Rivers begin high up on mountains and run downhill to the sea. They start from underground springs or melting glaciers. A river's route is divided into three parts. The first part is called the upper course, where the river flows steeply downhill and the current is fast. The water carries sand, gravel and rocks down with it.
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In the middle course the river flows along a gentler slope. It travels more slowly but it still wears away rock and sand from its banks.
In the lower course the river slows down and gets wider. The sand and rock it has been carrying are now worn down to tiny particles called silt.