River delta

A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth 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. 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. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply.[5] They are also ecologically important, with different species' assemblages depending on their landscape position.

Sacramento Delta at flood stage, 2009
Sacramento (California) Delta at flood stage, early-March 2009

Formation

Delta Formation
Delta forms where river meets lake[6]

River deltas form when a river carrying sediment reaches either (1) a body of water, such as a lake, ocean, or reservoir, (2) another river that cannot remove the sediment quickly enough to stop delta formation, or (3) an inland region where the water spreads out and deposits sediments. The tidal currents also cannot be too strong, as sediment would wash out into the water body faster than the river deposits it. The river must carry enough sediment to layer into deltas over time. The river's velocity decreases rapidly, causing it to deposit the majority, if not all, of its load. This alluvium builds up to form the river delta.[7] 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 deposits. 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 slope of the river channel decreases, it becomes unstable for two reasons. First, gravity makes the water flow in the most direct course down slope. If the river breaches its natural levees (i.e., during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.[8] Second, as its slope gets lower, the amount of shear stress on the bed decreases, which results in deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. This makes it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope. Often when the channel does this, some of its flow remains in the abandoned channel. When these channel-switching events occur, a mature delta develops a distributary network.

Another way these distributary networks form is from 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. 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 results 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, and Yangtze discharging along passive continental margins.[9] This phenomenon is due to three big factors: topography, basin area, and basin elevation.[9] Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate overtime 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 smaller 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.[9][10] 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.[10] Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.[9] However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches.[9]

Types of deltas

Lower Mississippi River landloss over time
Lower Mississippi River land loss over time
Mississippi Delta Lobes
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,[11] depending on the strength of each.[12] 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.[13]

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.[11] 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.[5] For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels.[14][15]

Ganges River Delta, Bangladesh, India
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.[16] Tidal deltas behave differently from a river- and wave-dominated deltas, which tend to have a few main distributaries. Once a wave- 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.[16]

Gilbert deltas

A Gilbert delta (named after Grove Karl Gilbert) is a type of 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.[17] [18] While some authors describe both lacustrine and marine locations of Gilbert deltas,[17] 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).[19]

Gilbert himself first described this type of delta on Lake Bonneville in 1885.[19] 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.

Tidal freshwater deltas

A tidal freshwater delta[20] is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".[21] 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.[22][23] 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.[24] 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.[25][26][27]

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

OkavangoDelta
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 Inner Niger Delta and Peace–Athabasca Delta are notable examples. The Amazon also has an inland delta before the island of Marajó, and the Danube has one in the valley on the Slovak-Hungarian border between Bratislava and Iža.

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.

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.

Sedimentary structure

Line5066 - Flickr - NOAA Photo Library
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.[17][28]

  • 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).[29] 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.[29] 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.[30]

Examples of deltas

EbroRiverDelta ISS009-E-09985
The Ebro River delta at the Mediterranean Sea

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.

The St. Clair River delta, between the Canadian province of Ontario and the U.S. state of Michigan, is the largest delta emptying into a body of fresh water.

Other deltas

Ecological threats to deltas

Human activities, such as the creation of dams for hydroelectric power or to create reservoirs can radically alter delta ecosystems. Dams block sedimentation, which can cause the delta to erode away. The use of water upstream can greatly increase salinity levels as less fresh water flows to meet the salty ocean water. 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 ecological devastation caused to deltas by damming and diversion of water. Construction, irrigation, and land alteration have impacted delta formation. As humans have altered surface roughness, runoff, and groundwater storage, studies have shown river delta retreat. However, historical data documents show that during the Roman Empire and Little Ice Age (times where there was considerable anthropogenic pressure), there were significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat.[31]

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.[32] 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.

