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 meters 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 years, scientists have debated which river is longer. Measuring a river is difficult because it is hard to pinpoint its exact beginning.

The Amazon River is about 6,437 kilometers (4,000 miles) long, while the Nile River is about 6,650 kilometers (4,135 miles) long. There is no debate, however, that the Amazon is the largest river by volume.

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, which carry moisture over land and release it as fresh water in precipitation. This fresh water feeds rivers and smaller streams. The movement of water between land, ocean and air is called the water cycle. The water cycle constantly replenishes freshwater sources, which is essential for almost all living things.

Anatomy of a River

The beginning of a river is called its source or headwaters. The source may be a melting glacier, such as the Gangotri Glacier, which is the main 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.

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. Some rivers have many 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, such as a large 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. Rivers have sculpted many landforms over time, like the Colorado River carving out the Grand Canyon in the U.S. state of Arizona. This happens through two processes. One process is called weathering, or the breaking down of rocks from water movement and other processes. The other is erosion, which happens when wind or water move the broken-down particles from weathering. 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. This movement of water in a river is called a current. Little by little, the current of a river tears away rocks and soil along its bed and carries them downstream. This carves a narrow, V-shaped valley through the riverbed. Rapids and waterfalls are common in 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 material and soil from upstream and from the banks of the river's meanders. Because of this, a river deposits very fertile soil along its flood plain. A flood plain is the area next to the river that is subject to flooding.

Some rivers deposit so much sediment that it partially blocks the flow of the river to the sea, forming a delta. Deltas almost always have fertile soil. For instance, the Nile Delta and the Ganges Delta are the chief agricultural areas for Egypt and Bangladesh, respectively.

Rivers Through History

Rivers have always been important to people. The world’s first great civilizations arose in the fertile flood plains of rivers: the Nile in Egypt, the Sindu (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 and contributed to the spread of colonialism. For example, the Volga River in Eastern Europe allowed people from Scandinavian and Russian cultures, near the source of the river, to trade goods and ideas with people from 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 the English explorer Henry Hudson, , whose navigation and understanding of the river helped Europeans seize land from Indigenous people during the colonial period.

When towns and industries developed along the rivers, the rushing waters supplied power to operate machinery. Factories operated mills powered by the Thames in England and the Mississippi in the United States.

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 Buenos Aires, Argentina; Cairo, Egypt; Kolkata, India; and Shanghai, China. Rivers continue to provide transportation routes, water for drinking and irrigating farmland, and power for homes and industries.

However, rivers around the world are facing challenges to their survival. Some challenges are caused by human-made dams, which are barriers that stop or divert the flow of water along a river. People build dams for a variety of purposes. Some dams prevent flooding or curb erosion. Others provide water supplies for agricultural purposes, such as irrigation, or for human consumption in nearby rural or urban areas. Dams can also allow for easier navigation, particularly in inland areas. Since the 1880s, dams have also been used to generate electricity. Hydroelectric power plants are found all over the world, harnessing the energy of flowing water to produce electricity. About 7% of all power in the United States comes from hydroelectric plants. One-sixth of the world’s energy came from hydroelectric power, or hydropower, in 2020. China is the world’s largest producer of hydropower.

Hydropower is a renewable energy source because water is constantly replenished through precipitation. Because hydroelectric plants do not burn fossil fuels, they do not emit greenhouse gases. However, the dams that generate hydroelectric power 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. Dams also break up rivers, which makes it harder for fish and other animals to travel, and reservoirs created by dams can destroy previously existing habitats.

Pollution is another problem facing rivers worldwide. Humans’ extensive use of rivers has contributed to their pollution. River pollution comes from garbage that is dumped or washed into rivers and from improper disposal of toxic wastes from factories. Agricultural runoff containing fertilizers and pesticides also pollutes rivers. 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 region. In 1969, the oily pollution in the Cuyahoga from the dumping of industrial waste was so great that the river actually caught fire from what was likely a flare or sparks from a train overhead.

In the United States, this incident helped fuel the environmental movement of the 1970s. After the Environmental Protection Agency was founded in January 1970, it began regulating the treatment of rivers and other bodies of water with the Clean Water Act of 1972. This act stops businesses and local governments from putting sewage and waste into rivers, which had been legal prior to this legislation. It also instituted limits to the allowable levels of pollutants. Still, some argue that the Clean Water Act did not do enough to clean up rivers, in part because there are legal loopholes that industries have used to challenge the act. Many of the world’s most significant rivers, such as the Rhine in Europe, the Ganges in India and the Mississippi in the United States face extreme pollution or degradation.

