What is a river system?

Definition of a river system:

A river system, also known as a drainage basin or watershed, is an area of land where all the precipitation that falls drains off and collects into a common outlet, such as a river, bay, or other body of water. It includes the main river and all of its tributaries.

Answer: A river system is the network of a river and all its tributaries that drain a particular area of land.