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A river delta is a
landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws ...

landform
created by
deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * List of deposed politicians, Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual ...
of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is cate ...

sediment
that is carried by a
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

river
as the flow leaves its
mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

mouth
and enters slower-moving or stagnant water. This occurs where a river enters an
ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
,
sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
sea
,
estuary An estuary is a partially enclosed Coast, coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime envir ...

estuary
,
lake A lake is an area filled with water, localized in a basin, surrounded by land Land is the solid surface of Earth that is not permanently submerged in water. Most but not all land is situated at elevations above sea level (variable ove ...

lake
,
reservoir A reservoir (; from French ''réservoir'' ) is most commonly an enlarged natural or artificial lake A lake is an area filled with water, localized in a basin, surrounded by land Land is the solid surface of Earth that is not per ...

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 Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is cate ...

sediment
, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human
civilization  A civilization (or civilisation) is a complex society A complex society is a concept that is shared by a range of disciplines including anthropology, archaeology, history and sociology to describe a stage of social formation. The concep ...

civilization
, as they are major agricultural production centers and population centers. They can provide
coastline The coast, also known as the coastline or seashore, is defined as the area where land meets the sea or ocean, or as a line that forms the boundary between the land and the ocean The ocean (also the sea or the world ocean) is the body ...

coastline
defense and can impact drinking water supply. They are also
ecologically Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Topics of interest include the biodiversity ...
important, with different species' assemblages depending on their landscape position.


Etymology

A river delta is so named because the shape of the
Nile Delta The Nile Delta ( ar, دلتا النيل, or simply , ) is the delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, a landform at the mouth of a river * D (NATO phonetic alphabet: "Delta"), ...
approximates the triangular uppercase Greek letter delta. The triangular shape of the Nile Delta was known to audiences of
classical Athenian Attic Greek is the Greek dialect of the ancient region of Attica Attica ( el, Αττική, Ancient Greek ''Attikḗ'' or , or ), or the Attic peninsula, is a historical region that encompasses the city of Athens, the capital city, capital ...
drama Drama is the specific Mode (literature), mode of fiction Mimesis, represented in performance: a Play (theatre), play, opera, mime, ballet, etc., performed in a theatre, or on Radio drama, radio or television.Elam (1980, 98). Considered as a ...
; the tragedy ''
Prometheus Bound ''Prometheus Bound'' ( grc, Προμηθεὺς Δεσμώτης, ''Promētheús Desmṓtēs'') is an Ancient Greek tragedy traditionally ascribed to Aeschylus Aeschylus (, ; grc-gre, Αἰσχύλος ''Aiskhylos'', ; c. 525/524 – c. 456/ ...

Prometheus Bound
'' by
Aeschylus Aeschylus (, ; grc-gre, Αἰσχύλος ''Aiskhylos'', ; c. 525/524 – c. 456/455 BC) was an ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kin ...
refers to it as the "triangular Nilotic land", though not as a "delta".
Herodotus Herodotus ( ; grc, Ἡρόδοτος, Hēródotos, ; BC) was an ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), ge ...
's description of Egypt in his ''Histories'' mentions the Delta fourteen times, as "the Delta, as it is called by the
Ionians The Ionians (; el, Ἴωνες, ''Íōnes'', , ''Íōn'') were one of the four major s that the considered themselves to be divided into during the ; the other three being the , , and . The was one of the of the , together with the and ...
", including describing the outflow of
silt Silt is granular material A granular material is a conglomeration of discrete solid, macroscopic scale, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when gra ...
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 Polybius (; grc-gre, Πολύβιος, ; ) was a Greek historian of the Hellenistic period The Hellenistic period covers the period of Mediterranean history between the death of Alexander the Great in 323 BC and the emergence of the ...

Polybius
likened the land between the
Rhône The Rhône ( , ; german: Rhone ; wae, Rotten ; it, Rodano ; frp, Rôno ; oc, Ròse ) is one of the major rivers of Europe and has twice the average discharge of the Loire The Loire (, also ; ; oc, Léger; la, Liger) is the longest r ...