As lowlands often adjacent to urban areas, deltas often comprise extensive industrial and commercial areas as well as agricultural land. These uses are often in conflict. The Fraser Delta in British Columbia, Canada, includes the Vancouver Airport and the Roberts Bank Superport and the Annacis Island industrial zone, and a mix of commercial, residential and agricultural land. Space is so limited in the Lower Mainland region, and in British Columbia in general, which is very mountainous, that the Agricultural Land Reserve was created to preserve agricultural land for food production.

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.[33]

Delta in Ismenius Lacus

Delta in Ismenius Lacus quadrangle, as seen by THEMIS.

Delta in Lunae Palus

Delta in Lunae Palus quadrangle, as seen by THEMIS.

Delta in Margaritifer Sinus

Delta in Margaritifer Sinus quadrangle as seen by THEMIS.

Distributary fan-delta

Probable delta in Eberswalde crater, as seen by Mars Global Surveyor. Image in Margaritifer Sinus quadrangle.

See also

  • Alluvial fan – A fan- or cone-shaped deposit of sediment crossed and built up by streams
  • Avulsion (river) – The rapid abandonment of a river channel and formation of a new channel
  • Estuary – A partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea
  • 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

References

  1. ^ Miall, A. D. 1979. Deltas. in R. G. Walker (ed) Facies Models. Geological Association of Canada, Hamilton, Ontario.
  2. ^ Elliot, T. 1986. Deltas. in H. G. Reading (ed.). Sedimentary environments and facies. Backwell Scientific Publications, Oxford.
  3. ^ 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.
  4. ^ 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. doi:10.1002/esp.189. ISSN 1096-9837.
  5. ^ 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. doi:10.1016/j.margeo.2014.12.004.
  6. ^ "How a Delta Forms Where River Meets Lake". 2014-08-12. Retrieved 2017-12-12.
  7. ^ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Modeling". pasternack.ucdavis.edu. Retrieved 2017-06-12.
  8. ^ Slingerland, R. and N. D. Smith (1998), "Necessary conditions for a meandering-river avulsion," Geology (Boulder), 26, 435–438.
  9. ^ 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. doi:10.1086/629606. JSTOR 30068527.
  10. ^ 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. doi:10.1016/S0037-0738(00)00041-5.
  11. ^ 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.
  12. ^ Perillo, G. M. E. 1995. Geomorphology and Sedimentology of Estuaries. Elsevier Science B.V., New York.
  13. ^ Orton, G.J.; Reading, H.G. (1993). "Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size". Sedimentology. 40 (3): 475–512. doi:10.1111/j.1365-3091.1993.tb01347.x.
  14. ^ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Hydrometeorology". pasternack.ucdavis.edu. Retrieved 2017-06-12.
  15. ^ Pasternack, Gregory B.; Hinnov, Linda A. (October 2003). "Hydrometeorological controls on water level in a vegetated Chesapeake Bay tidal freshwater delta". Estuarine, Coastal and Shelf Science. 58 (2): 367–387. doi:10.1016/s0272-7714(03)00106-9.
