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, estuary, lake or another river. A delta is formed only when a channel deposits sediment into another body of water. It builds up sediment outwards into the flat area which the river's flow encounters (as a deltaic deposit) transported by the water and set down as the currents slow. Deltaic deposits of larger, heavily-laden rivers are characterized by the main channel dividing amongst often substantial land masses into multiple streams known as distributaries. These divide and come together again to form a maze of active and inactive channels. This hydrogeologic formation is known as a delta. A delta can sometimes be misinterpreted as an alluvial fan. The two terms, however, are not interchangeable. A delta is formed in water and an alluvial fan occurs on land.

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

[edit] Delta formation

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

The deposit at the mouth of a river is usually roughly triangular in shape. The triangular shape and the increased width at the base are due to blocking of the river mouth, with resulting continual formation of distributaries at angles to the original course. These distributaries start out flowing fairly fast, but slow as more sediment is deposited and ultimately, the water flows elsewhere. Change in the depositional process is directly related to a river or streams competency (the size of particles it can transport) and its capacity ( the amount of material it can move). This change in flow affects the particle size in the suspended and bed loads, the size of the particles decrease as the flow slows and the larger particles are deposited. This deposition goes on continually in a cyclic fashion, creating alternating sediment beds of coarse and fine grain deposits. As a channel deposits sediment in one location, it becomes more difficult for the channel to reach the body of water. As this occurs, the channel will change course to go by the way of steepest gradient. This happens continuously as the channel moves back and forth from the mouth of the river. As sediment is laid down in this fashion, the predictable fan-shape of the delta is formed. 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, more deposition occurs and a delta may take on a multi-lobed shape which resembles a bird's foot. An example of a river dominated delta is the Mississippi River delta. Another type of delta is a wave dominated delta. Deposition is still occurring on this landform, however, waves erode the outer edge of the structure, giving it an even more identifiable delta shape. An example of this is the Nile River delta. The formation of a delta consists of three main forms: the topset, foreset/frontset, and bottomset.The bottomset beds are created from the suspended sediment that settles out of the water as the river flows into the body of water and loses energy. The suspended load is carried out the furthest into the body of water than all other types of sediment creating a turbidite. These beds are laid down in horizontal layers and consist of smaller grains. The foreset beds in turn build over the bottomset beds as the main delta form advances. The foreset beds consist of the bed load that the river is moving along which consists of larger sediments that roll along the main channel. When it reaches the edge of the form, the bed load rolls over the edge, and builds up in steeply angled layers over the top of the bottomset beds. The angle of the outermost edge of the delta is created by the sediments angle of repose. As the forsets build outward (which make up the majority of the delta) they pile up and miniature landslides occur. This slope is created in this fashion as the bedload continues to be deposited and the delta moves outward. In cross section, one would see the foresets lying in angled, parallel bands, showing each stage of the creation of the delta. The topset beds in turn overlay the foresets, and are horizontal layers of smaller sediment size that form as the main channel of the river shifts elsewhere and the larger particles of the bed load no longer are deposited. As the channels move across the top of the delta, the suspended load settles out in horizontal beds over the top. 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 ( $Δ$ ). Ganges/Brahmaputra combination (this delta spans most of Bangladesh and West Bengal) is the world's largest delta, and empties into the Bay of Bengal. Other rivers with notable deltas include the Sacramento-San Joaquin, the Mississippi, the Rhine, the Rhône, the Danube, the Ebro, the Volga, the Lena, the Tigris-Euphrates, the Indus, the Krishna-Godavari, the Truckee, the Carson River, the Kaveri, the Ayeyarwady, and the Mekong.

A Gilbert delta is a specific type of delta that only occurs in fresh water and with coarse sediments. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta. Other rivers, particularly those located on coasts with significant tidal range, do not form a delta but enter into the sea in the form of an estuary. Notable examples include the Saint Lawrence River 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 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. The Amazon has also an inland delta before the island of Marajo.