Rhône
and
Isère Isère ( , ; frp, Isera; oc, Isèra, ) is a landlocked department Department may refer to: * Departmentalization, division of a larger organization into parts with specific responsibility Government and military *Department (country subdivi ...
rivers to the Nile Delta, referring to both as islands, but did not apply the word delta. According to the Roman geographer
Strabo Strabo''Strabo'' (meaning "squinty", as in strabismus Strabismus is a condition in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be pre ...

Strabo
, the Cynic philosopher
Onesicritus of Astypalaea Alexander the Great receives a visit from Thalestris, queen of the Amazons">Thalestris.html" ;"title="Alexander the Great receives a visit from Thalestris">Alexander the Great receives a visit from Thalestris, queen of the Amazons, one of the lege ...
, who accompanied
Alexander the Great Alexander III of Macedon ( grc-gre, Αλέξανδρος}, ; 20/21 July 356 BC – 10/11 June 323 BC), commonly known as Alexander the Great, was a king (''basileus ''Basileus'' ( el, βασιλεύς) is a Greek term and title A title ...

Alexander the Great
's conquests in
India India, officially the Republic of India (Hindi Hindi (Devanagari: , हिंदी, ISO 15919, ISO: ), or more precisely Modern Standard Hindi (Devanagari: , ISO 15919, ISO: ), is an Indo-Aryan language spoken chiefly in Hindi Belt, ...

India
, reported that
Patalene Patalene was an ancient area of Indian subcontinent, now in modern Pakistan, that corresponds to the area of Sind. The Indo-Greeks are mentioned in ancient sources as having occupied the areas of the Patalene (Sindh Sindh (; sd, سن ...
(the delta of the
Indus River The Indus ( ) is a transboundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the highest number of these rivers, including t ...

Indus River
) was "a delta". (). The Roman author
Arrian Arrian of Nicomedia (; Ancient Greek, Greek: ''Arrianos''; la, Lucius Flavius Arrianus; ) was a Greek people, Greek historian, public servant, military commander and philosopher of the Roman Greece, Roman period. ''The Anabasis of Alex ...

Arrian
's ''Indica'' states that "the delta of the land of the Indians is made by the
Indus river The Indus ( ) is a transboundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the highest number of these rivers, including t ...

Indus river
no less than is the case with that of Egypt". As a
generic term Trademark distinctiveness is an important concept in the law governing trademark A trademark (also written trade mark or trade-markThe styling of ''trademark'' as a single word is predominantly used in the United States and Philippines on ...
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 Edward Gibbon (; 8 May 173716 January 1794) was an English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval Eng ...

Edward Gibbon
.


Formation

River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a
reservoir A reservoir (; from French ''réservoir'' ) is most commonly an enlarged natural or artificial lake A lake is an area filled with water, localized in a basin, surrounded by land Land is the solid surface of Earth that is not per ...

reservoir
. When the flow enters the standing water, it is no longer confined to its
channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austr ...
and expands in width. This flow expansion results in a decrease in the
flow velocityIn continuum mechanics the flow velocity in fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodyna ...
, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and is deposited as
alluvium Alluvium (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of t ...
, which builds up to form the river delta. Over time, this single channel builds a deltaic lobe (such as the bird's-foot of the Mississippi or
Ural river The Ural (russian: Урал, ), known as Yaik (russian: Яик, ba, Яйыҡ, translit=Yayıq, ; kk, Жайық, translit=Zhayyk, جايىق, ) before 1775, is a river flowing through Russia Russia ( rus, link=no, Россия, Rossiy ...
deltas), pushing its mouth into the standing water. As the deltaic lobe advances, the
gradient In vector calculus Vector calculus, or vector analysis, is concerned with differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Prod ...

gradient
of the river channel becomes lower because the river channel is longer but has the same change in elevation (see
slope In mathematics, the slope or gradient of a line Line, lines, The Line, or LINE may refer to: Arts, entertainment, and media Films * ''Lines'' (film), a 2016 Greek film * ''The Line'' (2017 film) * ''The Line'' (2009 film) * ''The Line'', ...

slope
). As the gradient of the river channel decreases, the amount of
shear stress Shear stress, often denoted by (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...

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 A floodplain or flood plain or bottomlands is an area of land adjacent to a river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows int ...
. This destabilizes the river channel. If the river breaches its natural
levees file:River Levee Cross Section Figure.svg, Components of a levee: file:Sacramento River Levee.jpg, The side of a levee in Sacramento, California A levee (), dike (American English), dyke (Commonwealth English), embankment, floodbank, or stop ...

