How is a river basin formed?

The main river along with all its tributaries that drain an area forms a river basin or the catchment area. A river basin is made up of many different watersheds. A watershed is a small version of a river basin. Every stream and tributary has its own watershed, which drains to a larger stream or wetland. These streams, ponds, wetlands, and lakes are part of a river basin. Every river is part of a network of watersheds that make up a river system’s entire drainage basin. All the water in the drainage basin flows downhill toward bigger rivers. 

Further Reading:

• What are the Parts of a River Called?
• The Major Indian River Systems