River delta

Source: Wikipedia, the free encyclopedia.
Lena river delta
Satellite image of the Amazon Delta captured by NASA in 2005.
NASA satellite photograph of the Nile Delta (shown in false color)

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

geologic timescales, they are also important carbon sinks.[7]

Etymology

A river delta is so named because the shape of the

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]

Formation

A delta forms where a river meets a lake.[10]

River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a

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
).

Sacramento–San Joaquin (California) Delta at flood stage, early March 2009

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

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

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.

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,

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

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

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

Orinoco River, which he visited in 1800.[41] Other prominent examples include the Inner Niger Delta,[42] Peace–Athabasca Delta,[43] the Sacramento–San Joaquin River Delta,[44] and the Sistan delta of Iran.[45] The Danube has one in the valley on the Slovak–Hungarian border between Bratislava and Iža.[46]

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.

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.[48][49]

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.[25][50]

Existential threats to deltas

Human activities in both deltas and the

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]

The extensive anthropogenic activities in deltas also interfere with

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.[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

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.

The Kokemäki River (Kokemäenjoki) flows through the city of Pori in Satakunta, Finland. Its delta, where the delta islands remain between the distributaries, starts near the centre.

Urban areas and human habitation tends to locate in lowlands near water access for transportation and

Yangtze River Delta, European Low Countries and the Greater Tokyo Area
.

Examples

See also: Category:River deltas

The Ganges–Brahmaputra Delta, which spans most of Bangladesh and West Bengal and empties into the Bay of Bengal, is the world's largest delta.[66]

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.[67]

See also

References

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Bibliography

External links

The Wikibook Historical Geology has a page on the topic of: Deltas
Wikimedia Commons has media related to River deltas.
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