A landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.
The Greek historian Herodotus coined the term delta for the Nile River delta because the sediment deposited at its mouth had the shape of the upper-case Greek letter Delta.