Runoff and Drainage Basins

Streams are part of the Hydrologic Cycle, carrying precipitation back to the oceans as surface runoff.

Average Annual Precipitation in the USA.

Water that enters the streams and makes up the runoff comes from direct overland flow and from water that moved underground before being discharged into streams.
Surface Runoff = Precipitation – Infiltration Loss – Evaporation – Transpirations

Factors affecting the amount of runoff:
Amount of rainfall – desert areas have greater evaporation and infiltration so the proportion of precipitation going to runoff is low;
Human activity – the more land that is paved, the greater the runoff.
Vegetation removal – reduces the amount of transpiration.

Streams are a part of a drainage network. Each stream has an area that it drains called a drainage basin or watershed.

Drainage basins are separated by divides (high points between basins).

See the Clayborn Creek example for details.