How river deltas form
Streams and rivers drain carrying sediment in a watershed or catchment basin. Deltas begin to form when it carries too much sediment and clogs the entrance into the basin.
At this point, the stream loses energy because of the increased sediment deposits. This forces the basin entrance to widen forming a delta.
Deltas take different shapes. If you have too much sediment deposits, a feature like a bird-foot delta forms.
Deltas are fertile and often overcrowded
When area flood, sediments spread across the region. When water recedes, soils are rich and fertile for agriculture purposes.
In this diagram, the river enters a lake. The water flow decelerates with sediments dropping in. A delta forms depositing a prism of sediments that gradually tapers out toward the lake interior.
This diagram depicts a vertical cross section through geological layers deposited by rivers, deltas and lakes. Deposits from a series of successive deltas build out increasingly high in elevation as they migrate toward the center of the basin.