The Amazon River: How the World's Mightiest River Works

A River Without Equal

The Amazon River crosses almost the entire width of South America, from the snowy peaks of the Andes in Peru to the Atlantic Ocean on the coast of Brazil. By the amount of water it carries, it is the largest river in the world by a huge margin. On average, more than 200,000 cubic metres of water flow out of its mouth every second. That is more than the next seven largest rivers in the world put together, and roughly one-fifth of all the fresh water that rivers deliver to the world's oceans.

Its length is a different story. The Amazon is either the longest or the second-longest river on Earth, and the title is disputed with the Nile in Africa. Most reference works give the Amazon a length of about 6,400 kilometres and the Nile about 6,650 kilometres, but some researchers have measured the Amazon at close to 7,000 kilometres. Why scientists cannot agree is one of the more surprising parts of the river's story.

Where the Amazon Begins and Ends

Sources high in the Andes

The Amazon starts as small streams of meltwater and rain in the Andes of southern Peru, less than 200 kilometres from the Pacific Ocean. Even so, all that water ends up flowing thousands of kilometres east to the Atlantic, because the high wall of the Andes blocks the way to the nearby Pacific.

For decades, the most distant source was considered to be a stream on the slopes of Nevado Mismi, a mountain in the Arequipa region of Peru, whose waters feed the Apurímac River. In 2014, a study argued that the Mantaro River, also in the Peruvian Andes, reaches farther from the mouth and should count as the true source. The debate shows how hard it is to pick a single starting point for a river fed by countless mountain streams.

Many names for one river

The river changes its name several times along the way. In Peru, two great rivers, the Marañón and the Ucayali, meet and form what is usually called the Amazon. After crossing into Brazil, the river is known locally as the Solimões. Only below the city of Manaus, where it joins the Rio Negro, do Brazilians call it the Amazonas.

A mouth the size of a sea

Near the Atlantic, the Amazon spreads into a vast estuary, a wide zone where the river meets the ocean. In the middle of it lies Marajó, one of the largest islands in the world surrounded by both river and sea water, roughly as large as Switzerland. The river pushes so much fresh water into the Atlantic that the sea surface stays less salty for a long distance offshore. In 1500, the Spanish navigator Vicente Yáñez Pinzón sailed into this area and was astonished to find drinkable water far out at sea, so he named it Mar Dulce, the Sweet Sea.

Why the Length Is Disputed

Measuring a river sounds simple, but it requires two decisions: where it starts and where it ends. Both are tricky for the Amazon.

• The source: choosing the Mantaro instead of the Apurímac headwaters adds dozens of kilometres to the total.
• The mouth: the Amazon has no single clear end. Researchers must decide whether to follow the main channel north of Marajó or to include the channels south of the island that connect to the Pará River estuary. Including them makes the river much longer.
• The path: rivers wind and shift. Every bend and side channel changes the measurement, and the Amazon moves its channels over time.
• The tools: estimates depend on the maps or satellite images used and on how finely the curves are traced.

In 2007, a team of Brazilian researchers using satellite images calculated a length of nearly 7,000 kilometres, which would make the Amazon slightly longer than the Nile. Many geographers have not accepted this method, so most encyclopedias still list the Nile as longer. Whatever the final answer, there is no argument about volume: no other river comes close.

The Size of the Amazon Basin

A river's drainage basin is all the land whose rainwater eventually flows into it. The Amazon basin covers about 7 million square kilometres, roughly 40 percent of South America. It stretches across parts of Brazil, Peru, Bolivia, Colombia, Ecuador, Venezuela and Guyana, with the largest share, about 60 percent, in Brazil.

More than a thousand tributaries

Over 1,100 tributaries feed the Amazon, and several of them would be among the world's great rivers on their own. The Madeira is the longest tributary, at over 3,000 kilometres. Other major ones include the Rio Negro, the Japurá, the Purus, the Juruá, the Tapajós and the Xingu. The Rio Negro alone ranks among the largest rivers in the world by the volume of water it carries.

Why so much water?

The basin lies across the equator, where warm, moist air rises and produces heavy rain almost all year. Moist winds from the Atlantic carry water vapour inland, and the rainforest itself recycles a large part of it. Trees pull water from the soil and release it through their leaves, a process called transpiration. This vapour forms new clouds that rain again farther west. Scientists sometimes call these moving streams of moisture flying rivers, because they carry water through the air to other parts of South America, including farming regions far to the south.

Rivers of Three Colours

One of the most striking features of the Amazon system is that its rivers do not all look the same. Scientists group them into three types based on the colour and chemistry of their water.

• White-water rivers, such as the Solimões and the Madeira, start in the Andes and carry huge loads of sediment eroded from young mountains. Their water looks milky brown, like coffee with milk, and is rich in nutrients.
• Black-water rivers, such as the Rio Negro, drain sandy, flat forest lands. Decaying leaves release natural compounds called tannins, which stain the water the colour of strong tea and make it acidic. Because of this acidity, there are fewer mosquitoes along many black-water rivers.
• Clear-water rivers, such as the Tapajós and the Xingu, flow from ancient, worn-down rock plateaus. They carry little sediment, so their water often looks greenish and transparent.

The Meeting of Waters

Near Manaus, the dark Rio Negro meets the sandy-coloured Solimões. Instead of mixing right away, the two flow side by side in the same channel for several kilometres, with a sharp line between them. This famous sight, called the Meeting of Waters, happens because the rivers differ in temperature, speed and the amount of material dissolved and suspended in them. The colder, faster and denser Solimões and the warmer, slower Negro only gradually blend together.

A River That Rises and Falls

The Amazon is not the same size all year. During the rainy season, water levels rise dramatically. At Manaus, the difference between low and high water is typically around 10 metres. In the dry season the main river may be a few kilometres wide in many places, but during floods parts of it spread out to more than 40 kilometres across.

