An illustration of a river's course, from source to mouth, showing typical features Rivers are natural channels that carry rain, melted ice and snow downhill from mountains and uplands to lowlands, lakes and seas. The world’s longest rivers are the Nile in Africa and the Amazon in South America, both about 6600 kilometres (4100 miles) long. Rivers shape the land as they flow over rocks of varying hardness, widening and deepening their valleys by erosion. The faster they flow, the greater their erosive power, and the larger the rocks and amount of sediment they can transport.
A typical river starts as a spring gushing out of the ground, as a melting glacier, or as rainwater collecting in small brooks and streams. The river’s upper waters usually flow fast and steep. The swift current washes away soil or mud so the bed is stony and the banks are bare. Gradually the slope eases and the river flows more slowly in its middle reaches, widening as smaller rivers, called tributaries, add to it.
The Amazon is so wide and fast-flowing, it carries more water than the Nile—which is about the same length—plus the next five longest rivers in the world put together.
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