Deltas are formed at the end of rivers (river mouth) as it flows into the sea
Rivers carry a lot of sediment (mud)
The river flows into a calm sea
This slows the river down
Which makes the river drop its sediment
This deposited sediment builds up over years creating a delta
The river is forced to split up
The smaller rivers made by deltas are called distributaries
Examples of Deltas are Nile Delta, Ganges Delta