Why do rivers always flow downhill?

How it works

A river begins at its source, often high in the hills. It might start as a spring, melting snow or rainwater trickling off the slopes. Gravity acts like an invisible hand pulling the water downhill, and the water always takes the lowest path it can find. Little streams meet and join, and bit by bit they grow into a big river.

As it flows, a river works like a conveyor belt, carrying sand, mud and pebbles along. Where the water runs fast it wears away the banks; where it slows down it drops what it is carrying, so rivers slowly grow bends. The place where a river meets the sea is called its mouth, and the mud it drops there can build new flat land.

For a very long time, people have settled beside rivers. A river gives water to drink and for crops, fish to catch, and a road for boats to carry goods far away. That is why so many of the world's big cities have grown up along a river.

Where to see it in New Zealand

New Zealand's longest river is the Waikato. It flows out of Lake Taupō in the middle of the North Island, runs north through the city of Hamilton, which is built on both banks, and finally reaches the Tasman Sea on the west coast.

Try it together

After rain, go outside with an adult and watch where the water runs on a sloping path or yard. Then build a small hill in a sandpit and pour water slowly from the top with a watering can. Watch the little rivers form and see where they join.