A river is a large, natural stream of flowing water. Rivers are found on every continent and on nearly every kind of land. Some flow all year round. Others flow seasonally or during wet years. A river may be only kilometers long, or it may span much of a continent.

The longest rivers in the world are the Nile in Africa and the Amazon in South America. Both rivers flow through many countries. For centuries, scientists have debated which river is longer. Measuring a river is difficult because it is hard to pinpoint its exact beginning and end. Also, the length of rivers can change as they meander, are dammed, or their deltas grow and recede.

The Amazon is estimated to be between 6,259 kilometers (3,903 miles) and 6,800 kilometers (4,225 miles) long. The Nile is estimated to be between 5,499 kilometers (3,437 miles) and 6,690 kilometers (4,180 miles) long. There is no debate, however, that the Amazon carries more water than any other river on Earth. Approximately one-fifth of all the fresh water entering the oceans comes from the Amazon.

Rivers are important for many reasons. One of the most important things they do is carry large quantities of water from the land to the ocean. There, seawater constantly evaporates. The resulting water vapor forms clouds. Clouds carry moisture over land and release it as precipitation. This freshwater feeds rivers and smaller streams. The movement of water between land, ocean, and air is called the water cycle. The water cycle constantly replenishes Earth’s supply of fresh water, which is essential for almost all living things.

Anatomy of a River

No two rivers are exactly alike. Yet all rivers have certain features in common and go through similar stages as they age.

The beginning of a river is called its source or headwaters. The source may be a melting glacier, such as the Gangotri Glacier, the source of the Ganges River in Asia. The source could be melting snow, such as the snows of the Andes, which feed the Amazon River. A river’s source could be a lake with an outflowing stream, such as Lake Itasca in the U.S. state of Minnesota, the source of the Mississippi River. A spring bubbling out of the ground can also be the headwaters of a river. The source of the Danube River is a spring in the Black Forest of Germany.

From its source, a river flows downhill as a small stream. Precipitation and groundwater add to the river’s flow. It is also fed by other streams, called tributaries. For instance, the Amazon River receives water from more than 1,000 tributaries. Together, a river and its tributaries make up a river system. A river system is also called a drainage basin or watershed. A river’s watershed includes the river, all its tributaries, and any groundwater resources in the area.

The end of a river is its mouth. Here, the river empties into another body of water—a larger river, a lake, or the ocean. Many of the largest rivers empty into the ocean.

The flowing water of a river has great power to carve and shape the landscape. Many landforms, like the Grand Canyon in the U.S. state of Arizona, were sculpted by rivers over time. This process is called weathering or erosion.

The energy of flowing river water comes from the force of gravity, which pulls the water downward. The steeper the slope of a river, the faster the river moves and the more energy it has.

The movement of water in a river is called a current. The current is usually strongest near the river’s source. Storms can also increase the current. A swift current can move even large boulders. These break apart, and the pieces that are carried in the moving water scrape and dig into the river bottom, or bed.

Little by little, a river tears away rocks and soil along its bed, and carries them downstream. The river carves a narrow, V-shaped valley. Rapids and waterfalls are common to rivers, particularly near their sources.

Eventually, the river flows to lower land. As the slope of its course flattens, the river cuts less deeply into its bed. Instead, it begins to wind from side to side in looping bends called meanders. This action widens the river valley.

At the same time, the river begins to leave behind some of the rocks, sand, and other solid material it collected upstream. This material is called sediment. Once the sediment is deposited, it is called alluvium. Alluvium may contain a great deal of eroded topsoil from upstream and from the banks of its meanders. Because of this, a river deposits very fertile soil on its flood plain. A flood plain is the area next to the river that is subject to flooding.

The deepest part of a river bed is called a channel. The channel is usually located in the middle of a river. Here, the current is often strong. In large rivers, ships travel in channels. Engineers may dredge, or dig, deeper channels so more water can flow through the river or the river can transport larger ships.

Near the end of its journey, the river slows and may appear to move sluggishly. It has less energy to cut into the land, and it can no longer carry a heavy load of sediment. Where the river meets the ocean or a lake, it may deposit so much sediment that new land, a delta, is formed.