  16. ^ a b Fagherazzi S., 2008, Self-organization of tidal deltas, Proceedings of the National Academy of Sciences, vol. 105 (48): 18692–18695,
  17. ^ a b c Characteristics of deltas. (Available archived at [1] – checked Dec 2008.)
  18. ^ Bernard Biju-Duval, J. Edwin Swezey. "Sedimentary Geology". Page 183. ISBN 2-7108-0802-1. Editions TECHNIP, 2002. Partial text on Google Books.
  19. ^ a b "Geological and Petrophysical Characterization of the Ferron Sandstone for 3-D Simulation of a Fluvial-deltaic Reservoir". By Thomas C. Chidsey, Thomas C. Chidsey, Jr (ed), Utah Geological Survey, 2002. ISBN 1-55791-668-3. Pages 2–17. Partial text on Google Books.
  20. ^ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: Tidal Freshwater Deltas". pasternack.ucdavis.edu. Retrieved 2017-06-12.
  21. ^ Pasternack, G. B. 1998. Physical dynamics of tidal freshwater delta evolution. Ph.D. Dissertation. The Johns Hopkins University, 227pp, 5 appendices.
  22. ^ Pasternack, Gregory B.; Hilgartner, William B.; Brush, Grace S. (2000-09-01). "Biogeomorphology of an upper Chesapeake Bay river-mouth tidal freshwater marsh". Wetlands. 20 (3): 520–537. doi:10.1672/0277-5212(2000)020<0520:boaucb>2.0.co;2. ISSN 0277-5212.
  23. ^ Pasternack, Gregory B; Brush, Grace S (2002-03-01). "Biogeomorphic controls on sedimentation and substrate on a vegetated tidal freshwater delta in upper Chesapeake Bay". Geomorphology. 43 (3–4): 293–311. doi:10.1016/s0169-555x(01)00139-8.
  24. ^ Pasternack, Gregory B.; Brush, Grace S. (1998-09-01). "Sedimentation cycles in a river-mouth tidal freshwater marsh". Estuaries. 21 (3): 407–415. doi:10.2307/1352839. ISSN 0160-8347. JSTOR 1352839.
  25. ^ Gottschalk, L. C. (1945). "Effects of soil erosion on navigation in upper Chesapeake Bay". Geographical Review. 35 (2): 219–238. doi:10.2307/211476. JSTOR 211476.
  26. ^ Brush, G. S. (1984). "Patterns of recent sediment accumulation in Chesapeake Bay (Virginia-Maryland, U.S.A.) tributaries". Chemical Geology. 44 (1–3): 227–242. doi:10.1016/0009-2541(84)90074-3.
  27. ^ Orson, R. A.; Simpson, R. L.; Good, R. E. (1992). "The paleoecological development of a late Holocene, tidal freshwater marsh of the upper Delaware River estuary". Estuaries. 15 (2): 130–146. doi:10.2307/1352687. JSTOR 1352687.
  28. ^ D.G.A Whitten, The Penguin Dictionary of Geology (1972)
  29. ^ a b Robert L. Bates, Julia A. Jackson, Dictionary of Geological Terms AGI (1984)
  30. ^ Hori, K. and Saito, Y. Morphology and Sediments of Large River Deltas. Tokyo, Japan: Tokyo Geographical Society, 2003
  31. ^ Maselli, Vittorio; Trincardi, Fabio (2013-05-31). "Man made deltas". Scientific Reports. 3: 1926. doi:10.1038/srep01926. ISSN 2045-2322. PMC 3668317. PMID 23722597.
  32. ^ "Mineral Photos – Sand and Gravel". Mineral Information Institute. 2011. Archived from the original on 2011-10-06. Retrieved 2011-11-02.
  33. ^ 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. E12S15

