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72-769: The Ganges Delta (also known the Ganges-Brahmaputra Delta , the Sundarbans Delta or the Bengal Delta ) is a river delta predominantly covering the Bengal region of the Indian subcontinent , consisting of Bangladesh and the Indian state of West Bengal . It is the world's largest river delta and it empties into the Bay of Bengal with the combined waters of several river systems , mainly those of

144-432: 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

216-493: 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, and agricultural land is frequently 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

288-545: 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 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

360-533: A fan. The more often the flow changes course, the shape develops closer to an ideal fan because more rapid changes in channel position result in a 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

432-399: 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. A tidal freshwater delta is

504-545: A flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example. See endorheic basin . 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 . The formation of a delta is complicated, multiple, and cross-cutting over time, but in

576-573: A hypocynal delta dip at a very shallow angle, around 1 degree. 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 tends 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. The modern Mississippi River delta

648-439: 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. A good example of

720-423: A sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary". 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 its brackish junction with the mainstem estuary up to

792-447: 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 on small scale by crossbedding . Human activities in both deltas and the river basins upstream of deltas can radically alter delta environments. Upstream land use change such as anti-erosion agricultural practices and hydrological engineering such as dam construction in

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864-451: Is a cladogram showing the relationships of the major extant crocodile groups based on molecular studies, excluding separate extinct taxa: Caiman [REDACTED] Melanosuchus [REDACTED] Paleosuchus [REDACTED] Alligator [REDACTED] Crocodylus [REDACTED] Mecistops [REDACTED] Osteolaemus [REDACTED] Gavialis [REDACTED] Tomistoma [REDACTED] Below

936-659: Is a compound of krokè ( ' pebbles ' ), and drilos/dreilos ( ' worm ' ), although drilos is only attested as a colloquial term for ' penis ' . It is ascribed to Herodotus , and supposedly describes the basking habits of the Egyptian crocodile. The form crocodrillus is attested in Medieval Latin . It is not clear whether this is a medieval corruption or derives from alternative Greco-Latin forms (late Greek corcodrillos and corcodrillion are attested). A (further) corrupted form cocodrille

1008-551: 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. A Gilbert delta (named after Grove Karl Gilbert ) is a type of fluvial-dominated delta formed from coarse sediments, as opposed to gently sloping muddy deltas such as that of

1080-545: 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. Crocodile Crocodiles ( family Crocodylidae ) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa , Asia , the Americas and Australia . The term “crocodile” is sometimes used more loosely to include all extant members of

1152-469: Is a maze of many river branches, the area is difficult to pass. Most islands are only connected with the mainland by simple wooden ferryboats. Bridges are rare. Some islands are not yet connected to the electric grid, so island residents tend to use solar cells for a bit of electric supply. Arsenic is a naturally occurring substance in the Ganga Delta that has detrimental effects on health and may enter

1224-1221: Is a more detailed cladogram of Crocodylidae, based on a 2021 study using paleogenomics that extracted DNA from the extinct Voay . Mecistops cataphractus West African slender-snouted crocodile Euthecodon † Brochuchus † Rimasuchus † Osteolaemus osborni Osborn's dwarf crocodile Osteolaemus tetraspis Dwarf crocodile Voay † Crocodylus anthropophagus † Crocodylus thorbjarnarsoni † Crocodylus palaeindicus † Crocodylus Tirari Desert † Crocodylus johnstoni Freshwater crocodile Crocodylus novaeguineae New Guinea crocodile Crocodylus mindorensis Philippine crocodile Crocodylus porosus Saltwater crocodile Crocodylus siamensis Siamese crocodile Crocodylus palustris Mugger crocodile Crocodylus checchiai † Crocodylus falconensis † Crocodylus suchus West African crocodile Crocodylus niloticus Nile crocodile Crocodylus moreletii Morelet's crocodile Crocodylus rhombifer Cuban crocodile Crocodylus intermedius Orinoco crocodile Crocodylus acutus American crocodile Alternatively, some morphological studies have recovered Mecistops as

