A river delta is a landform that forms at the mouth of a river, where the river flows into an ocean, sea, estuary, lake, or reservoir.Deltas form from the deposition of sediment carried by a river as the flow leaves its mouth. Over long periods, this deposition builds the characteristic geographic pattern of a river delta.
In other words, you can say that a river delta is a landform that forms from the deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment.
The following are conditions necessary for formation of river delta:
- The river must have large load. This will be possible if there is active erosion in the upper and middle stages. The river needs to have enough material to deposit at the river mouth otherwise all materials will be eroded by ocean current or being deposited before they reach the river mouth.
- There should not be extensive deposition in the middle stage e.g. presence of lake in between or high evaporation rate (first). If there is a extensive deposition at middle of the river there will be no enough load to be deposited at the river mouth and hence no delta will be formed.
- The river’s load must be deposited faster than it can be removed by the action of currents and tides.
- Presence of shallow adjoining sea or continental shelf.
- The velocity of a river must be sufficiently low to allow most of its load to be deposited in the river’s mouth.