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

Bridge River Delta Provincial Park

Bridge River Delta Provincial Park is a provincial park in British Columbia, Canada, located 65 kilometres north of Pemberton, British Columbia, or 80 km west of Lillooet, British Columbia. The park, which is 992 ha. in size, was established in 2010.

Clifton Court Forebay

Clifton Court Forebay is a reservoir in the San Joaquin River Delta region of eastern Contra Costa County, California, 17 mi (27 km) southwest of Stockton. The estuary region the forebay is located in is only 1m to 3m above mean sea level.

Colorado River Delta

The Colorado River Delta is the region where the Colorado River flows into the Gulf of California (also known as the Sea of Cortez). The delta is part of a larger geologic region called the Salton Trough. Historically, the interaction of the river's flow and the ocean's tide created a dynamic environment, supporting freshwater, brackish, and saltwater species. Within the delta region, the river split into multiple braided channels and formed complex estuary and terrestrial ecosystems. Use of water upstream and the accompanying reduction of fresh water flow has resulted in loss of most of the wetlands of the area, as well as drastic changes to the aquatic ecosystems. However, a scheme is currently in place which aims to rejuvenate the wetlands by releasing a pulse of water down the river delta.

Indus River Delta

The Indus River Delta (Urdu: سندھ ڈیلٹا‎, Sindhi: سنڌو ٽِڪور‎), forms where the Indus River flows into the Arabian Sea, mostly in the Southern Sindh province of Pakistan with a small portion in the Kutch Region of the Western tip of India. The delta covers an area of about 41,440 km² (16,000 square miles), and is approximately 210 km (130 mi) across where it meets the sea. The active part of the delta is 6,000 km2 in area (2,300 sq mi). The climate is arid, the region only receives between 25 and 50 centimetres (9.8 and 19.7 in) of rainfall in a normal year. The delta is home to the largest arid mangrove forests in the world, as well as many birds, fish and the Indus Dolphin.

Since the 1940s, the delta has received less water as a result of large-scale irrigation works capturing large amounts of the Indus water before it reaches the delta. The result has been catastrophic for both the environment and the local population. As a result, the 2010 Pakistan floods were considered "good news" for the ecosystem and population of the river delta as they brought much needed fresh water.The population of the active part of the delta was estimated at 900,000 in 2003. Most of the population depends on agriculture and fishing. Mangrove forests provide fuel wood. Many former settlements in the delta have been abandoned as result of lack of water in the Indus and the encroaching Arabian Sea.

Mekong Delta

The Mekong Delta (Vietnamese: Đồng bằng Sông Cửu Long, "Nine Dragon river delta" or simply Vietnamese: Đồng Bằng Sông Mê Kông, "Mekong river delta"), also known as the Western Region (Vietnamese: Miền Tây) or the South-western region (Vietnamese: Tây Nam Bộ) is the region in southwestern Vietnam where the Mekong River approaches and empties into the sea through a network of distributaries. The Mekong delta region encompasses a large portion of southwestern Vietnam of over 40,500 square kilometres (15,600 sq mi). The size of the area covered by water depends on the season. Before 1975, Mekong Delta is part of Republic of Vietnam. Mekong Delta is home of the IV Corps region during Vietnam War. IV Corps is the only corps in South Vietnam that VC didn't attack significantly until the last President Duong Van Minh surrender to North Vietnam.

Today, the region comprises 12 provinces: Long An, Đồng Tháp, Tiền Giang, An Giang, Bến Tre, Vĩnh Long, Trà Vinh, Hậu Giang, Kiên Giang, Sóc Trăng, Bạc Liêu, and Cà Mau, along with the province-level municipality of Cần Thơ.

The Mekong Delta has been dubbed as a "biological treasure trove". Over 1,000 animal species were recorded between 1997 and 2007 and new species of plants, fish, lizards, and mammals have been discovered in previously unexplored areas, including the Laotian rock rat, thought to be extinct.

Mississippi River Delta

The Mississippi River Delta is the river delta at the confluence of the Mississippi River with the Gulf of Mexico, in Louisiana in the southeastern United States. It is a three-million-acre (4,700 sq mi; 12,000 km2) area of land that stretches from Vermilion Bay on the west, to the Chandeleur Islands in the east, on Louisiana's southeastern coast. It is part of the Louisiana coastal plain, one of the largest areas of coastal wetlands in the United States. The Mississippi River Delta is the 7th largest river delta on Earth (USGS) and is an important coastal region for the United States, containing more than 2.7 million acres (4,200 sq mi; 11,000 km2) of coastal wetlands and 37% of the estuarine marsh in the conterminous U.S. The coastal area is the nation's largest drainage basin and drains about 41% of the contiguous United States into the Gulf of Mexico at an average rate of 470,000 cubic feet per second (3,500,000 US gal/s; 13,000,000 L/s).

Mobile–Tensaw River Delta

The Mobile–Tensaw River Delta is the largest river delta and wetland in Alabama. It encompasses approximately 260,000 acres (110,000 ha) in a 40-by-10-mile (64 km × 16 km) area and is the second largest delta in the contiguous United States.The delta's northernmost point is the confluence of the Tombigbee and Alabama rivers and follows a southerly direction that ultimately opens into the head of Mobile Bay through the Mobile, Tensaw, Apalachee, Middle, Blakeley, and Spanish rivers near the Battleship Parkway. It is contained within sections of Baldwin, Clarke, Mobile, Monroe, and Washington counties.