As the water rises, it pours into the surrounding forest. Two kinds of flooded forest form along the rivers. Várzea forests are flooded by nutrient-rich white water, while igapó forests are flooded by black water. For several months a year, fish swim between tree trunks and through the treetops of drowned forests. Some fish, like the tambaqui, eat fruits and seeds that fall into the water and later spread the seeds elsewhere, helping the forest grow.

The roaring tidal wave

Near the mouth, ocean tides push against the river's flow. During some of the strongest tides, a wave known as the pororoca rushes upstream. Its name comes from an Indigenous Tupi word linked to its roaring sound. The wave can reach several metres in height, and in some places surfers ride it for long distances.

Life in and Around the River

The Amazon flows through the largest tropical rainforest on Earth, covering about 5.5 million square kilometres. The basin is home to an enormous share of the world's known plant and animal species, and new ones are still being discovered every year.

Remarkable animals

• Fish: scientists have described more than 3,000 fish species in the Amazon basin, more than in any other river system. They include piranhas, catfish of all sizes and the arapaima, or pirarucu, one of the largest freshwater fish in the world, which can grow to about 3 metres long and must come to the surface to gulp air.
• The Amazon river dolphin: also called the boto or pink river dolphin, it turns pinker with age. Its flexible neck lets it twist through flooded forests in search of fish.
• The Amazonian manatee: the only manatee that lives only in fresh water. It grazes on water plants.
• The electric eel: despite its name, it is a knifefish, not a true eel, and it can produce powerful electric shocks to stun prey.
• Giant otters, black caimans and green anacondas also hunt along its banks and waterways.

Giant water lilies

The calm backwaters of the Amazon are home to Victoria amazonica, a giant water lily whose round floating leaves can grow wider than 2 metres. Their undersides are covered in sharp spines that protect them from hungry fish and manatees, and a network of ribs makes them strong enough to support small weights.

People and History Along the Amazon

The first inhabitants

People have lived in the Amazon for at least 11,000 years. Archaeologists have found evidence that ancient communities shaped the forest by planting useful trees such as Brazil nut and cacao. They also created terra preta, which means black earth in Portuguese: dark, fertile soils formed by adding charcoal, pottery fragments and food waste to the poor tropical ground. These soils stay fertile for centuries and are still farmed today. Hundreds of Indigenous peoples continue to live in the Amazon basin, speaking many different languages.

How the river got its name

The first European to travel down the river was the Spanish explorer Francisco de Orellana. In 1541 he set off from the Andes in present-day Ecuador as part of an expedition led by Gonzalo Pizarro, which was searching for cinnamon and gold. Orellana and his men floated downstream and reached the Atlantic in 1542. The expedition's chronicler, the friar Gaspar de Carvajal, wrote that they fought against warriors led by women. The Spaniards compared them to the Amazons, the female warriors of ancient Greek myths, and the name stuck to the river.

Cities and the rubber boom

In the late 19th and early 20th centuries, the world wanted rubber for bicycle and car tyres, and the Amazon was the main source of natural rubber, made from the sap of the rubber tree. The rubber boom made cities like Manaus in Brazil and Iquitos in Peru suddenly rich. Manaus built a grand opera house, the Teatro Amazonas, which opened in 1896 in the middle of the rainforest. The boom ended when rubber plantations in Southeast Asia, grown from seeds taken out of Brazil, began producing rubber more cheaply. The rubber era also brought great suffering to many Indigenous people who were forced to work collecting latex.

Today Manaus is a city of about two million people. Iquitos is often described as the largest city in the world that cannot be reached by road, so people and goods arrive by river or by air.

A Highway Made of Water

For people living in the Amazon, the river is the main road. Boats carry passengers, food, fuel and goods between towns and villages. The river is so deep and wide that ocean-going ships can sail about 1,500 kilometres inland to Manaus, and smaller seagoing vessels can travel all the way to Iquitos, about 3,600 kilometres from the sea.

One fact surprises many people: not a single bridge crosses the main channel of the Amazon. There are several reasons. Much of the river flows through areas with few roads and few people, so there is little demand for crossings. The banks are often soft and change shape, and floods spread the river over huge areas, making construction very difficult and expensive. Ferries do the job instead. There is, however, a long bridge across the Rio Negro at Manaus, opened in 2011.

A River Older Than It Looks

The Amazon has not always flowed east. Geologists have found evidence that, millions of years ago, rivers in this part of South America flowed west, toward the Pacific. As the Andes rose, they blocked that route. For a long time, water collected in huge wetlands and lakes in the western part of the continent. Eventually, around 10 million years ago, give or take a few million years, the water found a path east, and the modern Amazon began draining into the Atlantic. Scientists still study the exact timing, but this history helps explain why some Amazon fish and dolphins have relatives that came from the sea.

Challenges Facing the Amazon

The health of the river is closely tied to the health of the rainforest. Large areas of forest have been cleared for cattle ranching, farming, logging and mining. With fewer trees, less moisture is returned to the air, which can mean less rain and a weaker flow of flying rivers. Gold mining can release mercury into rivers, where it builds up in fish and harms the people who eat them. Large dams on tributaries, such as the Belo Monte dam on the Xingu, generate electricity but change the flow of water and the lives of fish and riverside communities.

Recent years have shown how sensitive the river system is. In 2023 and again in 2024, severe droughts dropped the Rio Negro at Manaus to its lowest levels since records began more than a century ago. Boats were stranded, villages were cut off, and in Lake Tefé more than a hundred river dolphins died when the shallow water became extremely hot. Scientists, governments and Indigenous communities are working to protect the forest, because keeping the Amazon healthy matters not only for South America but for the climate of the whole planet.