Question

Why do rivers form distributaries at the mouth?

Answer 1

The deposition of river's sediment at the mouth of the river in a large amount allows the river to form distrubutaries.

Answer 2

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 happens at places where a riven enters any another water bodies such as the ocean, seas, estuary, lake, reservoir or another river.