Niger Delta

The Niger Delta is the delta of the Niger River sitting directly on the Gulf of Guinea on the Atlantic Ocean in Nigeria. It is typically considered to be located within nine coastal southern Nigerian states, which include: all six states from the South South geopolitical zone, one state (Ondo) from South West geopolitical zone and two states (Abia and Imo) from South East geopolitical zone. Of all the states that the region covers, only Cross River is not an oil-producing state.

The Niger Delta is a very densely populated region sometimes called the Oil Rivers because it was once a major producer of palm oil. The area was the British Oil Rivers Protectorate from 1885 until 1893, when it was expanded and became the Niger Coast Protectorate. The delta is a petroleum-rich region and has been the center of international controversy over pollution.

Nile Delta

The Nile Delta (Arabic: دلتا النيل‎ Delta n-Nīl or simply الدلتا ed-Delta) is the delta formed in Northern Egypt (Lower Egypt) where the Nile River spreads out and drains into the Mediterranean Sea. It is one of the world's largest river deltas—from Alexandria in the west to Port Said in the east, it covers 240 km (150 mi) of Mediterranean coastline and is a rich agricultural region. From north to south the delta is approximately 160 km (99 mi) in length. The Delta begins slightly down-river from Cairo.The Nile Delta is an area of the world that lacks detailed ground truth data and monitoring stations. Despite the economic importance of the Nile Delta, it could be considered as one of the most data-poor regions with respect to sea level rise.

Orinoco Delta

The Orinoco Delta is a vast river delta of the Orinoco River, located in eastern Venezuela.

Pearl River Delta

The Pearl River Delta Metropolitan Region (PRD, Chinese: 珠江三角洲城市圈; pinyin: Zhūjiāng Sānjiǎozhōu Chéngshìquān) is the low-lying area surrounding the Pearl River estuary, where the Pearl River flows into the South China Sea. It is one of the most densely urbanized regions in the world, and is often considered as a megacity. It is now the wealthiest region in South China and one of the wealthiest in the whole China along with Yangtze River Delta in East China and Jingjinji in North China. The region's economy is referred to as Pearl River Delta Economic Zone, it is also part of the Guangdong-Hong Kong-Macau Greater Bay Area.

The PRD is a megalopolis, with future development into a single mega metropolitan area, yet itself is at the southern end of a larger megalopolis running along the southern coast of China, which include metropolises such as Chaoshan, Zhangzhou-Xiamen, Quanzhou-Putian and Fuzhou. The nine largest cities of the PRD had a combined population of 57.15 million at the end of 2013, comprising 53.69% of the provincial population. According to the World Bank Group, the PRD has become the largest urban area in the world in both size and population.The west side of this region, along with Chaoshan, was also the source of much Chinese emigration from the 19th to the mid 20th centuries, including to the Western world, where they formed many Chinatowns. Today, much of the Chinese diaspora in the US, Canada, Australia, Latin America, and much of Southeast Asia traces their ancestry to the west side of this region. Its dominant language is Cantonese.

Pearl River Delta Economic Zone

The Pearl River Delta Economic Zone (simplified Chinese: 珠江三角洲经济区; traditional Chinese: 珠江三角洲經濟區; pinyin: Zhūjiāng Sānjiǎozhōu Jīngjìqū) (once called 粤江平原), is in the Pearl River Delta region, the expansive delta lands of the Pearl River at the South China Sea. It consists of Guangzhou, Shenzhen, Zhuhai, Foshan, Dongguan, Zhongshan, Jiangmen, and parts of Huizhou and Zhaoqing, has been the most economically dynamic region of the Chinese Mainland since the launch of China's reform programme in 1979. Adjacent Hong Kong and Macau are not part of the economic zone.

