What Is A River Delta?

Magnificent braided river delta with red and green flora showing at low tide. Alaska, Lower Cook Inlet, Kachemak Bay.

What Is A River Delta And Where Is It Formed?

A river delta is formed at the mouth of a river where the river deposits the sediment load carried by it and drains into a slower moving or static body of water. This usually occurs when the river joins a sea, estuary, ocean, lake, reservoir or in rare cases a slower moving river. The deltas of rivers are usually highly fertile areas and thus some of the most densely populated settlements of humans are based on such deltas. Some example of major deltas of the world is the Nile River delta and the Ganges River delta.

How Are River Deltas Formed?

There are several criteria for a river to form a delta.

First of all, the river should carry enough sediment to deposit at its mouth to form the delta.

Second, the tidal currents of the river must be weak so that the river is unable to carry the sediments into the body of water that it joins.

Third, the river must drain into a body of water that is either static or has a slow flow rate to prevent the fast removal of sediments carried by the river at its mouth.

When a river enters its last phase before it drains into another body of water, it is usually no longer confined to the channel in which it flows but its waters spread out as it enters another body of water. The spreading out of the river decreases its velocity and this, in turn, reduces its capacity to carry the great load of sediments with it. Hence, the sediments get deposited on the river bed and river banks. As the sediments build up over time, the river no longer flows through a single channel since the heavy deposits of sediments impede the flow of water. Thus, the river branches out and forms distributaries. All the distributaries of a river continue to flow downwards towards the body of water into which the river empties its waters. Thus, a fan-shaped river delta eventually develops with various distributaries and sedimental land forms.

How Many Types Of Deltas Are There?

Wave-dominated deltas

In wave-dominated deltas, the shape of the delta is controlled by wave-driven sediments. In such deltas, wave action deflects much of sediment at the river mouth along the coastline. Waves also cause deltas to retract by removing sediments from the river mouth.

Tide-dominated deltas

In tide-dominated deltas, tides control the shape of the delta. Erosion plays a significant role in shaping such deltas. Distributaries are formed in such deltas when there is sufficient water in the region like during storm surges and floods. These distributaries gradually silt up and finally become non-existent. The Ganges river delta is an example of the tide-dominated delta.

Gilbert deltas

A Gilbert delta is formed when the sediments deposited to form the delta are coarse in nature. Such deltas usually form at places where a mountain river joins a freshwater lake. While some suggest that Gilbert deltas are formed at both marine and lacustrine locations, others suggest that they are formed when the river waters blend more easily with the waters of a freshwater lake. Usually in cases of rivers entering the sea or a salt lake, the waters of the river float on top of the denser sea water. Several creeks that flow into the Okanagan Lake in British Columbia form such a delta at their mouths.

Inland deltas

In rare cases, a river might branch out at a specific location inland, deposit its sediments at the location, and then again rejoin to continue as a river. Such an area is called an inland delta. Such deltas often occur on former lake beds. An example of an inland delta is the Peace–Athabasca Delta.

The Largest Delta In The World

The Ganges/Brahmaputra combination delta that occupies most of Bangladesh and parts of West Bengal, India is the largest delta in the world.

The largest delta draining into a freshwater body is the St. Clair River delta which drains into the Lake St. Clair.

Importance Of Deltas

Deltas are usually highly fertile areas and support extensive crop cultivation. Sand and gravel are also quarried from deltas and are utilized for a variety of purposes like road and building construction. The nearness to the sea and the easy means of navigation of goods and people also makes some deltas of the world important industrial hubs. Thus, large settlements often grow up in the delta regions. For example, the Ganga-Brahmaputra delta region in Bangladesh and West Bengal, India supports an extremely high density of population.

Threats To The Deltas Of The World

Delta ecosystems are radically altered by human activities like the creation of dams and hydroelectric power stations. Dams alter the natural flow of rivers and the sedimentation rate in the delta. Lower volumes of sediments now reach the delta leading to the gradual erosion of the delta region. Also, the salinity levels of the deltaic waters increase as less fresh water enters the ocean due to extraction upstream of the dam. Some of the worst affected deltas of the world include the Nile Delta and the Colorado River Delta.

Do Deltas Also Exist In Mars?

Researchers have found that deltas are formed even in Martian lakes. The presence of deltas in the planet thus signify that once large amounts of water existed there.


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