Deltas are  landforms formed at the mouth of a river, where the river meets a body of water with a lower velocity than the river (e.g. lake or sea),  resulting in the reduction in the river’s capacity to transport sediment.

Deltas are dynamic areas that change quickly due to the erosion of unstable land during storm and flood events and the creation of new land. Deltas are fertile areas which often support large populations due to their agricultural productivity. Examples include the Ganges delta in Bangladesh and the Nile delta in Egypt.

The Nile Delta

Deltas are formed in the following way:

1.     Rivers typically contain their highest sediment loads near their mouth, where they meet seas and oceans. As the river enters a body of water its velocity reduces as does its carrying capacity; 

2.     At the mouth of the river the flow of water becomes increasingly lateral. This lateral movement of water reduces hydraulic radius and increases wetted perimeter;

3.     This causes sediment to be deposited, as does flocculation where clay sediments join together, gain in mass and sink.

4.     Over time sediment builds up. This can create small islands that split the channel in the same was as braided streams; 

5.     This continues until the river splits as more islands form;

6.     If sediment is fine grained bird’s foot deltas can be created. If sediment is coarse grained arcuate deltas form.