RIVERSMost rivers flow downhill from hills or mountains to the sea. They start as fast-flowing streams, which join together to create small rivers. These often cascade down steep slopes, cutting deep valleys, until they reach flatter ground. Here, regular seasonal flooding creates broad, fertile floodplains. Each river wanders across its floodplain, growing in size but flowing more slowly, until it reaches the estuary or delta where it spills into the sea.
180
TRIBUTARIES
▼The main stream of a river is joined by other streams, called tributaries. They all add to the flow, especially after heavy rain and during the spring thaw when mountain snow melts. At such times, they turn into torrents, carrying masses of gravel downhill and flowing in “braided” (plaited) patterns. These braided streams flow down from Aoraki (Mount Cook), New
Zealand, to join the Hopkins River.SOURCE
▼All rivers have a source. It may be a lake, a swamp, or a spring bubbling out of the ground, which feeds the main stream of the young river. In high mountains, the source may be a stream of meltwater pouring from the end of a glacier.▼UPPER COURSE
The upland part of a river is known as its upper course. It is usually fast-flowing, with a rocky bed, rapids, and even waterfalls. Here the Churun River cascades over the rim of the flat-topped Auyantepui mountain in Venezuela, South America, at Angel Falls – the highest waterfall on Earth, at 979 m (3,212 ft).180_181_Rivers.indd 180 03/01/19 12:10 PM