River Basins. 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. 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.