood stage, early March 2009.

River deltas are important in human civilization, as they are major agricultural production centers and population centers.[5] They can provide coastline defense and can impact drinking water supply.[6] They are also ecologically important, with different species' assemblages depending on their landscape position.

Etymology

A river delta is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta. The triangular shape of the Nile Delta was known to audiences of classical Athenian drama; the tragedy Prometheus Bound by Aeschylus refers to it as the "triangular Nilotic land", though not as a "delta".[7] Herodotus's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it is called by the Ionians", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle.[7] Despite making comparisons to other river-systems' deltas, Herodotus did not describe them as "deltas".[7] The Greek historian Polybius likened the land between the Rhône and Isère rivers to the Nile Delta, referring to both as islands, but did not apply the word delta.[7] According to the Roman geographer Strabo, the Cynic philosopher Onesicritus of Astypalaea, who accompanied Alexander the Great's conquests in India, reported that Patalene (the delta of the Indus River) was "a delta".[7] (Koinē Greek: καλεῖ δὲ τὴν νῆσον δέλτα, romanized: kalei de tēn nēson délta, lit.'he calls the island a delta').[7] The Roman author Arrian's Indica states that "the delta of the land of the Indians is made by the Indus river no less than is the case with that of Egypt".[7]

As a generic term for the landform at the mouth of river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of Edward Gibbon.[8]

Formation

 
A delta forms where a river meets a lake[9]

River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a reservoir. When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and is deposited as alluvium, which builds up to form the river delta.[10][11] Over time, this single channel builds a deltaic lobe (such as the bird's-foot of the Mississippi or Ural river deltas), pushing its mouth into the standing water. As the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope).

As the gradient of the river channel decreases, the amount of shear stress on the bed decreases, which results in the deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. This destabilizes the river channel. If the river breaches its natural levees (such as during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a steeper, more stable gradient.[12] Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up a mature delta with a distributary network.

Another way these distributary networks form is from the deposition of mouth bars (mid-channel sand and/or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth-bar, which splits the river into two distributary channels.[13][14] A good example of the result of this process is the Wax Lake Delta.

In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan. The more often the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position result in more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's-feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape.

Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the Mississippi, Nile, Amazon, Ganges, Indus, Yangtze, and Yellow River discharging along passive continental margins.[15] This phenomenon is due mainly to three factors: topography, basin area, and basin elevation.[15] Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate over time to form large river deltas. Topography along active margins tend to be steeper and less widespread, which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf.

There are many other lesser factors that could explain why the majority of river deltas form along passive margins rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, and volcanic activity resulting in delta formation to exist closer to the sediment source.[15][16] When sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion, delta lobe switching, and auto cyclicity.[16] Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.[15] However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches.[15]

Types

 
Delta lobe switching in the Mississippi Delta, 4600 yrs BP, 3500 yrs BP, 2800 yrs BP, 1000 yrs BP, 300 yrs BP, 500 yrs BP,× current

Deltas are typically classified according to the main control on deposition, which is a combination of river, wave, and tidal processes,[17][18] depending on the strength of each.[19] The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river.[20]

Fluvial-dominated deltas

Fluvial-dominated deltas are found in areas of low tidal range and low wave energy.[21] Where the river water is nearly equal in density to the basin water, the delta is characterized by homopycnal flow, in which the river water rapidly mixes with basin water and abruptly dumps most of its sediment load. Where the river water has higher density than basin water, typically from a heavy load of sediment, the delta is characterized by hyperpycnal flow in which the river water hugs the basin bottom as a density current that deposits its sediments as turbidites. When the river water is less dense than the basin water, as is typical of river deltas on an ocean coastline, the delta is characterized by hypopycnal flow in which the river water is slow to mix with the denser basin water and spreads out as a surface fan. This allows fine sediments to be carried a considerable distance before settling out of suspension. Beds in a hypocynal delta dip at a very shallow angle, around 1 degree.[21]

Fluvial-dominated deltas are further distinguished by the relative importance of the inertia of rapidly flowing water, the importance of turbulent bed friction beyond the river mouth, and buoyancy. Outflow dominated by inertia tend to form Gilbert type deltas. Outflow dominated by turbulent friction is prone to channel bifurcation, while buoyancy-dominated outflow produces long distributaries with narrow subaqueous natural levees and few channel bifurcations.[22]

The modern Mississippi River delta is a good example of a fluvial-dominated delta whose outflow is buoyancy-dominated. Channel abandonment has been frequent, with seven distinct channels active over the last 5000 years. Other fluvial-dominated deltas include the Mackenzie delta and the Alta delta.[13]

