An illustration of a river's course from source to mouthRivers are natural water channels. They carry rain or melted snow and ice downhill to lowlands, lakes and seas. The force of running water wears away the rocks. This is called erosion. Over many years, rivers gradually cut away the ground to make valleys and gorges. They carry the rock fragments away downstream, breaking them up into even tinier pieces. These sink to the river bottom as gravel, silt or mud.
A river starts as a spring, rainwater collecting on sodden ground, or as meltwater from a glacier. Near its source, the river, often called a brook or a stream, flows quickly. Its waters wash away soil and mud so that the stream bed is bare rock. Streams eventually join together to form a larger, slower river.
Tributaries and meanders
The world’s two longest rivers, the Nile in Africa and the Amazon in South America, are both about 6600 kilometres (4100 miles) long.
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