River delta

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Nile River delta, as seen from Earth orbit. Photo courtesy of NASA.

A delta is a landform where the mouth of a river flows into an ocean, sea, desert, estuary or lake, building outwards (as a deltaic deposit) from sediment carried by the river and deposited as the water current is dissipated. Deltaic deposits of larger, heavily-laden rivers are characterized by the river channel dividing into multiple streams (distributaries), these divide and come together again to form a maze of active and inactive channels. A related notion is estuaries, which are another type of river mouth.

1 Delta formation
2 List of deltas
3 See also
4 External link

[edit] Delta formation

This arcuate delta has formed on the south-west coastline of Greenland, near Narsarsuaq.

A deposit at the mouth of a river usually roughly triangular in shape. The triangular shape and the increased width at the base are due to blocking of the river mouth by silt, with resulting continual formation of distributaries at angles to the original course. Herodotus the great historian used this term for the Nile river delta because the sediment deposit at its mouth had the shape of upper-case Greek letter Delta: Δ.

Where delta formation is river-dominated and less subject to tidal or wave action, a delta may take on a multi-lobed shape which resembles a bird's foot. The Mississippi Delta is an example of this type.

The Ganges river delta in India and Bangladesh is one of the most fertile regions in the world.

The most famous delta is that of the Nile River, and it is this delta from which the term is derived, because the Nile delta has a very characteristic triangular shape, like the (upper-case) Greek letter delta ( $Δ$ ). Other rivers with notable deltas include the Amazon, the Ganges/Brahmaputra combination (this delta spans most of Bangladesh and West Bengal), the Niger, the Mississippi, the Sacramento-San Joaquin, the Rhine, the Rhône, the Danube, the Ebro, the Volga, the Lena, the Tigris-Euphrates, the Indus, the Krishna-Godavari, the Kaveri, the Ayeyarwady, and the Mekong.

In rare cases the river delta is located inside a large valley and is called an inverted river delta. Sometimes a river will divide into multiple branches in an inland area, only to rejoin and continue to the sea; such an area is known as an inland delta, and often occur on former lake beds. The Niger Inland Delta is the most notable example. These rock formations, which sometimes contain coal, cap the thick series of sedimentary rocks of the Allegheny Plateau in eastern North America.