Even though rivers hold only a tiny fraction of the Earth's total water, they have always been essential to human civilization. Rivers carry fresh water to people, plants, and animals all across the Earth. They provide people with transportation and waterpower. They also shape the land by carving out valleys and canyons.
How rivers flow
A river begins as a tiny trickle on a slope—the result of rain, melting snow or glaciers, or a spring. (A spring is an opening at or near the surface of the Earth through which water from underground sources emerges.) As the trickle runs downhill, it combines with other trickles to form a larger, swifter creek. Picking up speed and volume as it flows, the water eventually becomes a river.
As it flows, the river receives water from streams that flow into it. These streams are called tributaries. The river and its tributaries make up a river system. The area of land from which the water of a river system is drawn is called a basin. Some rivers have very large basins while others of equal length drain much smaller basins. Thus, though the Nile and the Amazon are about the same length, the basin of the Amazon is more than twice as big as that of the Nile.
- Erosion by the Colorado River has created new landforms.
Most rivers have upper, middle, and lower courses. The upper course starts at a high elevation and rushes so rapidly that it carves a narrow channel in the land. The river carries material from the soil and rock that it erodes, or wears away. A swiftly moving upper course may even move boulders in this way. The moving water and the material it carries wear away even more rock and soil. Over thousands or millions of years, the river creates canyons and deep valleys in this way. The Grand Canyon, formed by the Colorado River, and the great gorge of the Zambezi below Victoria Falls show the changes a river can make in the Earth's surface.
In the middle course, the river descends to gentler slopes. Here the water flows more slowly. The soil and rock particles—silt, gravel, and sand—begin to sink to the riverbed. There they form a layer called sediment. Some of this material builds up to form sandbars and islands.
The river flows even more gently in its lower course. More material sinks to the bottom and builds up, making the lower course quite shallow. Heavy rain or melting snow rushing from the upper and middle courses can cause the lower course to overflow. When the flood recedes, a layer of sediment is left on the surrounding countryside. In the case of rivers that flood every year, such as the Nile, the layers of sediment pile up to form wide, fertile pieces of land called floodplains. The fertility of the soil attracts human settlers, but the flooding makes life on the river dangerous. People build embankments, or dikes, along the river to protect the floodplain and their homes.
When the river reaches the sea, it drops its remaining load of silt, gravel, and sand at its mouth. This material builds an ever-growing triangular-shaped area called a delta. Here the river branches into several channels, called distributaries, that empty into the ocean.
Rivers and human life
Rivers have always been of great use to humans. The earliest civilizations, including those of Mesopotamia and Egypt in the Middle East and the Indus Valley in India, developed near rivers. The rivers provided early humans with water to drink and fish to eat. When people learned to build and use boats, rivers supplied a cheap and easy way to travel. The Nile and the Rhine have been highways of trade for many centuries. Floodplains provided fertile soil for crops, and the system called irrigation allowed people to use rivers to water their fields.
People still depend on rivers for much of their water supply. Cities need huge amounts of water every day for industrial and home use. In many places rivers are used to produce electric power. Hydroelectric dams, such as the Hoover Dam on the Colorado River, harness the swift flow of rivers to create great amounts of electricity.
Some human activities are harmful to rivers. Large factories built alongside rivers use enormous amounts of water for cooling and other purposes. Then they return the water to the river at overheated temperatures. The unnaturally hot water disturbs the ecology of the river and kills fish. Industries also dump sulfur, lead, and other harmful chemical waste into rivers. Cities near rivers contribute to the problem by releasing their wastes into the water. Another source of river pollution is the use of chemical fertilizers or pesticides (insect-killing substances) on surrounding land. These chemicals get into the groundwater and then can enter a river.
Pollution does not just affect the creatures that live in the water. When people eat fish taken from polluted streams, the pollution passes into their bodies and can cause cancer or other health problems. (See also pollution.)