A landform shaped due to deposition of sediments by a river at its mouth is a delta. This story brings you more information on river deltas through facts and examples of delta landforms across the world.
Waves And Tides Limit Delta Formation
The Columbia river deposits huge amounts of sediments, but the strong wave action and currents disperse all the sediments hindering any delta formation. Similarly, intense tidal action in the Atlantic prevents the Amazon river from forming a delta.
Human settlement goes hand-in-hand with sources of water, rivers being the most common. Based on this principle, we can also understand why river deltas become easily populated; sometimes, despite the risk of flooding.
Water is a major agent of erosion. Rivers cut across and erode different types of landforms, carrying a heavy load of sediments along with them. The point where they relieve themselves of this load is generally at the river-ocean or river-sea confrontation.
What Is A Delta?
Delta is a landform at the mouth of a river, which is shaped by the continuous process of sediment deposition by the river. Delta formations are found where a river flows into an ocean, sea, lake, or an estuary. An inland delta is formed when the river water spreads out and deposits sediments.
How Is It Formed?
A river stream does not remain bound by its channel when it meets the ocean or sea. The mixing of the flow of a river with the ocean or sea water widens its spread, and the velocity of the river flow decreases. Thus, the capacity of the stream to carry sediment decreases too, leaving behind deposits.
The heavier and coarser sediments are deposited much earlier, near to the land, unlike the finer sediments being carried away into the ocean water. Siltation occurs at the river mouth. The single channel-like consistent deposition of silt leads to the formation of a deltaic lobe (a projection of the delta into the sea), like the bird's foot delta.
Distributaries: As rivers move towards the large water bodies, their velocity decreases. Sediment buildup due to deposition hinders the natural flow of rivers. This results into the river stream breaching its banks and forming a separate flow or channel. These are the distributaries of a river.
Gradient of the river flow influences the distributary network. This channel-switching is observed at mature delta formations. The Lena river meeting the Arctic ocean in Russia is a vivid example of avulsion.
Sand bars or mouth bars: These are formed at the mouth of a river and also shape the distributary network. The river water traverses around this sand bar. Maximum deposition occurs at the higher end of the sand bar, resulting in diversion of the water flow in 2 directions, leading to two distributaries. The Wax Lake delta in Louisiana is an example of this.
The upper part of a delta affected by the river flow is the Upper Delta Plain, whereas, the low lying areas of the delta, influenced more by tides and waves, is the Lower Delta Plain.
Similar to any energy transfer process, from warm to cool, or high to low, the process of sedimentation also tries to balance out the amount of deposition in one place compared to another. River channels change course from high deposition areas to low deposition areas, so as to level the delta into a plain.
Frequent channel switching and a large distributary network is accompanied by a more uniform delta, which gives rise to what is known as the alluvial fan delta or fan-shaped delta. It is a cone shaped deposition of riverine sediments by interwoven streams.
Types of Delta Landforms
The velocity of the river flow, the nature and load of sediments, and the action of tides and waves on the coast, together determine the formation of delta. River deltas are primarily classified according to their shape into:
Arcuate: These are triangular or fan-shaped deltas formed on coasts with shallow waters. This shape is effected due to the distributary network of river streams. Also, constant wave action results into a smooth delta landform. Avulsion is observed more than the other types of delta.
Cuspate: It is a tooth-shaped delta. The river Tiber of Italy forms a cuspate delta at its mouth in the Tyrrhenian Sea. Heavy wave action hits the river flow on a flat coastline. This pushes the sediment back on both sides of the river mouth. This results into a tooth-shaped point forming the delta. Cuspate forelands are formed due to longshore drifts.
Bird-foot: As the name suggests, when viewed from above, these deltas look like a bird's foot. One major channel of river extending into the sea or ocean is its characteristic feature. Other major distributaries look like a bird's foot with separated fingers. The shelf here is broad and shallow, but it deepens abruptly around the delta.
Estuarine: Though not of a particular shape, this is a type of delta formed inland, within the coastline. The river empties all its sediments into a long estuary. This estuary gets filled up eventually with deposition of sediments. The Seine river of France is a prime example of estuarine delta.
Examples of River Deltas Across the World
Ganga-Brahmaputra or the Sunderban Deltais the largest delta landform in the world on the wetlands of the state of West Bengal in India and Bangladesh. It is an example of a tide dominated delta, which is also known as Green delta, due to it high fertility. It has a dendritic structure, appearing like tree branches when viewed from above.
East coast of Borneo Island, Indonesia is known for its peculiar shape, which makes it look like big fat fingers from above.
Nile riverdelta region is characterized by intense agricultural activity due to its fertile depositions. The Nile forms an example of an arcuate delta.
TheTiber riverforms a cuspate delta when it enters the Tyrrhenian sea near Rome, Italy.
TheMississippi riverdelta is a classic example of the bird's foot delta. It is also known as a lobic delta due to the deltaic lobes being formed there.
Theriver Rhonein Europe is an example of a composite arcuate and cuspate delta in the Mediterranean sea.
The Gulf of Riga is a bay in the Baltic sea with the Daugava, Lielupe, Gauja, Salaca and Parnu rivers forming the unique delta.
TheRio de la Plata―confluence of the Uruguay and Parana rivers―forms an estuarine delta in the Atlantic ocean, on the borders of Argentina and Uruguay.
The River Ebrois a delta in the Mediterranean Sea influenced by tidal action. It also is a wetland under the Ramsar Convention of Wetlands.
TheLena riverdelta in the Laptev Sea in the Arctic Ocean appears like a beautiful tree when viewed from above. A dense network of distributaries makes it an arcuate delta.
Other examples of famous delta landforms across the world include the Niger delta, the Tigris-Euphrates delta, and deltas formed by the rivers Danube, Rhine, Volga, Indus, Mekong, Yangtze, and Orinoco.