A drainage basin is the area drained by a river and its tributaries.

A diagram showing a drainage basin.

A satellite picture of southeastern Australia. The Murray and Darling basins are shown by green dashed lines.

The boundary of a river basin is called the watershed. The river network normally occupies less than 5% of the total drainage basin, and even in times of flood it is rare for surface water to cover more than a fifth of the basin.

The headwaters form at the edge of the drainage basin, then join together to make a master, or trunk stream.