Rivers and river systems

A river is fed by precipitation. Some rain soaks into the ground and seeps slowly into streams, some runs straight off the surface, and some is released gradually from melting snow and glaciers. Springs, where groundwater emerges, feed streams even during dry spells.

All the land that drains into a particular river is called its drainage basin or watershed. The Amazon has the largest basin in the world, covering around 40 percent of South America, and it carries more water than any other river, roughly a fifth of all the fresh water that rivers deliver to the oceans.

• Upper course: steep, fast and narrow, cutting down into rock to make V-shaped valleys, waterfalls and rapids.
• Middle course: the gradient eases, the valley widens and the river begins to swing in bends called meanders.
• Lower course: wide and slow on a flat floodplain, depositing sediment as its energy falls.
• Mouth: the river meets a lake or sea, forming an estuary or a delta.

Flowing water erodes by dragging and bouncing rocks along its bed, dissolving minerals and simply wearing away soft material. A river carries its load of sediment in solution, in suspension and by rolling larger stones along the bottom.

Given enough time, rivers can cut extraordinary landscapes. The Colorado River carved the Grand Canyon over millions of years as the surrounding plateau was slowly uplifted. On flat ground, meanders migrate across the floodplain, and a bend can be cut off to leave a curved oxbow lake.

When a river floods, water spills across the land beside it, slows down and drops sediment. Repeated floods build fertile floodplains, which is why so many farming civilizations grew beside rivers: Egypt on the Nile, Mesopotamia between the Tigris and Euphrates, the Indus valley, and early China along the Yellow River.

Where a river enters the sea with a heavy load of sediment, it can build a delta, a fan of land crossed by branching channels. The Ganges-Brahmaputra delta in Bangladesh and India is the world's largest. Deltas are fertile and densely populated but low-lying, which makes them vulnerable to storms, sinking land and sea level rise.

The Nile and the Amazon are the two longest rivers in the world, each roughly 6,400 to 6,700 kilometres long. Which is longer depends on where you decide each one's true source lies and how you measure its winding course, and geographers still disagree.

The Yangtze in China is the longest river flowing entirely within one country, and the Mississippi-Missouri system is the longest in North America. Rivers that cross borders, such as the Nile, the Mekong and the Danube, require international agreements over how their water is shared.

People have straightened, dammed, dredged and diverted rivers for thousands of years. Dams provide hydroelectric power, store water for irrigation and cities, and control floods. China's Three Gorges Dam on the Yangtze is the largest hydroelectric station in the world by installed capacity.

These benefits carry costs. Dams block fish migration, trap sediment that would otherwise replenish deltas, and flood valleys, displacing communities. Heavy water use can leave rivers running dry before they reach the sea, and pollution from farms, industry and sewage harms river life. Many countries now work to restore natural floodplains and remove obsolete dams.