Rivers of Europe

The longest river in Europe is the Volga. It flows approximately 3,700 kilometers (2,299 miles) across Russia and empties into the Caspian Sea. The Volga has been used for many years to transport timber and grain. The river is also known for its sturgeon called the beluga sturgeon (Huso huso), a type of large fish whose eggs are used to make a famous delicacy: Russian caviar. However, these fish are in a major decline. Heavy metals from agriculture and other industries have polluted the river and killed many fish. Dams that break up the river and make it harder for fish to navigate the river have decreased the number of beluga sturgeons as well.

The Thames, in England, is one of Europe’s most historic rivers. Along its banks is the city of London, a bustling urban area that has stood for more than a thousand years. In the first century C.E., 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 in terms of transport is the Rhine, which runs from the Alps in Switzerland, through Germany and the Netherlands, and empties into the North Sea. It flows through industrial regions, which has contributed to its rising pollution levels. About 6,000 toxic chemicals have been found in the river, and some efforts, such as a program to revive the native salmon population, have not yet had great success. Still, organizations like Rhine Clean Up are monitoring the river and working toward restoring its natural vibrancy.

Rivers of Asia

Asia’s longest river is the Yangtze, in China. It flows from the Tanggula Mountains, between Tibet and China’s Qinghai province, and empties into the East China Sea 6,300 kilometers (3,915 miles) later. The Yangtze River Basin is major source of agricultural products, including more than 66% of rice produced in the country. It is home to hundreds of species of fish, which, in addition to contributing to a vibrant ecosystem, supports a large fishing industry in the Yangtze Delta.

The Yangtze River is the home of the world’s most powerful hydroelectric power plant, the Three Gorges Dam. The dam spans 2,335 meters (7,660 feet) and set a world record in 2020 for producing about 111.8 terawatt hours of electricity. In addition to producing renewable energy, the dam has a human-made reservoir that is designed to reduce flooding downstream.

Though the dam’s engineers have been admired for building the mega dam, a significant technical accomplishment, there has been controversy surrounding the Three Gorges Dam. More than 1.3 million people were forced out of their homes to make way for the dam and its reservoir. Experts and journalists claim that many of these people did not receive the compensation they were promised in return for being displaced. Additionally, significant archaeological sites, including those of ancient tribes like the Ba ethnic group, were destroyed in order to build the dam.

The dam has also increased earthquakes and landslides in the region. Dams with very large reservoirs, like the Three Gorges Dam, can 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.

The Ganges is the most significant river on Asia's Indian subcontinent. It is sacred to the followers of the Hindu religion. Hindus have long worshipped the river as a goddess, Ganga Ma (Mother Ganges). Hindus believe the river’s water purifies the soul. River access has, however, been a source of tension and conflict. Some Hindu people on the political right have tried to block non-Hindus from using the river.

Hindus, Muslims and other local communities use the Ganges every day for agriculture and as a source of drinking water. At the same time, the Ganges has high levels of pollution from industries like agriculture and textile plants, as well as pollution from sewage. The Ganges faces other threats too. Rising temperatures from human-caused climate change are also expected to increase the number and severity of droughts affecting the river. If people and governments do not take greater action to mitigate some of these harmful effects, experts believe the 40 million people who rely on the river may need to migrate away from the Ganges watershed.

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 where the earliest civilizations formed. The earliest evidence of agriculture—farming and domestication of animals—appears in the Fertile Crescent. Similar to the Ganges, the rich cultural history of the rivers is at risk because of pollution from sewage and conflict and droughts from climate change. A combination of increasing temperatures due to climate change and strain from extracting water for agricultural purposes in an arid climate has led to decreasing water levels. Scientists have warned that the Euphrates River could go dry by 2040.

Rivers of North America

The Mississippi is the chief river system 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. Because the Mississippi and Missouri rivers are close in length, experts debate which one is the longest in North America, though most sources say the Missouri River is the longest. But, because the Missouri flows into the Mississippi, it is safe to say that the Mississippi river system is the longest in North America and the fourth longest in the world.

People throughout history, starting with Indigenous Americans, have used the Mississippi for commerce and agriculture. The word “Mississippi” comes from an Anishinaabe word (as transcribed by a French colonizer) for “Great River.” Indigenous people have long had settlements along the river, including a major urban center now called Cahokia. This city, which experts believe developed around 800 C.E. and dismantled between 1300 and 1400, had a population as high as 20,000. It covered an area of around 16 square kilometers (6 square miles)—larger than London was at the time—and the Mississippi River was instrumental in the culture being able to thrive.

Later, European colonizers began exploring the river and started to use it for commerce, eventually pushing many Indigenous people off their ancestral lands with coercive treaties or force in order to use the river for their own benefit. During the 1800s, people in the United States used the river for the slave trade. The river was also a feature of the Underground Railroad, as enslaved people in Missouri, for example, would brave crossing the river into the free state of Illinois.

