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.