Rivers

A river is a permanent stream of water flowing in a depression called the channel; the channel is the lowest part of the river valley. The place where a river begins is its source, and where it flows into an ocean, sea, lake or another river is its mouth; smaller rivers joining the main river from the side are tributaries. The land area whose water all flows into one river is the river basin, and the boundary between neighbouring basins is the watershed; the largest basin in the world belongs to the Amazon (about 7 million km²). Mountain rivers flow fast in narrow, deep valleys; plain rivers flow slowly in a wide channel. When a river drops from a height a waterfall forms: the highest is Angel Falls in Venezuela (979 m), Victoria Falls is about 108 m and Niagara about 50 m. Rivers are fed by rain, snow, glaciers and groundwater; the Amu Darya and Zarafshan are fed mainly by snow and glacier water, and the Syr Darya by mixed sources. People have built reservoirs, canals and power stations on rivers; we must not waste or pollute water.