1296-590: Is also an important activity in the delta region, with fish being a major source of food for many of the people in the area. In the last decades of the 20th century, scientists helped the poor people of the delta to improve fish farming methods. By turning unused ponds into viable fish farms and improving methods of raising fish in existing ponds, many people can now earn a living raising and selling fish. Using new systems, fish production in existing ponds has increased 800%. Shrimp are farmed in containers or cages that are submerged in open water. Most are exported. As there

1368-457: Is an oceanic dolphin which enters the delta from the Bay of Bengal. The Ganges river dolphin is a true river dolphin , but is extremely rare and considered endangered . Trees found in the delta include sundari , garjan ( Rhizophora spp.), bamboo , mangrove palm ( Nypa fruticans ), and mangrove date palm ( Phoenix paludosa ). The Ganges Delta lies at the junction of three tectonic plates:

1440-591: Is called by the Ionians ", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle. Despite making comparisons to other river systems deltas, Herodotus did not describe them as "deltas". 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. According to

1512-642: Is estimated that 30,000 chital ( Axis axis ) are in the Sundarbans part of the delta. Birds found in the delta include kingfishers , eagles , woodpeckers , the shalik ( Acridotheres tristis ), the swamp francolin ( Francolinus gularis ), and the doel ( Copsychus saularis ). Two species of dolphin can be found in the delta: the Irrawaddy dolphin ( Orcaella brevirostris ) and the Ganges river dolphin ( Platanista gangetica gangetica ). The Irrawaddy dolphin

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1584-571: Is found in Old French and was borrowed into Middle English as cocodril(le) . The Modern English form crocodile was adapted directly from the Classical Latin crocodīlus in the 16th century, replacing the earlier form. The use of -y- in the scientific name Crocodylus (and forms derived from it) is a corruption introduced by Laurenti (1768). Crocodylidae was named as a family by Georges Cuvier in 1807. It belongs to

1656-534: Is good for agriculture. It is composed of a labyrinth of channels , swamps , lakes, and flood plain sediments (chars). The Gorai-Madhumati River , one of the distributaries of the Ganges, divides the Ganges Delta into two parts: the geologically young, active, eastern delta, and the older, less active, western delta. Around 280 million (180 million Bangladesh and 100 million West Bengal, India) people live on

1728-548: 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". Herodotus 's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it

1800-726: Is their much higher levels of aggression . Crocodile size , morphology , behaviour and ecology differ somewhat among species . However, they have many similarities in these areas as well. All crocodiles are semiaquatic and tend to congregate in freshwater habitats such as rivers , lakes , wetlands and sometimes in brackish water and saltwater . They are carnivorous animals, feeding mostly on vertebrates such as fish , reptiles , birds and mammals , and sometimes on invertebrates such as molluscs and crustaceans , depending on species and age. All crocodiles are tropical species that, unlike alligators, are very sensitive to cold . They separated from other crocodilians during

1872-416: The 14,600 square kilometres (5,600 sq mi) protected. Where the delta meets the Bay of Bengal, Sundarbans mangroves form the world's largest mangrove ecoregion, covering an area of 20,400 square kilometres (7,900 sq mi) in a chain of 54 islands. They derive their name from the predominant mangrove species, Heritiera fomes , which are known locally as sundri or sundari . Animals in

1944-891: The Brahmaputra River and the Ganges River . It is also one of the most fertile regions in the world, thus earning the nickname the Green Delta . The delta stretches from the Hooghly River in the west as far as the Meghna River in the east. The Ganges Delta has the shape of a triangle and is considered to be an "arcuate" (arc-shaped) delta. It covers more than 105,000 km (41,000 sq mi) and lies mostly in Bangladesh and India, with rivers from Bhutan , Tibet , and Nepal draining into it from

