A river delta is the result of a gradual build up of sediment on the continental shelf at the mouth of the river. Deltas can build up, with the aid of deposition due to flooding, until they are higher than the water around them. This results in the river cutting channels through the delta to reach the now further-away ocean.
The name is derived the the Greek letter delta because of the similarity between the shape of the letter and the triangular shape of the deposited sediments. However, some deltas are known as bird's foot deltas because of their shape (e.g. Mississippi).
Deltas can also form where rivers join another river, or where they flow into lakes.