Hydrological Cycle

Rivers exist because of the hydrological cycle. The sun evaporates water from oceans and lakes. That moisture rises, cools, condenses into clouds, and falls as precipitation. Gravity then pulls rain and snowmelt downhill across the land, concentrating it into channels.

A river's watershed (or drainage basin) is the total area of land from which it collects water. The Amazon's watershed covers 7 million square kilometers — roughly the size of Australia.