River delta. A river delta is a landform that is formed at the mouth of a river, where the river flows into an ocean, sea, estuary, lake, or reservoir.Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river.iver delta. A river delta is a landform that is formed at the mouth of a river, where the river flows into an ocean, sea, estuary, lake, or reservoir.