A drainage basin is an open system.

A drainage basin is the catchment area that of a river and it’s tributaries

Definitions

Source - Where the river begins

Channel - Where the river flows

Tributary - A small river or stream that joins a larger river

Mouth - Where the river meets the sea

Confluence - The point at which two rivers meet

Watershed - The area of highland (mountains) forming the edge of a river basin. Acts as a wall.