Rivers of the World

The Parts of a River

Every river has a beginning and an end. Its source, or headwaters, may be a spring, a lake, a melting glacier or simply the farthest little stream. Its mouth is where it empties into a lake, a bigger river or the sea. Along the way, geographers use a few key words.

1. Source. Where the river begins, often high in hills or mountains.
2. Tributary. A smaller stream or river that flows into a bigger one.
3. Confluence. The place where two rivers meet and join.
4. Meander. A looping bend. Rivers often meander where the land is flatter.
5. Floodplain. Flat, low land beside a river that floods from time to time. Floods leave fresh mud behind, which makes the soil good for farming.
6. Delta. Land built from mud and sand dropped where a river slows down at its mouth. The river often splits into several channels there.
7. Mouth. Where the river ends and its water flows into the sea, a lake or another river.

Drainage Basins and Divides

A drainage basin, also called a watershed or catchment, is all the land that drains water to the same river or outlet. Wherever you are standing, you are standing in one. The high ground that separates one basin from the next is called a drainage divide. Rain falling on one side of a divide ends up in one river; rain falling on the other side ends up in a different one.

Big basins hold many smaller ones, like nesting bowls. Because everything in a basin drains to the same place, what happens upstream, such as farming, building or pollution, affects the water downstream.

The World's Great Rivers

There is more than one way to call a river "the biggest." You can measure its length, how much water it carries or how much land it drains.

The Nile is usually listed as the longest river. The Amazon is the biggest by far in the other two ways. It drains about 7 million square kilometers, the largest basin on Earth, and it carries about one-fifth of all the water that runs off the world's land into the oceans.

The Yangtze is the longest river in Asia, and its basin is home to hundreds of millions of people. The Mississippi and its tributaries drain about 40 percent of the continental United States.

How Do We Know How Long a River Is?

You might think a river's length is easy to look up. It isn't. Different sources give different numbers for the same river, because measuring one raises tricky questions:

• Where is the source? A big river has many headwater streams. Which one counts as the "real" start? For the Amazon, scientists still argue about which stream in the Andes is the true source.
• Where is the mouth? The Amazon spreads into a huge estuary with several outlets. Where the river ends and the ocean begins is partly a choice.
• How closely do you follow the bends? Measuring every small meander on a detailed map gives a longer answer than a rougher map does.
• Rivers change. Floods, dams and new channels change a river's path over time.

That is why some researchers argue the Amazon is longer than the Nile. Even for one river, official numbers vary. The National Park Service says the Mississippi is about 2,350 miles long from Lake Itasca to the Gulf of Mexico (which the U.S. government has called the Gulf of America since 2025), and it notes that the U.S. Geological Survey gives 2,300 miles and the EPA 2,320 miles. When you see a river length, check who measured it.

Rivers and the First Cities

Some of the world's earliest cities and states grew up in river valleys. Rivers gave people water to drink and water for crops. Floods spread rich mud across the floodplains, and boats carried goods and ideas. Four famous river-valley civilizations were:

• Mesopotamia, between the Tigris and Euphrates rivers in today's Iraq. Its name means "land between the rivers."
• Ancient Egypt, along the Nile.
• The Indus Valley civilization, along the Indus River in today's Pakistan and northwest India.
• Early China, in the valley of the Huang He, or Yellow River.

Today, rivers still shape where people live. Many great cities sit on rivers, including Cairo on the Nile, Shanghai at the mouth of the Yangtze and Budapest on the Danube.

Dams and River Power

A dam is a wall across a river that holds back water in a lake called a reservoir. Dams can store water for farms and cities, control floods and spin turbines to make hydroelectric power. They also flood land, can force people to move, trap the mud that builds up farmland and deltas, and block fish that swim upriver.

• Three Gorges Dam, China. Built across the Yangtze, it is the world's largest hydroelectric plant by power capacity, about 22,500 megawatts. The dam was completed in 2006, and the power plant was in full operation in 2012.
• Aswan High Dam, Egypt. Finished in 1970 on the Nile, it ended the river's yearly flood in Egypt. That made farming more reliable and protected towns, but the mud that once fed the fields and the delta now stays behind the dam.
• Grand Ethiopian Renaissance Dam (GERD), Ethiopia. Built on the Blue Nile near the border with Sudan and completed in 2025, it is Africa's largest hydroelectric dam, with a capacity of 5,150 megawatts. Ethiopia says it will bring electricity to millions of people. Egypt, which depends on the Nile for almost all of its water, has objected to how it was filled and run.

Shared Rivers

Rivers don't stop at borders, so many countries often share one river. What one country does upstream affects the countries downstream. That makes rivers a reason for both cooperation and conflict.

• The Danube is about 2,857 km long and flows through 10 countries, more than any other river. Its basin reaches into 19 countries, making it the most international river basin in the world. Its countries work together through a commission, the ICPDR.
• The Nile basin is shared by 11 countries. Upstream countries like Ethiopia want to use the river for power and farming, while Egypt, far downstream, worries about losing water.
• The Mekong flows through six countries: China, Myanmar, Laos, Thailand, Cambodia and Vietnam. In 1995, the four downstream countries signed an agreement and set up the Mekong River Commission to share information and plan together. China and Myanmar are dialogue partners, not members.

Rivers Under Pressure

People take water from rivers for farms, homes and factories. Sometimes they take so much that a river runs dry before it reaches the sea.

• The Colorado River. It supplies water to millions of people in the U.S. Southwest and northern Mexico. Because of dams and water use, it rarely reaches its mouth at the Gulf of California. In 2014, the United States and Mexico released a planned "pulse flow" of water, and in May 2014 the river reached the sea for the first time in many years.
• Freshwater wildlife. The 2024 Living Planet Report found that monitored populations of freshwater animals, such as fish, frogs and river dolphins, fell by an average of 85 percent between 1970 and 2020. That is the average change in the size of the populations scientists track, not the share of all animals lost. Dams, pollution, too much water use and overfishing are among the causes.

There is good news too. The Danube countries have cut pollution by working together, and the Colorado pulse flow showed that planned releases can bring water back to a dry delta for a time.

Links to Other Units

• Landforms and Water: the water cycle, valleys, deltas and waterfalls.
• Oceans and Seas: where river water ends up.
• Food and Farming: irrigation and the first farms in river valleys.
• Natural Hazards: floods and how people prepare for them.

Self-Guided Quiz: Rivers of the World

Ten questions. Choose one answer for each, then check your score. You can try as many times as you like.

Credits

Written by Marco for Jugami's World Geography course. The maps and diagrams are original SVG illustrations made for this page from the data in the sources. Land and river outlines come from Natural Earth (public domain). Lengths, flows and areas are rounded, and different sources give different values.