The 2008-20 plan, released by China's National Development and Reform Commission, is designed to boost the pan—Pearl River Delta as a "center of advanced manufacturing and modern service industries," and as a "center for international shipping, logistics, trade, conferences and exhibitions and tourism." Goals include the development of two to three new cities in the Guangdong-Hong Kong-Macao Greater Bay Area, the development of 10 new multinational firms, and expansion of road, rail, seaport and airport capacities by 2020. They include construction of the 31-mile (50 km) Hong Kong–Zhuhai–Macau Bridge linking Hong Kong, Macau and the Pearl River Delta. The construction of 1,864 miles (3,000 km) of highways in the region was to be completed by 2012, and rail expansions of 683 miles (1,099 km) by 2012 and 1,367 miles (2,200 km) by 2020.

Red River Delta

The Red River Delta (Vietnamese: Đồng Bằng Sông Hồng, or Châu Thổ Sông Hồng) is the flat low-lying plain formed by the Red River and its distributaries merging with the Thái Binh River in northern Vietnam. The delta has the smallest area but highest population and population density of all regions. The region measuring some 15,000 square km is well protected by a network of dikes. It is an agriculturally rich area and densely populated. Most of the land is devoted to rice cultivation.Eight provinces together with two municipalities, the capital Hanoi and the port Haiphong form the delta. It has a population of almost 23 million.

The Red River Delta is the cradle of the Vietnamese nation. Water puppetry originated in the rice paddies here.

The region was bombed by United States warplanes during the Vietnam War.

The region was designated as the Red River Delta Biosphere Reserve as part of UNESCO's Man and the Biosphere Programme in 2004.

Sacramento–San Joaquin River Delta

The Sacramento–San Joaquin River Delta, or California Delta, is an expansive inland river delta and estuary in Northern California. The Delta is formed at the western edge of the Central Valley by the confluence of the Sacramento and San Joaquin rivers and lies just east of where the rivers enter Suisun Bay. The Delta is recognized for protection by the California Bays and Estuaries Policy. Sacramento–San Joaquin Delta was designated a National Heritage Area on March 12, 2019. The city of Stockton is located on the San Joaquin River on the eastern edge of the delta. The total area of the Delta, including both land and water, is about 1,100 square miles (2,800 km2).

The Delta was formed by the raising of sea level following glaciation, leading to the accumulation of Sacramento and San Joaquin River sediments behind the Carquinez Strait, the sole outlet from the Central Valley to San Pablo and San Francisco Bays and the Pacific Ocean. The narrowness of the Carquinez Strait coupled with tidal action has caused the sediment to pile up, forming expansive islands. Geologically, the Delta has existed for about 10,000 years, since the end of the last Ice Age. In its natural state, the Delta was a large freshwater marsh, consisting of many shallow channels and sloughs surrounding low islands of peat and tule.

Since the mid-19th century, most of the region has been gradually claimed for agriculture. Wind erosion and oxidation have led to widespread subsidence on the Central Delta islands; much of the Delta region today sits below sea level, behind levees earning it the nickname "California's Holland". Much of the water supply for central and southern California is also derived from here via pumps located at the southern end of the Delta, which deliver water for irrigation in the San Joaquin Valley and municipal water supply for southern California.

Suisun Bay

Suisun Bay ( sə-SOON) is a shallow tidal estuary (a northeastern extension of the San Francisco Bay) in northern California. It lies at the confluence of the Sacramento and San Joaquin Rivers, forming the entrance to the Sacramento–San Joaquin River Delta, an inverted river delta. Suisun Marsh, the tidal marsh land to the north, is the largest marsh in California. Grizzly Bay forms a northern extension of Suisun Bay. The bay is directly north of Contra Costa County.

The bay was named in 1811, after the Suisunes, a Native American tribe of the area. The word originates with the Patwin.