Not all rivers have deltas. The Amazon does not have a true delta, for instance. The strength of the tides and currents of the Atlantic Ocean prevent the build-up of sediment. Deltas almost always have fertile soil. The Nile Delta and the Ganges Delta are the chief agricultural areas for Egypt and Bangladesh, for instance.

Rivers Through History

Rivers have always been important to people. In prehistoric times, people settled along the banks of rivers, where they found fish to eat and water for drinking, cooking, and bathing.

Later, people learned that the fertile soil along rivers is good for growing crops. The world’s first great civilizations arose in the fertile flood plains of the Nile in Egypt, the Indus in southern Asia, the Tigris and the Euphrates in the Middle East, and the Huang (Yellow) in China.

Centuries later, rivers provided routes for trade, exploration, and settlement. The Volga River in Eastern Europe allowed Scandinavian and Russian cultures, near the source of the river, to trade goods and ideas with Persian cultures, near the mouth of the Volga in southern Europe. The Hudson River in the U.S. state of New York is named after English explorer Henry Hudson, who used the river to explore what was then the New World.

When towns and industries developed, the rushing water of rivers supplied power to operate machinery. Hundreds of factories operated mills powered by the Thames in England, the Mississippi in the United States, and the Ruhr in Germany.

Rivers remain important today. If you look at a world map, you will see that many well-known cities are on rivers. Great river cities include New York City, New York; Buenos Aires, Argentina; London, England; Cairo, Egypt; Kolkata, India; and Shanghai, China. In fact, rivers are usually the oldest parts of cities. Paris, France, for instance, was named after the Iron Age people known as the Parisii, who lived on the islands and banks of the Seine River, which flows through the city.

Rivers continue to provide transportation routes, water for drinking and for irrigating farmland, and power for homes and industries.

Rivers of Europe

The longest river in Europe is the Volga. It flows approximately 3,685 kilometers (2,290 miles) across Russia and empties into the Caspian Sea. The Volga has been used for centuries to transport timber from northern forests, grain from farms along its valley, and manufactured goods. The river is also known for its sturgeon, a type of large fish whose eggs are used to make a famous delicacy—Russian caviar.

The Thames, in England, is one of Europe’s most historic rivers. Along its banks stands the city of London, a bustling urban area for more than a thousand years. By 100 CE, London had already become an important Roman settlement and trading post. Because of its location on the river and near the seacoast, London became England’s principal city and trade center.

Europe’s busiest river is the Rhine, which runs from the Alps in Switzerland, through Germany and the Netherlands, and empties into the North Sea. It flows through many industrial and farming regions and carries barges laden with farm products, coal, iron ore, and a variety of manufactured goods.

Rivers of Asia

Asia’s longest and most important river is the Yangtze, in China. It flows from the Dangla Mountains, between Tibet and China’s Qinghai province. It empties in the East China Sea 6,300 kilometers (3,915 miles) later. The Yangtze is a highway for trade through the world’s most populous country.

The Yangtze is also an agricultural river. Its valley is a major rice-growing region, and its water is used to irrigate fields. Many Chinese live on the river in houseboats or sailboats called junks.

The Yangtze River is the home of the world’s most powerful hydroelectric power plant, the Three Gorges Dam. Eventually, the plant will be able to constantly produce 22,500 megawatts of power. China’s rural population will have access to affordable electricity for homes, businesses, schools, and hospitals. Creating the Three Gorges Dam was one of the largest engineering feats in history. Engineers dammed the Yangtze, creating a 39.3-cubic-kilometer (31.9 million acre-foot) reservoir, or artificial lake.

The Ganges is the greatest river on Asia's Indian subcontinent. It is sacred to the millions of followers of the Hindu religion. For thousands of years, Hindus have worshipped the river as a goddess, Ganga Ma (Mother Ganges). Hindus believe the river’s water purifies the soul and heals the body. Millions of people use the Ganges every day for bathing, drinking, and industry.