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.Slingerland, R. and N. D. Smith (1998), "Necessary conditions for a meandering-river avulsion," ''Geology'' (Boulder), 26, 435–438. 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 A distributary, or a distributary channel, is a stream A stream is a body of water with surface water flowing within the stream bed, bed and Bank (geography), banks of a Channel (geography), channel. The flow of a stream is controlled by ...
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 the result of this process is the . 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 Pyrenees An alluvial fan is an accumulation of sediments shaped like a section of a shallow cone, with its apex at a point source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in ...

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 Mississippi () is a U.S. state, state in the Southeastern United States, Southeastern region of the United States, bordered to the north by Tennessee; to the east by Alabama; to the south by the Gulf of Mexico; to the southwest by Louisiana; a ...

Mississippi
,
Nile The Nile, , Bohairic , lg, Kiira , Nobiin Nobiin, or Mahas, is a Northern Nubian languages, Nubian language of the Nilo-Saharan languages, Nilo-Saharan language family. "Nobiin" is the genitive case, genitive form of ''Nòòbíí'' ("Nub ...

Nile
,
Amazon Amazon usually refers to: * Amazons In Greek mythology, the Amazons (Ancient Greek: Ἀμαζόνες ''Amazónes'', singular Ἀμαζών ''Amazōn'') are portrayed in a number of ancient Greek, ancient epic poems and legends, such as the ...

Amazon
,
Ganges The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma River, Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian ...

Ganges
,
Indus#REDIRECT Indus River
{{Redirect category shell, {{R from move {{R from miscapitalisation {{R unprintworthy ...

Indus
,
Yangtze The Yangtze or Yangzi ( or ) is the longest river in Asia, the third-longest in the world and the longest in the world to flow entirely within one country. It rises at Jari Hill in the Tanggula Mountains The Tanggula ( Chinese:  ...
, and
Yellow River The Yellow River (Chinese: , Jin: uə xɔ Mandarin Mandarin may refer to: * Mandarin (bureaucrat), a bureaucrat of Imperial China (the original meaning of the word) ** by extension, any senior government bureaucrat A bureaucrat is ...
discharging along passive continental margins. This phenomenon is due mainly to three factors:
topography Topography is the study of the forms and features of land surface Relief map of Sierra Nevada, Spain Terrain or relief (also topographical Topography is the study of the forms and features of land surfaces. The topography of an ...
, 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 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 A continental shelf is a portion of a continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth descriptio ...

continental shelf
. There are many other lesser factors that could explain why the majority of river deltas form along
passive margins
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. 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, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches.


Types

Deltas are typically classified according to the main control on deposition, which is a combination of river,
wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...
, and processes,Galloway, W.E., 1975, Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in Brousard, M.L., ed., Deltas, Models for Exploration: Houston Geological Society, Houston, Texas, pp. 87–98.Nienhuis, J.H., Ashton, A.D., Edmonds, D.A., Hoitink, A.J.F., Kettner, A.J., Rowland, J.C. and Törnqvist, T.E., 2020. Global-scale human impact on delta morphology has led to net land area gain. Nature, 577(7791), pp.514-518. 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

Fluvial-dominated deltas are found in areas of low tidal range and low wave energy. Where the river water is nearly equal in density to the basin water, the delta is characterized by ''homopycnal flow'', in which the river water rapidly mixes with basin water and abruptly dumps most of its sediment load. Where the river water has higher density than basin water, typically from a heavy load of sediment, the delta is characterized by ''hypercynal flow'' in which the river water hugs the basin bottom as a density current that deposits its sediments as
turbidite A turbidite is the geologic Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly ...

turbidite
s. 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. 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 Buoyancy (), or upthrust, is an upward exerted by a that opposes the of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bo ...

buoyancy
. Outflow dominated by
inertia Inertia is the resistance of any physical object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the grasp of the senses ** Object (abstract), an ob ...

inertia
tend to form Gilbert type deltas. Dominance of 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 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.


Gilbert deltas

A Gilbert delta (named after
Grove Karl Gilbert Grove Karl Gilbert (May 6, 1843 – May 1, 1918), known by the abbreviated name ''G. K. Gilbert'' in academic literature, was an American geologist. Biography Gilbert was born in Rochester, New York and graduated from the University of Roches ...