Gilbert deltas

A Gilbert delta (named after Grove Karl Gilbert) is a type of fluvial-dominated[23] delta formed from coarse sediments, as opposed to gently-sloping muddy deltas such as that of the Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta.[24][25] It is commonly a result of homopycnal flow.[21] Such deltas are characterized by a tripartite structure of topset, foreset, and bottomset beds. River water entering the lake rapidly deposits its coarser sediments on the submerged face of the delta, forming steeping dipping foreset beds. The finer sediments are deposited on the lake bottom beyond this steep slope as more gently dipping bottomset beds. Behind the delta front, braided channels deposit the gently dipping beds of the topset on the delta plain.[26][27]

While some authors describe both lacustrine and marine locations of Gilbert deltas,[24] others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with the lakewater faster (as opposed to the case of a river falling into the sea or a salt lake, where less dense fresh water brought by the river stays on top longer).[28] Gilbert himself first described this type of delta on Lake Bonneville in 1885.[28] Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and forming prominent peninsulas at Naramata, Summerland, and Peachland.

Wave-dominated deltas

In wave dominated deltas, wave-driven sediment transport controls the shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coast line.[17] The relationship between waves and river deltas is quite variable and largely influenced by the deepwater wave regimes of the receiving basin. With a high wave energy near shore and a steeper slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from the river delta, causing the delta to retreat.[6] For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels.[29][30]

 
The Ganges Delta in India and Bangladesh is the largest delta in the world, and one of the most fertile regions in the world.

Tide-dominated deltas

Erosion is also an important control in tide-dominated deltas, such as the Ganges Delta, which may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure.[31] Tidal deltas behave differently from river-dominated and wave-dominated deltas, which tend to have a few main distributaries. Once a wave-dominated or river-dominated distributary silts up, it is abandoned, and a new channel forms elsewhere. In a tidal delta, new distributaries are formed during times when there is a lot of water around – such as floods or storm surges. These distributaries slowly silt up at a more or less constant rate until they fizzle out.[31]

Tidal freshwater deltas

A tidal freshwater delta[32] is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".[33] Drowned coastal river valleys that were inundated by rising sea levels during the late Pleistocene and subsequent Holocene tend to have dendritic estuaries with many feeder tributaries. Each tributary mimics this salinity gradient from their brackish junction with the mainstem estuary up to the fresh stream feeding the head of tidal propagation. As a result, the tributaries are considered to be "subestuaries". The origin and evolution of a tidal freshwater delta involves processes that are typical of all deltas[4] as well as processes that are unique to the tidal freshwater setting.[34][35] The combination of processes that create a tidal freshwater delta result in a distinct morphology and unique environmental characteristics. Many tidal freshwater deltas that exist today are directly caused by the onset of or changes in historical land use, especially deforestation, intensive agriculture, and urbanization.[36] These ideas are well illustrated by the many tidal freshwater deltas prograding into Chesapeake Bay along the east coastline of the United States. Research has demonstrated that the accumulating sediments in this estuary derive from post-European settlement deforestation, agriculture, and urban development.[37][38][39]

Estuaries

Other rivers, particularly those on coasts with significant tidal range, do not form a delta but enter into the sea in the form of an estuary. Notable examples include the Gulf of Saint Lawrence and the Tagus estuary.

Inland deltas

 
Okavango Delta

In rare cases the river delta is located inside a large valley and is called an inverted river delta. Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an inland delta, and often occurs on former lake beds. The term was first coined by Alexander von Humboldt for the middle reaches of the Orinoco River, which he visited in 1800.[40] Other prominent examples include the Inner Niger Delta,[41] Peace–Athabasca Delta,[42] the Sacramento–San Joaquin River Delta,[43] and the Sistan delta of Iran.[44] The Danube has one in the valley on the Slovak-Hungarian border between Bratislava and Iža.[45]

In some cases, a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example.[46] See endorrheic basin.

Mega deltas

The generic term mega delta can be used to describe very large Asian river deltas, such as the Yangtze, Pearl, Red, Mekong, Irrawaddy, Ganges-Brahmaputra, and Indus.[47][48]

Sedimentary structure

 
Delta on Kachemak Bay at low tide

The formation of a delta is complicated, multiple, and cross-cutting over time, but in a simple delta three main types of bedding may be distinguished: the bottomset beds, foreset/frontset beds, and topset beds. This three part structure may be seen in small scale by crossbedding.[24][49]