The historic Tigris and Euphrates river system flows from Turkey through Syria and Iraq and into the Persian Gulf. The rivers lie in an area called the Fertile Crescent. The region between the two rivers, known as Mesopotamia, is the so-called “cradle of civilization.” The earliest evidence of civilization and agriculture—farming and domestication of animals—appears in the Fertile Crescent.

Rivers of North America

In North America, rivers served as highways for native tribes and, later, for European explorers.

French explorers began traveling the St. Lawrence and other rivers of Canada in the 1500s. They found an abundance of fish and other wildlife, and they encountered Native American tribes who hunted beaver. The explorers took beaver pelts back to Europe, where they were used to make fashionable hats. Soon, hunters explored and traveled networks of rivers in North America in search of beaver pelts. The establishment of trading posts along the rivers later opened the way for permanent European settlers.

The St. Lawrence River is still a major waterway. The river, which empties into the Atlantic, is linked to the Great Lakes by the St. Lawrence Seaway—a series of canals, locks, dams, and lakes. The St. Lawrence Seaway allows oceangoing ships to enter the interior of the continent.

The Mississippi is the chief river of North America. It flows approximately 3,766 kilometers (2,340 miles) through the heart of the United States, from its source in Minnesota to its delta in Louisiana and the Gulf of Mexico.

Spanish and French explorers first traveled the Mississippi in the 1500s and 1600s. In 1803, the United States bought almost the entire Mississippi River Valley from France as part of the Louisiana Purchase. After that, the Mississippi was widely traveled by traders and settlers on rafts, boats, and barges.

With the introduction of the steamboat, a new, industrial, era began on the Mississippi. Paddle wheelers carried trade goods up and down the river. Soon, workboats were joined by cruise ships and other luxurious passenger vessels. Writer Mark Twain, who was once a steamboat pilot on the river, described this era in his book Life on the Mississippi.

Over time, the Mississippi increased in importance as a trade route. Today, it carries cargo ships and barges in lines that may extend for more than a kilometer. Large quantities of petroleum, coal, and other bulky goods are conveyed on the river by massive barges pushed by powerful towboats.

North America’s Colorado River is famous for forming the Grand Canyon in Arizona. For millions of years, the river has cut its way through layers of rock to carve the canyon. Long ago, the river flowed through a flat plain. Then the Earth’s crust began to rise, lifting the land. The river began cutting into the land. The Grand Canyon is now about one and a half kilometers (one mile) deep at its deepest point, and 29 kilometers (18 miles) wide at its widest.

Rivers of South America

The strength of the Amazon River in South America dwarfs other rivers on the planet. The amount of water flowing through the Amazon is greater than the amount carried by the Mississippi, the Yangtze, and the Nile combined.

The Amazon begins as an icy stream high in the Andes mountains of Peru. It flows through Brazil and empties into the Atlantic Ocean. The Amazon and its tributaries drain a basin that covers an area equal to three-fourths of the contiguous United States.

The first Europeans to see the Amazon were Spanish explorers, who traveled it in the 1500s. They encountered a group of people who all appeared to be women, or so the story goes. The explorers called them Amazons, after female warriors described in Greek mythology. The name Amazon was later given to the river.

For much of its course, the Amazon flows through the world’s largest tropical rain forest. The region has abundant and unusual wildlife, including flesh-eating fish called piranhas; huge fish called pirarucu, which can weigh more than 125 kilograms (275 pounds); and giant snakes called anacondas.

Some Amazon tribes remain independent of Western culture. The Tagaeri people, for instance, continue to live a nomadic life based around the Amazon and its tributaries in the rain forest of Ecuador. Because of the demand for timber from the rain forest, the land of the indigenous people of the Amazon is shrinking. Today, there are fewer than 100 Tagaeri living in the rain forest.

Rivers provide energy to many South American communities. The Itaipú Dam crosses the Paraná River on the Brazil-Paraguay border. Construction of the dam required the labor of thousands of workers and cost more than $12 billion. The dam’s power plant can regularly produce some 12,600 megawatts of electricity. The huge reservoir formed by the dam supplies water for drinking and for irrigation.

Rivers of Africa

Africa’s two largest rivers are the Nile and the Congo.