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 the Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta.Characteristics of deltas
(Available archived a

– checked Dec 2008.)
It is commonly a result of homopycnal flow. 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 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 Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. The Western Interior Seaway preceded Lake Bonneville. Lake Bonneville was a pluvial lake that formed in response to an increase in precipitati ...
in 1885."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. . Pages 2–17
Partial text
on Google Books.
Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into
Okanagan Lake Okanagan Lake is a large, deep lake in the Okanagan Valley of British Columbia ( en, Splendour without diminishment) , image_map = British Columbia in Canada 2.svg , Label_map = yes , coordinates = , ...

Okanagan Lake
in
British Columbia ( en, Splendour without diminishment) , image_map = British Columbia in Canada 2.svg , Label_map = yes , coordinates = , official_lang = None , Slogan = Beautiful British C ...

British Columbia
and forming prominent
peninsulas A peninsula ( la, paeninsula from ' "almost" and ' "island") is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrange ...

peninsulas
at
Naramata Naramata is a community within the Regional District of Okanagan–Similkameen in British Columbia, Canada. Naramata is situated in the Okanagan, Okanagan Valley on the southeast shore of Lake Okanagan, to the north of Penticton. In 2010, Naramat ...
, Summerland, and
Peachland Peachland is a district municipality of over 5000 residents in the Okanagan Valley. It is located on the west side of Okanagan Lake in British Columbia ( en, Splendour without diminishment) , image_map = British Columbia in Can ...
.


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


Tide-dominated deltas

Erosion In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...

Erosion
is also an important control in tide-dominated deltas, such as the
Ganges Delta The Ganges Delta (also known as the Sundarbans Delta or the Bengal Delta) is a river delta A river delta is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body. Landforms toget ...
, which may be mainly submarine, with prominent
sandbars A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji In oceanography Oceanography (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the cl ...
and ridges. This tends to produce a "dendritic" structure.Fagherazzi S., 2008, Self-organization of tidal deltas, Proceedings of the National Academy of Sciences, vol. 105 (48): 18692–18695, 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 surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood Coastal flooding normally occurs when dry and low-lying land is submerged by seawater. The range of a coastal flooding is a result of the elevation of floodwater that penet ...

storm surge
s. These distributaries slowly silt up at a more or less constant rate until they fizzle out.


Tidal freshwater deltas

A tidal freshwater delta is 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 The Pleistocene ( , often referred to as the ''Ice Age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the earth’s most recent period of repeated glaciations. Before a change finally ...
and subsequent
Holocene The Holocene ( ) is the current geological epoch In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age (geology), age but shorter than a period (geology), period. The current epoch is the Holocene E ...
tend to have dendritic estuaries with many feeder tributaries. Each tributary mimics this salinity gradient from their brackish junction with the mainstem
estuary An estuary is a partially enclosed Coast, coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime envir ...

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 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 the onset of or changes in historical land use, especially
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

deforestation
,
intensive agriculture Intensive agriculture, also known as intensive farming (as opposed to extensive farming Extensive farming or extensive agriculture (as opposed to intensive farming) is an agricultural Agriculture is the science, art and practice of cultiv ...
, and
urbanization Urbanization (or urbanisation) refers to the population shift from rural File:Rural landscape in Finland.jpg, A rural landscape in Lappeenranta, South Karelia, Finland. 15 July 2000. In general, a rural area or a countryside is a geographi ...
. These ideas are well illustrated by the many tidal freshwater deltas prograding into
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary An estuary is a partially enclosed Coast, coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zo ...
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.


Estuaries

Other rivers, particularly those on coasts with significant
tidal range Tidal range is the height difference between high tide and low tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon an ...
, do not form a delta but enter into the sea in the form of an
estuary An estuary is a partially enclosed Coast, coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime envir ...

estuary
. Notable examples include the
Gulf of Saint Lawrence The Gulf of St. Lawrence (French language, French: ''Golfe du Saint-Laurent'') is the outlet of the North American Great Lakes via the St. Lawrence River into the Atlantic Ocean. The gulf is a semi-enclosed sea, covering an area of about and conta ...

