Rivers and Lakes

A river is a large stream of freshwater that flows across the land and eventually reaches the sea, a lake, or another river. Rivers begin high up — often in mountains — where rain and melting snow collect and trickle downhill. Water always flows downhill because of gravity, the same force that makes a ball roll down a slope, so every river journeys from high ground towards the lowest place it can reach.

The place where a river begins is called its source. As the river flows downhill, smaller streams called tributaries join it, making it wider and deeper. The place where the river reaches the sea is called its mouth. At the mouth, a river sometimes spreads out into many smaller channels and drops the mud it has carried, forming a flat, fan-shaped area of fertile land called a delta.