Today, the river is still used for commerce and around 175 million tons of freight go through the upper Mississippi each year. The largest port district in the world is Louisiana’s Mississippi River Delta. Because of writers like Mark Twain, the Mississippi has cultural significance that is known throughout the United States and the world.

However, the Mississippi is under threat from environmental concerns. Runoff from urban areas and farmland has flowed into the river. This runoff gathers in the Gulf of Mexico, where the river meets the sea. It pollutes the water to the point that there is not enough oxygen to support life. This “dead zone” kills fish and harms the ecosystem. The nearly 800 animal species that depend on the river for life are at extreme risk, and the Mississippi is one of the most endangered rivers in America.

The St. Lawrence River is another major waterway in North America, spanning 1,197 kilometers (744 miles). The river, which empties into the Atlantic Ocean, is linked to the Great Lakes by the St. Lawrence Seaway, a series of canals, locks, dams and lakes. The St. Lawrence Seaway allows ocean-bound ships to enter the interior of the continent.

An Indigenous tribe, likely Iroquoian, had permanent settlements along the St. Lawrence River in what is now Quebec and Montreal in the late 1500s. However, when French colonizers began taking control of the land and founding their own cities in the early 1600s, the tribe left the settlement and were possibly absorbed by neighboring nomadic tribes like the Innu, either by war or by choice. Colonizers continued to develop commerce along the river, making it a major contributor to the success of the timber trade in the region. Today, the larger system that includes the St. Lawrence River ships 200 million net tons of cargo each year.

One environmental concern facing the St. Lawrence River is invasive species. Invasive species are non-native plants and animals that disrupt the local ecosystem. In recent years, the Saint Regis Mohawk Tribe, who are indigenous to the area, were monitoring the river and discovered an invasive fish called tench (Tinca tinca). Tenches are native to Europe and Western Asia. These fish disturb the St. Lawrence ecosystem in many ways, including competing with native fish for food. The Saint Regis Mohawk Tribe continues to monitor the situation and look for ways to stop the species from creating permanent damage.

Rivers of South America

The Amazon River in South America transports the greatest volume of water of any river in the world and is at least 6,400 kilometers (4,000 miles) long. It begins as an icy stream high in the Andes Mountains of Peru. It flows through Brazil and empties into the Atlantic Ocean. While most sources state that it is the second-longest river in the world, experts have debated where the river truly begins, and some claim it is longer than the Nile and therefore the world’s longest river.

For much of its course, the Amazon flows through the world’s largest tropical rainforest. The region has abundant wildlife, including flesh-eating fish called red bellied piranhas (Pygocentrus nattereri); huge fish called pirarucu (Arapaima gigas), which can weigh up to 200kg (445 pounds); and giant snakes called green anacondas (Eunectes murinus).

Colonial history has had a large effect on the Amazon. The name “Amazon” comes from a Spanish colonizer who claimed to have encountered Indigenous tribes with female warriors. He called these women “Amazons” after the female warrior figures from Greek myths. Though the Eurocentric name has spread throughout the world, locals have names for different sections of the river. For example, Peruvians refer to their section of the Amazon as Marañón River. While colonialism has resulted in many Indigenous people along the Amazon losing their way of life, others have found ways to retain their traditional cultures. The Tagaeri people, for instance, continue to live a nomadic life based around the Amazon and its tributaries in the rainforest of Ecuador. However, the ancestral lands of the Tagaeri are being taken away from them by oil companies using the land to expand oil drilling to fulfill high worldwide demand. Because of pressure from land encroachment and violence against them, representatives from the Tagaeri community are publicly advocating for their right to live and move freely without interference.

Rivers provide energy to many South American communities, sometimes at a cost to the environment. The Itaipú Dam crosses the Paraná River on the Brazil-Paraguay border. Construction of the dam was a technical and architectural challenge and cost more than $12 billion. The dam’s power plant can produce a maximum of 14,000 megawatts of electricity. But this electricity production has downsides. Guaíra Falls, formerly located on the Upper Paraná River, was one of the biggest waterfalls on the planet and was a popular tourist destination. The falls, above which was a permanent rainbow, were destroyed when the dam was built and a reservoir was created. There are social issues exacerbated by the creation of the dam as well. Brazil and Paraguay built the dam together and share the energy it produces. However, they have a treaty that says that Paraguay must sell the energy it doesn’t use back to Brazil at a lower price than other countries. Paraguay believes this treaty is unfair, increasing tensions between the two countries.

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.

One of the earliest civilizations in the world developed along the Lower Nile, the northern portion of the river. 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.