  • The bottomset beds are created from the lightest suspended particles that settle farthest away from the active delta front, as the river flow diminishes into the standing body of water and loses energy. This suspended load is deposited by sediment gravity flow, creating a turbidite. These beds are laid down in horizontal layers and consist of the finest grain sizes.
  • The foreset beds in turn are deposited in inclined layers over the bottomset beds as the active lobe advances. Foreset beds form the greater part of the bulk of a delta, (and also occur on the lee side of sand dunes).[50] The sediment particles within foreset beds consist of larger and more variable sizes, and constitute the bed load that the river moves downstream by rolling and bouncing along the channel bottom. When the bed load reaches the edge of the delta front, it rolls over the edge, and is deposited in steeply dipping layers over the top of the existing bottomset beds. Under water, the slope of the outermost edge of the delta is created at the angle of repose of these sediments. As the foresets accumulate and advance, subaqueous landslides occur and readjust overall slope stability. The foreset slope, thus created and maintained, extends the delta lobe outward. In cross section, foresets typically lie in angled, parallel bands, and indicate stages and seasonal variations during the creation of the delta.
  • The topset beds of an advancing delta are deposited in turn over the previously laid foresets, truncating or covering them. Topsets are nearly horizontal layers of smaller-sized sediment deposited on the top of the delta and form an extension of the landward alluvial plain.[50] As the river channels meander laterally across the top of the delta, the river is lengthened and its gradient is reduced, causing the suspended load to settle out in nearly horizontal beds over the delta's top. Topset beds are subdivided into two regions: the upper delta plain and the lower delta plain. The upper delta plain is unaffected by the tide, while the boundary with the lower delta plain is defined by the upper limit of tidal influence.[51]

Existential threats to deltas

Human activities in both deltas and the river basins upstream of deltas can radically alter delta environments.[52] Upstream land use change such as anti-erosion agricultural practices and hydrological engineering such as dam construction in the basins feeding deltas have reduced river sediment delivery to many deltas in recent decades.[53] This change means that there is less sediment available to maintain delta landforms, and compensate for erosion and sea level rise, causing some deltas to start losing land.[53] Declines in river sediment delivery are projected to continue in the coming decades.[54]

The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes.[55] People living on deltas often construct flood defences which prevent sedimentation from floods on deltas, and therefore means that sediment deposition can't compensate for subsidence and erosion. In addition to interference with delta aggradation, pumping of groundwater,[56] oil, and gas,[57] and constructing infrastructure all accelerate subsidence, increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining,[58] and cause saltwater intrusion.[59] There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through sedimentation enhancing strategies.

While nearly all deltas have been impacted to some degree by humans, the Nile Delta and Colorado River Delta are some of the most extreme examples of the devastation caused to deltas by damming and diversion of water.[60][61]

Historical data documents show that during the Roman Empire and Little Ice Age (times where there was considerable anthropogenic pressure), there was significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat.[62]

Deltas in the economy

Ancient deltas are a benefit to the economy due to their well sorted sand and gravel. Sand and gravel is often quarried from these old deltas and used in concrete for highways, buildings, sidewalks, and even landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone.[63] Not all sand and gravel quarries are former deltas, but for ones that are, much of the sorting is already done by the power of water.

Urban areas and human habitation tends to locate in lowlands near water access for transportation and sanitation.[64] This makes deltas a common location for civilizations to flourish due to access to flat land for farming, freshwater for sanitation and irrigation, and sea access for trade. Deltas often host extensive industrial and commercial activities as well as agricultural land which are often in conflict. Some of the world's largest regional economies are located on deltas such as the Pearl River Delta, Yangtze River Delta, European Low Countries and the Greater Tokyo Area.

Examples

The Ganges–Brahmaputra Delta, which spans most of Bangladesh and West Bengal, India empties into the Bay of Bengal, is the world's largest delta.[65]

The Selenga River delta in the Russian republic of Buryatia is the largest delta emptying into a body of fresh water, in its case Lake Baikal.

Deltas on Mars

Researchers have found a number of examples of deltas that formed in Martian lakes. Finding deltas is a major sign that Mars once had large amounts of water. Deltas have been found over a wide geographical range. Below are pictures of a few.[66]

See also

  • Alluvial fan – Fan-shaped deposit of sediment
  • Avulsion (river) – Rapid abandonment of a river channel and formation of a new channel
  • Estuary – Partially enclosed coastal body of brackish water
  • Levee – Ridge or wall to hold back water
  • Nile Delta – Delta produced by the Nile River at its mouth in the Mediterranean Sea
  • Regressive delta

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Bibliography

  • Renaud, F. and C. Kuenzer 2012: The Mekong Delta System – Interdisciplinary Analyses of a River Delta, Springer, ISBN 978-94-007-3961-1, doi:10.1007/978-94-007-3962-8, pp. 7–48
  • KUENZER C. and RENAUD, F. 2012: Climate Change and Environmental Change in River Deltas Globally. In (eds.): Renaud, F. and C. Kuenzer 2012: The Mekong Delta System – Interdisciplinary Analyses of a River Delta, Springer, ISBN 978-94-007-3961-1, doi:10.1007/978-94-007-3962-8, pp. 7–48
  • Ottinger, M.; Kuenzer, C.; LIU; Wang, S.; Dech, S. (2013). "Monitoring Land Cover Dynamics in the Yellow River Delta from 1995 to 2010 based on Landsat 5 TM". Applied Geography. 44: 53–68. doi:10.1016/j.apgeog.2013.07.003.

