river delta

A river delta is a

landform

created by deposition of

sediment

that is carried by a

river

as the flow leaves its

mouth

and enters slower-moving or stagnant water. This occurs where a river enters an

ocean

,

sea

,

estuary

,

lake

,

reservoir

, or (more rarely) another river that cannot carry away the supplied sediment. The size and shape of a delta is controlled by the balance between watershed processes that supply

sediment

, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human

civilization

, as they are major agricultural production centers and population centers. They can provide

coastline

defense and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position.

Etymology

A river delta 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 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 the Roman 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". (). The Roman author

Arrian

's ''Indica'' states that "the delta of the land of the Indians is made by the

Indus river

no less than is the case with that of Egypt".

As a generic term for the landform at the mouth of river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of

Edward Gibbon

.

Formation

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

reservoir

. When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and is deposited as alluvium, 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 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 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.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 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

Wax Lake Delta

.

In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan. The more often the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position result in more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's-feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape.

Alluvial fan

deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape.

Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the

Mississippi

,

Nile

,

Amazon

,

Ganges

,

Indus

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