A river delta is a
landform shaped like a triangle, created by the
deposition of
sediment that is carried by a
river and enters slower-moving or stagnant water.[1][2] This occurs at a
river mouth, when it 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 uppercase Greek letter
delta, Δ. The size and shape of a delta are 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.
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. On
geologic timescales, they are also important
carbon sinks.[7]
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 Atheniandrama; the tragedy Prometheus Bound by
Aeschylus refers to it as the "triangular Nilotic land", though not as a "delta".[8]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.[8] Despite making comparisons to other river-systems' deltas, Herodotus did not describe them as "deltas".[8] 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.[8] According to the Greek 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" (
Koinē Greek: καλεῖ δὲ τὴν νῆσον δέλτα, romanized: kalei de tēn nēson délta,
lit. 'he calls the island a delta').[8] 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".[8]
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.[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.[11][12] 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.[13] 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.[14][15] 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.[16] This phenomenon is due mainly to three factors:
topography, basin area, and basin elevation.[16] 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.[16][17] 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.[17] Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.[16] However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches.[16]
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,[18][19] depending on the strength of each.[20] 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.[21]
Fluvial-dominated deltas
Fluvial-dominated deltas are found in areas of low tidal range and low wave energy.[22] 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.[22]
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.[23]
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.[14]
Gilbert deltas
A Gilbert delta (named after
Grove Karl Gilbert) is a type of fluvial-dominated[24] 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.[25][26] It is commonly a result of homopycnal flow.[22] 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.[27][28]
While some authors describe both lacustrine and marine locations of Gilbert deltas,[25] 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).[29] Gilbert himself first described this type of delta on
Lake Bonneville in 1885.[29] 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.[18] 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.[30][31]
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.[32] 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.[32]
Tidal freshwater deltas
A tidal freshwater delta[33] is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".[34] 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.[35][36] 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.[37] 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.[38][39][40]
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.
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.[47] See
endorheic basin.
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.[25][50]
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).[51] 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.[51] 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.[52]
Existential threats to deltas
Human activities in both deltas and the
river basins upstream of deltas can radically alter delta environments.[53] 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.[54] 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.[54] Declines in river sediment delivery are projected to continue in the coming decades.[55]
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.[61][62]
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.[63]
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.[64] 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.[65] 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.
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.[67]
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