External links

  • Louisiana State University Geology – World Deltas
  • http://www.wisdom.eoc.dlr.de WISDOM Water related Information System for the Sustainable Development of the Mekong Delta
  • Wave-dominated river deltas on coastalwiki.org – A coastalwiki.org page on wave-dominated river deltas

river, delta, river, delta, landform, shaped, like, triangle, created, deposition, sediment, that, carried, river, enters, slower, moving, stagnant, water, this, occurs, where, river, enters, ocean, estuary, lake, reservoir, more, rarely, another, river, that,. A river delta is a landform shaped like a triangle created by deposition of sediment that is carried by a river and enters slower moving or stagnant water 1 2 This occurs where a river enters an ocean sea estuary lake reservoir or more rarely another river that cannot carry away the supplied sediment It is so named because its triangle shape resembles the Greek letter Delta The size and shape of a delta is controlled by the balance between watershed processes that supply sediment and receiving basin processes that redistribute sequester and export that sediment 3 4 The size geometry and location of the receiving basin also plays an important role in delta evolution The Ebro River delta at the Mediterranean Sea Sacramento San Joaquin California Delta at flood stage early March 2009 River deltas are important in human civilization as they are major agricultural production centers and population centers 5 They can provide coastline defense and can impact drinking water supply 6 They are also ecologically important with different species assemblages depending on their landscape position Contents 1 Etymology 2 Formation 3 Types 3 1 Fluvial dominated deltas 3 1 1 Gilbert deltas 3 2 Wave dominated deltas 3 3 Tide dominated deltas 3 4 Tidal freshwater deltas 3 5 Estuaries 3 6 Inland deltas 3 7 Mega deltas 4 Sedimentary structure 5 Existential threats to deltas 6 Deltas in the economy 7 Examples 8 Deltas on Mars 9 See also 10 References 11 Bibliography 12 External linksEtymology EditA river delta is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta The triangular shape of the Nile Delta was known to audiences of classical Athenian drama the tragedy Prometheus Bound by Aeschylus refers to it as the triangular Nilotic land though not as a delta 7 Herodotus s description of Egypt in his Histories mentions the Delta fourteen times as the Delta as it is called by the Ionians including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle 7 Despite making comparisons to other river systems deltas Herodotus did not describe them as deltas 7 The Greek historian Polybius likened the land between the Rhone and Isere rivers to the Nile Delta referring to both as islands but did not apply the word delta 7 According to the Roman geographer Strabo the Cynic philosopher Onesicritus of Astypalaea who accompanied Alexander the Great s conquests in India reported that Patalene the delta of the Indus River was a delta 7 Koine Greek kaleῖ dὲ tὴn nῆson delta romanized kalei de ten neson delta lit he calls the island a delta 7 The Roman author Arrian s Indica states that the delta of the land of the Indians is made by the Indus river no less than is the case with that of Egypt 7 As a generic term for the landform at the mouth of river the word delta is first attested in the English speaking world in the late 18th century in the work of Edward Gibbon 8 Formation Edit A delta forms where a river meets a lake 9 River deltas form when a river carrying sediment reaches a body of water such as a lake ocean or a reservoir When the flow enters the standing water it is no longer confined to its channel and expands in width This flow expansion results in a decrease in the flow velocity which diminishes the ability of the flow to transport sediment As a result sediment drops out of the flow and is deposited as alluvium which builds up to form the river delta 10 11 Over time this single channel builds a deltaic lobe such as the bird s foot of the Mississippi or Ural river deltas pushing its mouth into the standing water As the deltaic lobe advances the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation see slope As the gradient of the river channel decreases the amount of shear stress on the bed decreases which results in the deposition of sediment within the channel and a rise in the channel bed relative to the floodplain This destabilizes the river channel If the river breaches its natural levees such as during a flood it spills out into a new course with a shorter route to the ocean thereby obtaining a steeper more stable gradient 12 Typically when the river switches channels in this manner some of its flow remains in the abandoned channel Repeated channel switching events build up a mature delta with a distributary network Another way these distributary networks form is from the deposition of mouth bars mid channel sand and or gravel bars at the mouth of a river When this mid channel bar is deposited at the mouth of a river the flow is routed around it This results in additional deposition on the upstream end of the mouth bar which splits the river into two distributary channels 13 14 A good example of the result of this process is the Wax Lake Delta In both of these cases depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition This results in the smoothing of the planform or map view shape of the delta as the channels move across its surface and deposit sediment