River delta

A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.

The Greek historian Herodotus coined the term delta for the Nile River delta because the sediment deposited at its mouth had the shape of the upper-case Greek letter Delta: Δ.

River deltas form when a river carrying sediment reaches a body of standing water,such as a lake, ocean, or 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 deposits. Over time, this single channel will build a deltaic lobe (such as the bird's-foot of the Mississippi or Ural River deltas), pushing its mouth further 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 slope of the river channel decreases, it becomes unstable for two reasons. First, water under the force of gravity will tend to flow in the most direct course down slope. If the river could breach its natural levees (i.e., during a flood), it would spill out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope. Second, as its slope gets lower, the amount of shear stress on the bed will decrease, which will result in deposition of sediment within the channel and for the channel bed to rise relative to the floodplain. This will make it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope. Often when the channel does this, some of its flow can remain in the abandoned channel. When these channel switching events a mature delta will gain a distributary network.

Another way in which these distributary networks may 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 Louisiana.

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. It is closer to an ideal fan the more often the flow changes course because more rapid changes in channel position results 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 in their name, avulse frequently and more closely approximate an ideal fan shape.

Types of deltas

,  4600 yrs BP,  3500 yrs BP,  2800 yrs BP,  1000 yrs BP,  300 yrs BP,  500 yrs BP,  current]] Deltas are typically classified according to the main control on deposition, which is usually either a river, waves, or tides. These controls have a large effect on the shape of the resulting delta.

Wave-dominated deltas

In wave dominated deltas, wave erosion controls the shape of the delta, although deposition still outweighs the amount of erosion and the delta is able to advance into the sea. Deltas of this form, such as the Nile Delta, tend to have a characteristic Greek-capital-delta shape $\backslash left(\backslash Delta\backslash right)$.

in India and Bangladesh is the largest delta in the world and it is also one of the most fertile regions in the world.]]

Tide-dominated deltas

Erosion is also an important control in tide dominated deltas, such as the Ganges Delta, which may be mainly submarine, with prominent sand bars and ridges. This tends to produce a "dendritic" structure. While some authors describe both lacustrine and marine locations of Gilbert deltas, Elsewhere, similar structures can be found e.g. at the mouths of several creeks flowing into Okanagan Lake in British Columbia and forming prominent peninsulas at Naramata (), Summerland (), or Peachland ()

Estuaries

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.

Inland deltas

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 Inner Niger Delta is the most notable example. The Amazon has also an inland delta before the island of Marajó.

In some cases a river flowing into a flat arid area splits into channels which then evaporates as it progresses into the desert. Okavango Delta in Botswana is one well-known example.

Sedimentary structure

The formation of a delta consists of three main forms: the topset, foreset/frontset, and bottomset.

See also

Estuary

Mega delta

Alluvial fan

Formation of delta-shaped river basin

Avulsion (river)

References

External links

Louisiana State University Geology - World Deltas

Category:Aquatic ecology Category:Ecology Category:Coastal geography Category:Sedimentology Category:Fluvial landforms Category:Water streams