A river basin is an area of land drained by a river and its tributaries. River basins have typical features, these include: Tributaries - smaller rivers flowing into a larger river. A Watershed - an area of highland surrounding the river basin. A confluence - where a river joins another river. Source - the start of a river. Mouth - Where a river meets a lake, the sea or an ocean. DELTA: A delta is a landform that is formed at the mouth of a river, where that 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. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta. /OR/, A river basin is the entire area where a river has eroded away. A delta is where the river meets the sea, and fans out.