Because the sediment is laid down in this fashion the shape of these deltas approximates a fan The more often the flow changes course the shape develops as closer to an ideal fan because more rapid changes in channel position result in more uniform deposition of sediment on the delta front The Mississippi and Ural River deltas with their bird s feet are examples of rivers that do not avulse often enough to form a symmetrical fan shape Alluvial fan deltas as seen by their name avulse frequently and more closely approximate an ideal fan shape Most large river deltas discharge to intra cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the Mississippi Nile Amazon Ganges Indus Yangtze and Yellow River discharging along passive continental margins 15 This phenomenon is due mainly to three factors topography basin area and basin elevation 15 Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate over time to form large river deltas Topography along active margins tend to be steeper and less widespread which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf There are many other lesser factors that could explain why the majority of river deltas form along passive margins rather than active margins Along active margins orogenic sequences cause tectonic activity to form over steepened slopes brecciated rocks and volcanic activity resulting in delta formation to exist closer to the sediment source 15 16 When sediment does not travel far from the source sediments that build up are coarser grained and more loosely consolidated therefore making delta formation more difficult Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion delta lobe switching and auto cyclicity 16 Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment 15 However the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches 15 Types Edit Delta lobe switching in the Mississippi Delta 4600 yrs BP 3500 yrs BP 2800 yrs BP 1000 yrs BP 300 yrs BP 500 yrs BP current Deltas are typically classified according to the main control on deposition which is a combination of river wave and tidal processes 17 18 depending on the strength of each 19 The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river 20 Fluvial dominated deltas Edit Fluvial dominated deltas are found in areas of low tidal range and low wave energy 21 Where the river water is nearly equal in density to the basin water the delta is characterized by homopycnal flow in which the river water rapidly mixes with basin water and abruptly dumps most of its sediment load Where the river water has higher density than basin water typically from a heavy load of sediment the delta is characterized by hyperpycnal flow in which the river water hugs the basin bottom as a density current that deposits its sediments as turbidites When the river water is less dense than the basin water as is typical of river deltas on an ocean coastline the delta is characterized by hypopycnal flow in which the river water is slow to mix with the denser basin water and spreads out as a surface fan This allows fine sediments to be carried a considerable distance before settling out of suspension Beds in a hypocynal delta dip at a very shallow angle around 1 degree 21 Fluvial dominated deltas are further distinguished by the relative importance of the inertia of rapidly flowing water the importance of turbulent bed friction beyond the river mouth and buoyancy Outflow dominated by inertia tend to form Gilbert type deltas Outflow dominated by turbulent friction is prone to channel bifurcation while buoyancy dominated outflow produces long distributaries with narrow subaqueous natural levees and few channel bifurcations 22 The modern Mississippi River delta is a good example of a fluvial dominated delta whose outflow is buoyancy dominated Channel abandonment has been frequent with seven distinct channels active over the last 5000 years Other fluvial dominated deltas include the Mackenzie delta and the Alta delta 13 Gilbert deltas Edit A Gilbert delta named after Grove Karl Gilbert is a type of fluvial dominated 23 delta formed from coarse sediments as opposed to gently sloping muddy deltas such as that of the Mississippi For example a mountain river depositing sediment into a freshwater lake would form this kind of delta 24 25 It is commonly a result of homopycnal flow 21 Such deltas are characterized by a tripartite structure of topset foreset and bottomset beds River water entering the lake rapidly deposits its coarser sediments on the submerged face of the delta forming steeping dipping foreset beds The finer sediments are deposited on the lake bottom beyond this steep slope as more gently dipping bottomset beds Behind the delta front braided channels deposit the gently dipping beds of the topset on the delta plain 26 27 While some authors describe both lacustrine and marine locations of Gilbert deltas 24 others note that their formation is more characteristic of the freshwater lakes where it is easier for the river water to mix with the lakewater faster as opposed to the case of a river falling into the sea or a salt lake where less dense fresh water brought by the river stays on top longer 28 Gilbert himself first described this type of delta on Lake Bonneville in 1885 28 Elsewhere similar structures occur for example at the mouths of several creeks that flow into Okanagan Lake in British Columbia and forming prominent peninsulas at Naramata Summerland and Peachland Wave dominated deltas Edit In