
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 ...

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 ...

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 ...

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 ...

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.

,
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 ...

,
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 ...

,
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 ...

, 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 ...

, 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 ...

, 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 ...

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/ ...

'' 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 ...

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 ...

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 ...

, 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 ...

'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, ...

, 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 ...

) 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 ...

'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 ...

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 ...

.
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 ...

. 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 ...

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'', ...

).
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 ...

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 ...

(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 ...

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 ...

,
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 ...

,
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 ...

,
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 ...

,
Indus#REDIRECT Indus River
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,