2016-491: The East India Company , and related these practices to major ecological changes wrought about by forest and land management practices. Debjani Bhattacharyya has shown how Calcutta was constructed as an urban centre through tracing ecological changes wrought upon by colonial powers involving land, water and humans throughout the mid-18th to the early 20th centuries. In terms of recent scholarship that focuses more on

2088-465: The Eocene epoch, about 55 million years ago. Many species are at the risk of extinction , some being classified as critically endangered . The word crocodile comes from Ancient Greek κροκόδιλος (krokódilos)  'lizard', used in the phrase ho krokódilos tou potamoú , ' the lizard of the ( Nile ) river ' . There are several variant Greek forms of the word attested, including

2160-467: The Ganges River Basin, making it the most populous river basin in the world. Most of the Ganges Delta has a population density greater than 200/km (520 people per square mile), making it one of the most densely populated regions in the world. Three terrestrial ecoregions cover the delta. The Lower Gangetic plains moist deciduous forests ecoregion covers most of the delta region, although

2232-614: The Greater Tokyo Area . 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. 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 . Researchers have found a number of examples of deltas that formed in Martian lakes . Finding deltas

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2304-706: The Indian Plate , the Eurasian Plate , and the Burma Plate . The edge of the Eocene paleoshelf runs approximately from Kolkata to the edge of the Shillong Plateau . The edge of the paleoshelf marks the transition from the thick continental crust in the northwest to the thin continental or oceanic crust in the southeast. The enormous sediment supply from the Himalayan collision has extended

2376-532: 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. Historical data documents show that during the Roman Empire and Little Ice Age (times when 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. Ancient deltas benefit

2448-399: The food chain , especially in key crops such as rice. The Ganges Delta lies mostly in the tropical wet climate zone, and receives between 1,500 to 2,000 mm (59 to 79 in) of rainfall each year in the western part, and 2,000 to 3,000 mm (79 to 118 in) in the eastern part.. Hot, dry summers and cool, dry winters make the climate suitable for agriculture. In November 1970,

2520-559: The order Crocodilia , which includes the alligators and caimans (both members of the family Alligatoridae ), the gharial and false gharial (both members of the family Gavialidae ) as well as other, extinct, taxa. Although crocodiles, alligators, and the gharial are similar in appearance, they belong to separate biological families . The gharial, with its narrow snout , is easier to distinguish, while morphological differences are more difficult to spot in crocodiles and alligators. The most obvious external differences are visible in

2592-583: The Bengal/Ganges delta is that most of the scholarship is limited to the 18th to the 21st centuries, with a general dearth of ecological history of the region prior to the 18th century. One of the greatest challenges people living on the Ganges Delta may face in coming years is the threat of rising sea levels caused by climate change . An increase in sea level of 0.5 metres (1 ft 8 in) could result in six million people losing their homes in Bangladesh. Important gas reserves have been discovered in

2664-700: 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'). The Roman author Arrian 's Indica states that "the delta of

2736-503: The Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta. It is commonly a result of homopycnal flow. 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

2808-472: 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

2880-1256: The basins feeding deltas have reduced river sediment delivery to many deltas in recent decades. 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. Declines in river sediment delivery are projected to continue in the coming decades. The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes. People living on deltas often construct flood defences which prevent sedimentation from floods on deltas, and therefore means that sediment deposition can not compensate for subsidence and erosion . In addition to interference with delta aggradation , pumping of groundwater , oil , and gas , and constructing infrastructure all accelerate subsidence , increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining , and cause saltwater intrusion . 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,

2952-422: 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

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3024-450: 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. Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up

3096-488: The deadliest tropical cyclone of the twentieth century hit the Ganges Delta region. The 1970 Bhola cyclone killed 500,000 people (official death toll), with another 100,000 missing. The Guinness Book of World Records estimated the total loss of human life from the Bhola cyclone at 1,000,000. Another cyclone hit the delta in 1991, killing about 139,000 people. It also left many people homeless. People have to be careful on