wave dominated deltas wave driven sediment transport controls the shape of the delta and much of the sediment emanating from the river mouth is deflected along the coast line 17 The relationship between waves and river deltas is quite variable and largely influenced by the deepwater wave regimes of the receiving basin With a high wave energy near shore and a steeper slope offshore waves will make river deltas smoother Waves can also be responsible for carrying sediments away from the river delta causing the delta to retreat 6 For deltas that form further upriver in an estuary there are complex yet quantifiable linkages between winds tides river discharge and delta water levels 29 30 The Ganges Delta in India and Bangladesh is the largest delta in the world and one of the most fertile regions in the world Tide dominated deltas Edit Erosion is also an important control in tide dominated deltas such as the Ganges Delta which may be mainly submarine with prominent sandbars and ridges This tends to produce a dendritic structure 31 Tidal deltas behave differently from river dominated and wave dominated deltas which tend to have a few main distributaries Once a wave dominated or river dominated distributary silts up it is abandoned and a new channel forms elsewhere In a tidal delta new distributaries are formed during times when there is a lot of water around such as floods or storm surges These distributaries slowly silt up at a more or less constant rate until they fizzle out 31 Tidal freshwater deltas Edit A tidal freshwater delta 32 is a sedimentary deposit formed at the boundary between an upland stream and an estuary in the region known as the subestuary 33 Drowned coastal river valleys that were inundated by rising sea levels during the late Pleistocene and subsequent Holocene tend to have dendritic estuaries with many feeder tributaries Each tributary mimics this salinity gradient from their brackish junction with the mainstem estuary up to the fresh stream feeding the head of tidal propagation As a result the tributaries are considered to be subestuaries The origin and evolution of a tidal freshwater delta involves processes that are typical of all deltas 4 as well as processes that are unique to the tidal freshwater setting 34 35 The combination of processes that create a tidal freshwater delta result in a distinct morphology and unique environmental characteristics Many tidal freshwater deltas that exist today are directly caused by the onset of or changes in historical land use especially deforestation intensive agriculture and urbanization 36 These ideas are well illustrated by the many tidal freshwater deltas prograding into Chesapeake Bay along the east coastline of the United States Research has demonstrated that the accumulating sediments in this estuary derive from post European settlement deforestation agriculture and urban development 37 38 39 Estuaries Edit Other rivers particularly those on coasts with significant tidal range do not form a delta but enter into the sea in the form of an estuary Notable examples include the Gulf of Saint Lawrence and the Tagus estuary Inland deltas Edit Okavango Delta In rare cases the river delta is located inside a large valley and is called an inverted river delta Sometimes a river divides into multiple branches in an inland area only to rejoin and continue to the sea Such an area is called an inland delta and often occurs on former lake beds The term was first coined by Alexander von Humboldt for the middle reaches of the Orinoco River which he visited in 1800 40 Other prominent examples include the Inner Niger Delta 41 Peace Athabasca Delta 42 the Sacramento San Joaquin River Delta 43 and the Sistan delta of Iran 44 The Danube has one in the valley on the Slovak Hungarian border between Bratislava and Iza 45 In some cases a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert The Okavango Delta in Botswana is one example 46 See endorrheic basin Mega deltas Edit The generic term mega delta can be used to describe very large Asian river deltas such as the Yangtze Pearl Red Mekong Irrawaddy Ganges Brahmaputra and Indus 47 48 Sedimentary structure Edit Delta on Kachemak Bay at low tide The formation of a delta is complicated multiple and cross cutting over time but in a simple delta three main types of bedding may be distinguished the bottomset beds foreset frontset beds and topset beds This three part structure may be seen in small scale by crossbedding 24 49 The bottomset beds are created from the lightest suspended particles that settle farthest away from the active delta front as the river flow diminishes into the standing body of water and loses energy This suspended load is deposited by sediment gravity flow creating a turbidite These beds are laid down in horizontal layers and consist of the finest grain sizes The foreset beds in turn are deposited in inclined layers over the bottomset beds as the active lobe advances Foreset beds form the greater part of the bulk of a delta and also occur on the lee side of sand dunes 50 The sediment particles within foreset beds consist of larger and more variable sizes and constitute the bed load that the river moves downstream by rolling and bouncing along the channel bottom When the bed load reaches the edge of the delta front it rolls over the edge and is deposited in steeply dipping layers over the top of the existing bottomset beds Under water the slope of the outermost edge of the delta is created