On the west, Suisun Bay is drained by the Carquinez Strait, which connects to San Pablo Bay, a northern extension of San Francisco Bay. In addition to the major bridges at the Carquinez Strait, it is spanned in its center by the Benicia-Martinez Bridge and at its eastern end by the State Route 160 crossing (Antioch Bridge) between Antioch and Oakley.

It is the anchorage of the Suisun Bay Reserve Fleet, a collection of U.S. Navy and merchant reserve ships, which was created in the period following World War II. The Glomar Explorer was anchored here after recovering parts of a sunken Soviet submarine in the mid-1970s (see Project Azorian). Many ships were removed and sold for scrap in the 1990s. In 2010, plans were announced to remove the mothball fleet in stages, with final removal by 2017.

The Central Pacific Railroad built a train ferry that operated between Benicia and Port Costa, California from 1879 to 1930. The ferry boats Solano and Contra Costa were removed from service when the nearby Martinez railroad bridge was completed in 1930. From 1913 until 1954 the Sacramento Northern Railway, an electrified interurban line, crossed Suisun Bay with the Ramon, a distillate-powered train ferry.

On April 28, 2004, a petroleum pipeline operated by Kinder Morgan Energy Partners ruptured, initially reported as spilling 1,500 barrels (264m³) of diesel fuel in the marshes, but, this was later updated to about 2,950 barrels. Kinder Morgan pleaded guilty to operating a corroded pipeline (and cited for failing to notify authorities quickly after the spill was discovered) and paid three million dollars in penalties and restitution.

Tonkin

Tonkin (Vietnamese: Bắc Kỳ, historically Đàng Ngoài), also spelled Tongkin, Tonquin or Tongking, is in the Red River Delta Region of northern Vietnam.

Yangtze River Delta

The Yangtze River Delta or YRD (Chinese: 长江三角洲 or simply 长三角) is a triangle-shaped metropolitan region generally comprising the Wu Chinese-speaking areas of Shanghai, southern Jiangsu province and northern Zhejiang province. The area lies in the heart of the Jiangnan region (literally, "south of the River"), where Yangtze River drains into the East China Sea. The urban build-up in the area has given rise to what may be the largest concentration of adjacent metropolitan areas in the world. It covers an area of 99,600 square kilometres (38,500 sq mi) and is home to over 115 million people as of 2013, of which an estimated 83 million is urban. If based on the greater Yangtze River Delta zone, it has over 140 million people in this region. With about 1/10 of China's population and 1/5 of the country's GDP, the YRD is one of the fastest growing and richest regions in East Asia measured by purchasing power parity. Having a fertile soil, the Yangtze River Delta abundantly produces grain, cotton, hemp and tea. In 2018, the Yangtze River Delta had a GDP of approximately US$2.2 trillion , about the same size as Italy.

Yangtze River Delta Economic Zone

Yangtze River Delta Economic Zone or Yangtze River Delta Economic Region (Chinese: 长江三角洲经济区) is an economic region in China that encompasses Shanghai municipality, Jiangsu, Anhui and Zhejiang province.

The region accounts for 20 percent of China's Gross Domestic Product and is responsible for one third of its imports and exports.

Yellow River

The Yellow River or Huang He (listen ) is the second longest river in China, after the Yangtze River, and the sixth longest river system in the world at the estimated length of 5,464 km (3,395 mi). Originating in the Bayan Har Mountains in Qinghai province of Western China, it flows through nine provinces, and it empties into the Bohai Sea near the city of Dongying in Shandong province. The Yellow River basin has an east–west extent of about 1,900 kilometers (1,180 mi) and a north–south extent of about 1,100 km (680 mi). Its total drainage area is about 752,546 square kilometers (290,560 sq mi).

Its basin was the birthplace of ancient Chinese civilization, and it was the most prosperous region in early Chinese history. There are frequent devastating floods and course changes produced by the continual elevation of the river bed, sometimes above the level of its surrounding farm fields.

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