Gulf of Saint Lawrence
and the
Tagus The Tagus ( ; es, Tajo ; pt, Tejo ; see below) is the longest river in the Iberian Peninsula The Iberian Peninsula , ** * Aragonese and Occitan: ''Peninsula Iberica'' ** ** * french: Péninsule Ibérique * mwl, Península Eib ...

Tagus
estuary.


Inland deltas

In rare cases the river delta is located inside a large valley and is called an
inverted river delta{{noref, date=July 2008 An inverted river delta is special category of river delta in which the narrow end of the delta emerges on the seafront and the wide end is located further inland, so that with respect to the seafront, the locations of both e ...
. Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an ''inland delta'', and often occurs on former lake beds. The term was first coined by
Alexander von Humboldt Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 17696 May 1859) was a , , , , and proponent of philosophy and . He was the younger brother of the Prussian minister, philosopher, and (1767–1835). Humboldt's quantitative work ...

Alexander von Humboldt
for the middle reaches of the
Orinoco River The Orinoco () is one of the longest river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of i ...
, which he visited in 1800. Other prominent examples include the
Inner Niger Delta The Inner Niger Delta, also known as the Macina or Masina, is the inland delta of the Niger River. It is an area of fluvial wetlands, lakes and floodplains in the semi-arid Sahel area of central Mali, just south of the Sahara desert. Location a ...
, Peace–Athabasca Delta, the
Sacramento–San Joaquin River Delta from the north and the San Joaquin River from the south through Stockton, California, Stockton. , with the Sacramento River above and San Joaquin River below The Sacramento–San Joaquin River Delta, or California Delta, is an expansive inland r ...
, and the
Sistan Sistān ( fa, سیستان), known in ancient times as Sakastān ( fa, سَكاستان, "the land of the Saka The Saka, Śaka, Shaka, Śāka or Sacae ( ''Sakā''; Kharosthi: 𐨯𐨐 ''Saka''; Brahmi: , ''Śaka''; sa, शक, शा ...

Sistan
delta of Iran. The
Danube The Danube ( ; ) is the List of rivers of Europe#Longest rivers, second-longest river in Europe, after the Volga in Russia. It flows through much of Central Europe, Central and Southeastern Europe, from the Black Forest into the Black Sea. It ...

Danube
has one in the valley on the Slovak-Hungarian border between
Bratislava Bratislava (, also ; ; formerly ; hu, Pozsony) is the Capital city, capital and largest city of Slovakia. Officially, the population of the city is about 430,000; however, it is estimated to be more than 660,000 - approximately 150% of the off ...

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 The Okavango Delta (or Okavango Grassland) (formerly spelled "Okovango" or "Okovanggo") in Botswana Botswana (, also ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label=Setswana The Tswana language () is a Ban ...

Okavango Delta
in
Botswana Botswana (, also ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label=Setswana; Kalanga language, Kalanga: ''Hango yeBotswana''), is a landlocked country in Southern Africa. Botswana is topographically flat, with up to 70 ...

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 The Yangtze or Yangzi ( or ) is the longest river in Asia, the third-longest in the world and the longest in the world to flow entirely within one country. It rises at Jari Hill in the Tanggula Mountains The Tanggula ( Chinese:  ...

Yangtze
,
Pearl A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle (mollusc), mantle) of a living animal shell, shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pe ...
,
Red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength Image:dominant wavelength.png, frame, Dominant/complementary wavelength example on the CIE color ...
,
Mekong The Mekong or Mekong River is a trans-boundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the highest number of these river ...

Mekong
, Irrawaddy, Ganges-Brahmaputra, and
Indus#REDIRECT Indus River
{{Redirect category shell, {{R from move {{R from miscapitalisation {{R unprintworthy ...

Indus.


Sedimentary structure

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. *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 A turbidite is the geologic Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly ...

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).Robert L. Bates, Julia A. Jackson, ''Dictionary of Geological Terms'' American Geological Institute, AGI (1984) 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, wikt:subaqueous, 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. 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.