3168-411: The delta about 400 kilometres (250 mi) seaward since the Eocene . The sediment thickness southeast of the edge of the paleoshelf beneath the Ganges Delta can exceed 16 km (9.9 mi). Approximately two-thirds of the Bangladesh people work in agriculture and grow crops on the fertile floodplains of the delta. The major crops that are grown in the Ganges Delta are jute , tea, and rice. Fishing

3240-512: The delta include the Indian python ( Python molurus ), clouded leopard ( Neofelis nebulosa ), Indian elephant ( Elephas maximus indicus ) and crocodiles , which live in the Sundarbans. Approximately 1,020 endangered Bengal tigers ( Panthera tigris tigris ) are believed to inhabit the Sundarbans. The Ganges–Brahmaputra basin has tropical deciduous forests that yield valuable timber: sal , teak , and peepal trees are found in these areas. It

3312-480: The delta, despite risks from floods caused by monsoons , heavy run-off from the melting snows of the Himalayas , and North Indian Ocean tropical cyclones . A large part of the nation of Bangladesh lies in the Ganges Delta; many of the country's people depend on the delta for survival. It is believed that upwards of 300 million people are supported by the Ganges Delta; approximately 400 million people live in

3384-985: The delta, such as in the Titas and Bakhrabad gas fields. Several major oil companies have invested in exploration of the Ganges Delta region. To offset land loss , tidal river management has been implemented in the delta. This method has been implemented in 5 beels and resulted in benefits including decreased waterlogging , creation of agricultural areas , improved navigation and land creation. 22°42′N 89°40′E  /  22.700°N 89.667°E  / 22.700; 89.667 River delta River deltas are important in human civilization , as they are major agricultural production centers and population centers. They can provide coastline defence and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position. On geologic timescales , they are also important carbon sinks . A river delta

3456-695: The eastern part of the Bengal/Ganges Delta, Iftekhar Iqbal argues for the inclusion of the Bengal Delta as an ecological framework within which to study the dynamics of agrarian prosperity or decline, communal conflicts, poverty and famine, especially throughout the colonial period. Iqbal has tried to show how resistance movements such as the Faraizi movement can be studied in relation to colonial ecological management practices. A strong criticism of environmental history scholarship with regards to

3528-619: The economy due to their well-sorted sand and gravel . Sand and gravel are often quarried from these old deltas and used in concrete for highways , buildings, sidewalks, and landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone. 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 tend to be located in lowlands near water access for transportation and sanitation . This makes deltas

3600-414: 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. Over time, this single channel builds a deltaic lobe (such as

3672-504: The forests have mostly been cleared for agriculture and only small enclaves remain. Thick stands of tall grass, known as canebrakes, grow in wetter areas. The Sundarbans freshwater swamp forests ecoregion lies closer to the Bay of Bengal; this ecoregion is flooded with slightly brackish water during the dry season, and fresh water during the monsoon season. These forests, too, have been almost completely converted to intensive agriculture, with only 130 square kilometres (50 sq mi) of

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3744-506: 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 as well as processes that are unique to the tidal freshwater setting. 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

3816-466: The geomorphology and ecosystem. Deltas are typically classified according to the main control on deposition, which is a combination of river, wave , and tidal processes, depending on the strength of each. 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. Fluvial-dominated deltas are found in areas of low tidal range and low wave energy. Where

3888-409: The head, with crocodiles having narrower and longer heads, with a more V-shaped than a U-shaped snout compared to alligators and caimans. Another obvious trait is that the upper and lower jaws of the crocodiles are the same width, and the teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed; therefore, all teeth are visible, unlike an alligator, which possesses in

3960-406: 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. While some authors describe both lacustrine and marine locations of Gilbert deltas, others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with