One tributary of the Nile, the White Nile, flows from tiny streams in the mountains of Burundi through Lake Victoria, Africa’s largest lake. The other tributary, the Blue Nile, begins in Lake Tana, Ethiopia. The two join at Khartoum, Sudan. The Nile then flows through the Sahara Desert in Sudan and Egypt, and empties into the Mediterranean Sea. Because the area where the tributaries meet is close to the two sources of the Nile, the area is called the Upper Nile, even though it is farther south geographically. The Lower Nile runs through Egypt.

One of the earliest civilizations in the world developed along the Lower Nile. Ancient Egyptian civilization arose about 5,000 years ago. It was directly related to the Nile and its annual flooding. Each year, the river overflowed, spreading rich sediment across its broad flood plain. This made the land extremely fertile. Egyptian farmers were able to grow plentiful crops. In fact, ancient Egyptians called their land Kemet, which means “Black Land,” because of the rich, black soil deposited by the river.

Egyptians also used the Nile as a major transportation route to both the Mediterranean and the African interior. The Pschent, or double crown worn by Egyptian monarchs, combined symbolism from both the Upper Nile and Lower Nile. A tall, white crown shaped like a bowling pin represented the lands of the Upper Nile. This crown was combined with a pointy red crown that had a curly wire protruding from the front. The red color symbolized the red soils of Lower Egypt, while the curly wire represented a honeybee. When putting on the Pschent, an Egyptian ruler assumed leadership for the entire Nile.

The Nile provided enterprising Egyptians with material to form a powerful civilization. From papyrus, a tall reed that grew in the river, Egyptians made a sort of paper, as well as rope, cloth, and baskets. Egyptians also built great cities, temples, and monuments along the river, including tombs for their monarchs, or pharaohs. Many of these ancient monuments are still standing.

The Congo River flows across the middle of Africa, through a huge equatorial rain forest, before emptying in the Atlantic Ocean. The Congo is second only to the Amazon in terms of water flow. It is the deepest river in the world, with measured depths of more than 230 meters (750 feet). Huge urban areas, including the capital cities of Brazzaville, Republic of Congo, and Kinshasa, Democratic Republic of Congo, sit on the banks of the river.

In the Democratic Republic of Congo, the river is the principal highway for transporting goods such as cotton, coffee, and sugar. Boats traveling the river range from dugout canoes to large freighters.

The river also supplies an abundance of fish to central Africa. Fishermen use baskets and nets hung from high poles across rushing falls and rapids to catch fish. They also use more traditional nets operated from either onshore or on boats.

Rivers of Australia

Much of Australia is arid, but rivers still run through it. Australia’s principal rivers are the Murray and the Darling, both in the southeastern part of the continent. The Murray flows some 2,590 kilometers (1,610 miles) from the Snowy Mountains to a lagoon on the Indian Ocean. Near the town of Wentworth, the Murray is joined by the Darling, a 2,739-kilometer (1,702-mile) river that flows from the highlands of the eastern coast.

Indigenous Australians placed great importance on the Murray River. The Murray valley had the greatest population density on the continent before the arrival of Europeans in the 1600s.

By the mid-1800s, European farmers had settled along both rivers and some of their tributaries. Most Australian farmers raised sheep and cattle. Riverboats began plying the waters, and towns grew up along the banks.

Much of Australia’s farmland still lies within the Murray-Darling basin, where river water irrigates some 1.2 million hectares (3 million acres). The region is the chief supplier of the country’s agricultural exports—wool, beef, wheat, and oranges.

Polluted Rivers

For centuries, people have depended on rivers for many things. Rivers have provided waterways for shipping, convenient construction sites for cities, and fertile land for farming. Such extensive use of rivers has contributed to their pollution. River pollution has come from directly dumping garbage and sewage, disposal of toxic wastes from factories, and agricultural runoff containing fertilizers and pesticides.

