A river is a naturally winding watercourse that drains surplus water from a drainage basin. (Small rivers are called streams, brooks, and so on.)

At any one moment the world's rivers carry only one ten-thousandth of one percent of the water that exists in the oceans and in the atmosphere of the world. But this amount, some 40,000 cubic kilometres a year, is usually enough to provide the world's people with all the water that they need.