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.

Because rivers give fresh water, fertile soil, fish to eat, and an easy way to travel by boat, many of the world's greatest cities grew up along their banks. London sits on the Thames, Paris on the Seine, and the Ancient Egyptians built their whole civilisation along the River Nile. Rivers have been the lifeblood of human settlements for thousands of years.

A lake is a body of water surrounded by land on all sides. Most lakes are freshwater. They form in hollows scooped out by ancient glaciers, by volcanic craters, or in dips in the land where rivers or rainwater collect. A common mix-up is thinking all large bodies of water are seas — but a lake is enclosed by land, while a sea is part of the connected, salty ocean.