By the 1960s, many of the world’s rivers were so polluted that fish and other wildlife could no longer survive in them. Their waters became unsafe for drinking, swimming, and other uses. One of the most famous examples of a polluted river was the Cuyahoga. The Cuyahoga is a busy river in the U.S. state of Ohio that empties into Lake Erie. It is a major highway for goods and services from the Midwest to the Great Lakes. In 1969, the oily pollution in the Cuyahoga was so great that the river actually caught fire—something it had done more than a dozen times in the 19th and 20th centuries.

Since the 1969 fire, stricter laws have helped clean up polluted rivers. The laws have restricted the substances factories can dump into rivers, limited the amount of agricultural runoff, banned toxic pesticides such as DDT, and required treatment of sewage.

Although the situation in some parts of the world has improved, serious problems remain. The Citarum River in Indonesia, for instance, is often cited as the most polluted river in the world. Textile factories near the Citarum dump toxic wastes into the river. The garbage floating on top of the river is so thick that water is invisible.

Even after communities have limited river pollution, toxic chemicals may remain. Many pollutants take years to dissolve. The pollutants also build up in the river’s wildlife. Toxic chemicals may cling to algae, which are eaten by insects or fish, which are then eaten by larger fish or people. At each stage of the river’s food web, the amount of the toxic chemical increases.

In parts of North America and Europe, there is also the severe problem of acid rain. Acid rain develops when emissions from factories and vehicles mix with moisture in the air. The acid that forms can be toxic for many living things. Acid rain falls as rain and snow. It builds up in glaciers, streams, and lakes, polluting water and killing wildlife.

Environmentalists, governments, and communities are trying to understand and solve these pollution problems. To provide safe drinking water and habitats where fish and other wildlife can thrive, rivers must be kept clean.

Dams

A dam is a barrier that stops or diverts the flow of water along a river. Humans have built dams for thousands of years.

Dams are built for many purposes. Some dams prevent flooding or allow people to develop or “reclaim” land previously submerged by a river. Other dams are used to change a river’s course for the benefit of development or agriculture. Still others provide water supplies for nearby rural or urban areas. Many dams are used to provide electricity to local communities.

In 1882, the world’s first hydroelectric power plant was built on the Fox River in the U.S. city of Appleton, Wisconsin. Since then, thousands of hydroelectric plants have been built on rivers all over the world. These plants harness the energy of flowing water to produce electricity. About 7 percent of all power in the United States, and 19 percent of power in the world, comes from hydroelectric plants. China is the world’s largest producer of hydroelectric power.

Hydroelectric power is renewable because water is constantly replenished through precipitation. Because hydroelectric plants do not burn fossil fuels, they do not emit pollution or greenhouse gases. However, hydroelectric power does have some negative effects on the environment.

Dams and hydroelectric plants change the flow and temperature of rivers. These changes to the ecosystem can harm fish and other wildlife that live in or near the river. And although hydroelectric plants do not release greenhouse gases, rotting vegetation trapped in the dams’ reservoirs can produce them. Decaying plant material emits carbon dioxide, a major greenhouse gas, into the atmosphere.

Dams also have an effect on people living near the rivers. For example, more than 1.3 million people had to move from their homes to make way for China’s Three Gorges Dam and its reservoir. Human rights organizations claim that many of these people did not receive the compensation they were promised in return for being displaced.

In addition, dams can affect fish populations and the fertility of flood plains. Fish may not be able to migrate and spawn. Farmers that depended on the fertile flooding may be cut off from the river by a dam. This can harm the livelihood of fishermen and farmers who live along the river, as well as consumers who must pay higher prices for food.

Dams with very large reservoirs may also trigger earthquakes. Earthquakes happen when two or more of the tectonic plates that make up Earth’s crust slide against each other. The weight of the water in the reservoirs can cause existing cracks, or faults, in these plates to slip and create an earthquake.

River Management

River management is the process of balancing the needs of many stakeholders, or communities that depend on rivers. Rivers provide natural habitats for fish, birds, and other wildlife. They also provide recreation areas and sporting opportunities such as fishing and kayaking.

Industries also depend on rivers. Rivers transport goods and people across continents. They provide affordable power for millions of homes and businesses.

Farmers and agribusinesses often rely on rivers for transportation. Rivers also supply water for irrigation.

River managers must consider the needs of all the current and future stakeholders.