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.
The diagram below shows these features.
A drainage basin is an example of an open system because it is open to inputs from outside, such as precipitation, and is responsible for outputs out of the system, such as output of water into the sea and evaporation of water into the atmosphere.