Existential threats to deltas

Human activities in both deltas and the River Basin, river basins upstream of deltas can radically alter delta environments. Upstream land use change such as Erosion control, 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. 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 Geomorphology, geomorphological and Ecology, ecological delta processes. People living on deltas often construct Flood defence, flood defences which prevent sedimentation from floods on deltas, and therefore means that sediment deposition can't compensate for subsidence and erosion. In addition to interference with delta aggradation, pumping of Groundwater exploitation, groundwater, Oil Extraction, oil, and Gas extraction, gas, and constructing infrastructure all accelerate subsidence, increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining, and cause Salinisation, saltwater intrusion. There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through Sedimentation enhancing strategy, sedimentation enhancing strategies. While nearly all deltas have been impacted to some degree by humans, the
Nile Delta The Nile Delta ( ar, دلتا النيل, or simply , ) is the delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, a landform at the mouth of a river * D (NATO phonetic alphabet: "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 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.


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. Not all sand and gravel quarries are former deltas, but for ones that are, much of the sorting is already done by the power of water. Urban areas and human habitation tends to locate in lowlands near water access for transportation and sanitation. This makes deltas a common location for civilizations to flourish due to access to flat land for farming, freshwater for sanitation and irrigation, and sea access for trade. Deltas often host extensive industrial and commercial activities as well as agricultural land which are often in conflict. Some of the world's largest regional economies are located on deltas such as the Pearl River Delta, Yangtze River Delta Economic Zone, Yangtze River Delta, Low Countries, European Low Countries and the Greater Tokyo Area.


Examples

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


Other deltas

* Amazon Delta * Danube Delta * Ebro Delta * Tigris–Euphrates river system, Euphrates Delta * Fly River#Delta, Fly Delta * Ganges Delta, Ganges–Brahmaputra Delta * Konaseema, Godavari Delta * Indus River Delta * Irrawaddy Delta * Chola Nadu, Kaveri Delta * Diviseema, Krishna Delta * Lena Delta Wildlife Reserve, Lena Delta * Mackenzie River, Mackenzie Delta * Mahanadi River Delta * Mekong Delta * Mississippi River Delta * Niger Delta *
Nile Delta The Nile Delta ( ar, دلتا النيل, or simply , ) is the delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, a landform at the mouth of a river * D (NATO phonetic alphabet: "Delta"), ...
*
Okavango Delta The Okavango Delta (or Okavango Grassland) (formerly spelled "Okovango" or "Okovanggo") in Botswana Botswana (, also ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label=Setswana The Tswana language () is a Ban ...

Okavango Delta
* Orinoco Delta * Paraná Delta * Delta do Parnaíba Environmental Protection Area, Parnaíba Delta * Pearl River Delta * Po (river)#Po Delta, Po Delta * Red River Delta * Rhine–Meuse–Scheldt delta, Rhine Delta * Arles, Rhône Delta *
Sacramento–San Joaquin River Delta from the north and the San Joaquin River from the south through Stockton, California, Stockton. , with the Sacramento River above and San Joaquin River below The Sacramento–San Joaquin River Delta, or California Delta, is an expansive inland r ...
* St. Clair River, St. Clair Delta * Bilu Island, Salween Delta * Volga Delta * Yangtze Delta * Yellow River, Yellow River Delta (also known as Huanghe) * Yukon–Kuskokwim Delta * Zambezi#Delta, Zambezi Delta


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.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 File:Delta in Ismenius Lacus.jpg, Delta in Ismenius Lacus quadrangle, as seen by THEMIS. File:Delta in Lunae Palus.jpg, Delta in Lunae Palus quadrangle, as seen by THEMIS. File:Delta in Margaritifer Sinus.jpg, Delta in Margaritifer Sinus quadrangle as seen by THEMIS. File:Distributary fan-delta.jpg, Probable delta in Eberswalde crater, as seen by Mars Global Surveyor. Image in Margaritifer Sinus quadrangle.


See also

* * * * * *


References


Bibliography

* Renaud, F. and C. Kuenzer 2012: The Mekong Delta System – Interdisciplinary Analyses of a River Delta, Springer, , , 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, , , pp. 7–48 *


External links


Louisiana State University Geology
– World Deltas * http://www.wisdom.eoc.dlr.de WISDOM Water related Information System for the Sustainable Development of the Mekong Delta
Wave-dominated river deltas on coastalwiki.org
– A coastalwiki.org page on wave-dominated river deltas {{Authority control Aquatic ecology Ecology River deltas, Coastal geography Sedimentology Fluvial landforms Water streams Bodies of water Water