Today, the Nile is vital to life in Egypt and other countries along its path. About 95% of the population of Egypt lives within 20 kilometers (12.4 miles) of the river. Water from the Nile is used for drinking, agriculture and more. However, this demand has resulted in overuse and depletion of the Nile’s waters. With heat waves made longer and hotter by climate change, the water in the river could decrease by as much as 70% by 2100, according to experts.

The Congo River flows across the middle of Africa 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 220 meters (720 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 many of the countries that the Congo flows through, the river is the principal highway for transporting people, though goods are transported as well. When the countries of the Congo Basin took power back from colonial European rulers, the Europeans left the countries politically unstable and without infrastructure such as roads and trains. Today, people of the Congo continue to rely on the river to travel between cities that have poor or nonexistent transportation by land.

The river also supplies fish to central Africa. The Enya, an ethnic group who live by and rely on the river, use a traditional method of fishing. It involves putting woven baskets and nets on poles across rushing falls and rapids to catch fish. They also use more traditional nets operated from either onshore or on boats. But pollution, particularly from industrial sources, is threatening this tradition. Mining is a major source of pollution for the river. In 2021, a diamond mine in Angola leaked toxic chemicals into a tributary of the Congo, turning it red. In addition to contaminating and killing countless fish, an estimated 4,500 people became ill, and 12 people died.

Rivers of Australia

Much of Australia is arid, but rivers still run through it. Australia’s principal rivers are the Murray and the Baaka (Darling), both in the southeastern part of the continent, flowing through what is now Queensland, New South Wales, Victoria and South Australia. The Murray flows about 2,500 kilometers (1,553 miles) from the Snowy Mountains to the ocean. Near the town of Wentworth, the Murray is joined by the Baaka (Darling), a 2,739-kilometer (1,702-mile) river that flows from the highlands of the eastern coast.

Indigenous Australians place great importance on the Murray River and its tributaries. They not only live and work by the river, but it is an important part of their spiritual lives as well. Indigenous people of Australia have inhabited the land around these rivers for thousands of years. Artifacts from Indigenous people have been found at Lake Mungo, a dry lake that was formerly part of the river system, that date back 40,000 years. This makes the civilization there one of the oldest in the world.

When Europeans began colonizing the country in the 1800s, they used the Murray River for irrigation for agriculture and began to build dams and institute other river management practices, which altered its flow. These practices have led to environmental damage. Indigenous tribes like the Barkandji have been fighting for water rights to the river, arguing that they used the river sustainably for thousands of years, and they will be able to restore the river under their management. Leaders from the tribe say the New South Wales government often shuts them out of the process. There are successful stories of indigenous water management in the Murray system, however. The Nari Nari Tribal Council was able to secure water rights from the Australian government for their ancestral homeland and restored it to a thriving wetland.

River Management and the Future of Rivers

River management is the process of balancing the needs of many stakeholders, including people and communities, that depend on rivers. River management of the past has included dams, levees and locks, but these systems have often caused harm to the environment. For example, scientists say 40% of fish found in the United States are imperiled because of human-made changes to rivers.

For that reason, scientists are looking for ways to sustainably manage rivers in ways that restore their habitats or at least prevent further harm. An initiative working toward this goal is the Sustainable Rivers Program (SRP), a joint venture by the U.S. Army Corps of Engineers and the Nature Conservancy. This organization studies rivers and creates plans for better management, accounting for the way the natural flow of the river influences the ecosystem. In 2022, the program worked on 44 rivers across the United States. Scientists who work as water or environmental analysts, engineers and more contribute to the success of organizations like these. Individuals can also get involved by participating in projects for their local rivers, stopping runoff from leaving their own yards and advocating for conservation to local, state and federal governments.

While certain governments restrict or bar some Indigenous people from water management, in other places, scientists are looking to Indigenous expertise for guidance. In Peru, for example, Indigenous people use their knowledge of the mountains to dig amunas, or canals, in spots where water can collect. Today, people are building new amunas to guide water from the mountains into modern cities like Lima. This technology also costs significantly less than dams.

Some tribes are also experiencing success in removing dams and gaining water rights. After years of advocacy, the tribes along the Klamath River in Oregon in the United States had several dams removed between 2023 and 2024. These dams almost completely destroyed the salmon population in the river, which was essential to the diet and culture of Indigenous tribes like the Yurok. Now, experts expect river management guided by Indigenous groups will restore the salmon population. This same dam removal project also benefits the Shasta tribe, located in California. The dams created a reservoir that drowned culturally significant lands. The Shasta tribe now has access to those lands and will be able to manage the water system.

With all stakeholders working together, sustainable river management can begin to tackle the great challenges that rivers are facing and will continue to face in the future.