When a river enters the sea , its speed is reduced it gets divided and enters the sea forming a plain called delta.
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.