The term used to describe the water that reaches the river as a result of rainfall or snowmelt in the river basin.

Surface runoff from torrential rainfall in India. Notice that water is running off from steps and drives which are artificial surfaces, and yet it is not running off from the grass in the background. The grass is a permeable surface and can soak up large amounts of rainfall.

Runoff is mostly unseen, as seepage through soil and rocks. Water only flows directly to rivers after torrential thunderstorms or very long periods of less heavy rain. When it does so, floods occur.