4032-594: 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). Gilbert himself first described this type of delta on Lake Bonneville in 1885. Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and form prominent peninsulas at Naramata , Summerland , and Peachland . In wave-dominated deltas, wave-driven sediment transport controls

4104-583: The land of the Indians is made by the Indus river no less than is the case with that of Egypt". As a generic term for the landform at the mouth of the river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of Edward Gibbon . River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a reservoir . When

4176-685: The larger superfamily Crocodyloidea , which also includes additional extinct crocodile relatives. These all belong to the order Crocodilia , which also includes alligators and gharials . Crocodylidae is cladistically defined as a crown group composed of the last common ancestor of the Nile crocodile ( Crocodylus niloticus ), the Dwarf crocodile ( Osteolaemus tetraspis ), and all of its descendants. It contains two subfamilies : Crocodylinae and Osteolaeminae . Crocodylinae contains 13-14 living species, as well as 6 extinct species. Osteolaeminae

4248-525: The later form krokódeilos ( κροκόδειλος ) found cited in many English reference works. In the Koine Greek of Roman times, krokodilos and krokodeilos would have been pronounced identically, and either or both may be the source of the Latinized form crocodīlus used by the ancient Romans. It has been suggested, but it is not certain that the word crocodilos or crocodeilos

4320-474: The north. 67% of the delta is inside Bangladesh and only 33% belongs to West Bengal. Most of the delta is composed of alluvial soils made up by small sediment particles that finally settle down as river currents slowdown in the estuary. Rivers carry these fine particles with them, even from their sources at glaciers as fluvio-glacial . Red and red-yellow laterite soils are found as one heads farther east. The soil has large amounts of minerals and nutrients, which

4392-524: The onset of or changes in historical land use, especially deforestation , intensive agriculture , and urbanization . 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. Other rivers, particularly those on coasts with significant tidal range , do not form

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4464-487: 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

4536-541: The river delta as severe flooding also occurs. In 1998, the Ganges flooded the delta, killing about 1,000 people and leaving more than 30 million people homeless. The Bangladesh government asked for $ 900 million to help feed the people of the region, as the entire rice crop was lost. The history of the Bengal delta has been a concern of emerging scholarship by environmental historians. Indian historian Vinita Damodaran has extensively profiled famine management practices by

4608-579: The river delta, causing the delta to retreat. For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels. 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. Tidal deltas behave differently from river-dominated and wave-dominated deltas, which tend to have

4680-414: 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 a 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

4752-399: The sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches. At the mouth of a river, the change in flow conditions can cause the river to drop any sediment it is carrying. This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels. Landforms at the river mouth drastically alter

4824-514: The sediment source. 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. Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment. However,

4896-424: The shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coastline. 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

4968-656: 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. This phenomenon is due mainly to three factors: topography , basin area, and basin elevation. 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 tends to be steeper and less widespread, which results in sediments not having

5040-585: The upper jaw small depressions into which the lower teeth fit. Also, when the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. For hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define the species ' family . Crocodiles have more webbing on the toes of the hind feet and can better tolerate saltwater due to specialized salt glands for filtering out salt, which are present, but non-functioning, in alligators. Another trait that separates crocodiles from other crocodilians

5112-732: Was first coined by Alexander von Humboldt for the middle reaches of the Orinoco River , which he visited in 1800. Other prominent examples include the Inner Niger Delta , Peace–Athabasca Delta , the Sacramento–San Joaquin River Delta , and the Sistan delta of Iran. The Danube has one in the valley on the Slovak–Hungarian border between Bratislava and Iža . In some cases, a river flowing into

5184-576: Was named by Christopher Brochu in 2003 as a subfamily of Crocodylidae separate from Crocodylinae, and contains the two extant genera Osteolaemus and Mecistops , along with several extinct genera. The number of extant species within Osteolaeminae is currently in question. Recent molecular studies using DNA sequencing have shown crocodiles to be more closely related to the gavialids rather than to alligators , contrary to prior theories based on morphological studies alone. Below

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