at the angle of repose of these sediments As the foresets accumulate and advance subaqueous landslides occur and readjust overall slope stability The foreset slope thus created and maintained extends the delta lobe outward In cross section foresets typically lie in angled parallel bands and indicate stages and seasonal variations during the creation of the delta The topset beds of an advancing delta are deposited in turn over the previously laid foresets truncating or covering them Topsets are nearly horizontal layers of smaller sized sediment deposited on the top of the delta and form an extension of the landward alluvial plain 50 As the river channels meander laterally across the top of the delta the river is lengthened and its gradient is reduced causing the suspended load to settle out in nearly horizontal beds over the delta s top Topset beds are subdivided into two regions the upper delta plain and the lower delta plain The upper delta plain is unaffected by the tide while the boundary with the lower delta plain is defined by the upper limit of tidal influence 51 Existential threats to deltas EditHuman activities in both deltas and the river basins upstream of deltas can radically alter delta environments 52 Upstream land use change such as anti erosion agricultural practices and hydrological engineering such as dam construction in the basins feeding deltas have reduced river sediment delivery to many deltas in recent decades 53 This change means that there is less sediment available to maintain delta landforms and compensate for erosion and sea level rise causing some deltas to start losing land 53 Declines in river sediment delivery are projected to continue in the coming decades 54 The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes 55 People living on deltas often construct flood defences which prevent sedimentation from floods on deltas and therefore means that sediment deposition can t compensate for subsidence and erosion In addition to interference with delta aggradation pumping of groundwater 56 oil and gas 57 and constructing infrastructure all accelerate subsidence increasing relative sea level rise Anthropogenic activities can also destabilise river channels through sand mining 58 and cause saltwater intrusion 59 There are small scale efforts to correct these issues improve delta environments and increase environmental sustainability through sedimentation enhancing strategies While nearly all deltas have been impacted to some degree by humans the Nile Delta and Colorado River Delta are some of the most extreme examples of the devastation caused to deltas by damming and diversion of water 60 61 Historical data documents show that during the Roman Empire and Little Ice Age times where there was considerable anthropogenic pressure there was significant sediment accumulation in deltas The industrial revolution has only amplified the impact of humans on delta growth and retreat 62 Deltas in the economy EditAncient deltas are a benefit to the economy due to their well sorted sand and gravel Sand and gravel is often quarried from these old deltas and used in concrete for highways buildings sidewalks and even landscaping More than 1 billion tons of sand and gravel are produced in the United States alone 63 Not all sand and gravel quarries are former deltas but for ones that are much of the sorting is already done by the power of water Urban areas and human habitation tends to locate in lowlands near water access for transportation and sanitation 64 This makes deltas a common location for civilizations to flourish due to access to flat land for farming freshwater for sanitation and irrigation and sea access for trade Deltas often host extensive industrial and commercial activities as well as agricultural land which are often in conflict Some of the world s largest regional economies are located on deltas such as the Pearl River Delta Yangtze River Delta European Low Countries and the Greater Tokyo Area Examples EditSee also Category River deltas The Ganges Brahmaputra Delta which spans most of Bangladesh and West Bengal India empties into the Bay of Bengal is the world s largest delta 65 The Selenga River delta in the Russian republic of Buryatia is the largest delta emptying into a body of fresh water in its case Lake Baikal Deltas on Mars EditResearchers have found a number of examples of deltas that formed in Martian lakes Finding deltas is a major sign that Mars once had large amounts of water Deltas have been found over a wide geographical range Below are pictures of a few 66 Delta in Ismenius Lacus quadrangle as seen by THEMIS Delta in Lunae Palus quadrangle as seen by THEMIS Delta in Margaritifer Sinus quadrangle as seen by THEMIS Probable delta in Eberswalde crater as seen by Mars Global Surveyor Image in Margaritifer Sinus quadrangle See also Edit Wetlands portalAlluvial fan Fan shaped deposit of sediment Avulsion river Rapid abandonment of a river channel and formation of a new channel Estuary Partially enclosed coastal body of brackish water Levee Ridge or wall to hold back water Nile Delta Delta produced by the Nile River at its mouth in the Mediterranean Sea Regressive deltaReferences Edit Miall A D 1979 Deltas in R G Walker ed Facies Models Geological Association of Canada Hamilton Ontario Elliot T 1986 Deltas in H G Reading ed Sedimentary environments and facies Backwell Scientific Publications Oxford Blum M D Tornqvist T E 2000 Fluvial responses to climate and sea level change a review and look forward Sedimentology 47 2 48 doi 10 1046 j 1365 3091 2000 00008 x S2CID 140714394 a b Pasternack Gregory B Brush Grace S Hilgartner William B 2001 04 01 Impact of historic land use change on sediment delivery to a Chesapeake Bay subestuarine delta Earth Surface Processes and Landforms 26 4 409 427 Bibcode 2001ESPL 26 409P doi 10 1002 esp 189 ISSN 1096 9837 S2CID 129080402 Schneider Pia Asch Folkard 2020 Rice production and food security in Asian Mega deltas A review on characteristics vulnerabilities and agricultural adaptation options to cope with climate change Journal of Agronomy and Crop Science 206 4 491 503 doi 10 1111 jac 12415 ISSN 1439 037X a b Anthony Edward J 2015 03 01 Wave influence in the construction shaping and destruction of river deltas A review Marine Geology 361 53 78 Bibcode 2015MGeol 361 53A doi 10 1016 j margeo 2014 12 004 a b c d e f g Celoria Francis 1966 Delta as a geographical concept in Greek literature Isis 57 3 385 388 doi 10 1086 350146 JSTOR 228368 S2CID 143811840 Word Stories Unexpected Relatives for Xmas Druide January 2020 Archived from the original on 2020 10 22 Retrieved 2020 12 21 How a Delta Forms Where River Meets Lake Jet Propulsion Laboratory 2014 08 12 Retrieved 2017 12 12 Dr Gregory B Pasternack Watershed Hydrology Geomorphology and Ecohydraulics TFD Modeling pasternack ucdavis edu Retrieved 2017 06 12 Boggs Sam 2006 Principles of sedimentology and stratigraphy 4th ed Upper Saddle River N J Pearson Prentice Hall pp 289 306 ISBN 0131547283 Slingerland R and N D Smith 1998 Necessary conditions for a meandering river avulsion Geology Boulder 26 435 438 a b Boggs 2006 p 295 Leeder M R 2011 Sedimentology and sedimentary basins from turbulence to tectonics 2nd ed Chichester West Sussex UK Wiley Blackwell p 388 ISBN 9781405177832 a b c d e Milliman J D Syvitski J P M 1992 Geomorphic Tectonic Control of Sediment Discharge to the Ocean The Importance of Small Mountainous Rivers The Journal of Geology 100 5 525 544 Bibcode 1992JG 100 525M doi 10 1086 629606 JSTOR 30068527 S2CID 22727856 a b Goodbred S L Kuehl S A 2000 The significance of large sediment supply active tectonism and eustasy on margin sequence development Late Quaternary stratigraphy and evolution of the Ganges Brahmaputra delta Sedimentary Geology 133 3 4 227 248 Bibcode 2000SedG 133 227G doi 10 1016 S0037 0738 00 00041 5 a b Galloway W E 1975 Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems in Brousard M L ed Deltas Models for Exploration Houston Geological Society Houston Texas pp 87 98 Nienhuis J H Ashton A D Edmonds D A Hoitink A J F Kettner A J 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Piet Nguyen Trung Nam Ahmed Kantoush Sameh Van Binh Doan Duc Dung Do Tran Quang Tho van der Vegt Maarten 2019 12 10 Tidal amplification and salt intrusion in the Mekong Delta driven by anthropogenic sediment starvation Scientific Reports 9 1 18746 Bibcode 2019NatSR 918746E doi 10 1038 s41598 019 55018 9 ISSN 2045 2322 PMC 6904557 PMID 31822705 Ali Elham M El Magd Islam A 2016 03 01 Impact of human interventions and coastal processes along the Nile Delta coast Egypt during the past twenty five years The Egyptian Journal of Aquatic Research 42 1 1 10 doi 10 1016 j ejar 2016 01 002 ISSN 1687 4285 Witze Alexandra 2014 03 20 Water returns to arid Colorado River delta Nature News 507 7492 286 287 Bibcode 2014Natur 507 286W doi 10 1038 507286a PMID 24646976 Maselli Vittorio Trincardi Fabio 2013 05 31 Man made deltas Scientific Reports 3 1926 Bibcode 2013NatSR 3E1926M doi 10 1038 srep01926 ISSN 2045 2322 PMC 3668317 PMID 23722597 Mineral Photos Sand and Gravel Mineral Information Institute 2011 Archived from the original on 2011 10 06 Retrieved 2011 11 02 Management About the Author Stefan Trickster This City Knows 2017 05 22 Why are cities located where they are This City Knows Retrieved 2020 01 05 a href Template Cite web html title Template Cite web cite web a first has generic name help https www geol lsu edu WDD PUBLICATIONS HCBK04 DeltaData amp Images pdf bare URL PDF Irwin III R et al 2005 An intense terminal epoch of widespread fluvial activity on early Mars 2 Increased runoff and paleolake development Journal of Geophysical Research 10 E12S15Bibliography EditRenaud F and C Kuenzer 2012 The Mekong Delta System Interdisciplinary Analyses of a River Delta Springer ISBN 978 94 007 3961 1 doi 10 1007 978 94 007 3962 8 pp 7 48 KUENZER C and RENAUD F 2012 Climate Change and Environmental Change in River Deltas Globally In eds Renaud F and C Kuenzer 2012 The Mekong Delta System Interdisciplinary Analyses of a River Delta Springer ISBN 978 94 007 3961 1 doi 10 1007 978 94 007 3962 8 pp 7 48 Ottinger M Kuenzer C LIU Wang S Dech S 2013 Monitoring Land Cover Dynamics in the Yellow River Delta from 1995 to 2010 based on Landsat 5 TM Applied Geography 44 53 68 doi 10 1016 j apgeog 2013 07 003 External links Edit The Wikibook Historical Geology has a page on the topic of Deltas Wikimedia Commons has media related to River deltas Louisiana State University Geology World Deltas http www wisdom eoc dlr de WISDOM Water related Information System for the Sustainable Development of the Mekong Delta Wave dominated river deltas on coastalwiki org A coastalwiki org page on wave dominated river deltas Retrieved from https en wikipedia org w index php title River delta amp oldid 1138746911, wikipedia, wiki, book, books, library,

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