The Amazon River: The Giant River of South America, Explained

Short answer

• The Amazon River is in South America. It flows west to east across the northern part of the continent, through Peru, a short border stretch of Colombia, and Brazil. It is not in the United States or North America.
• Its length is at least 4,000 miles (6,400 km), measured from its headwaters in the Peruvian Andes, according to Encyclopaedia Britannica. Whether it edges past the Nile as the world's longest river is genuinely disputed, because the answer depends on which source stream and which mouth are counted.
• It carries more water than any other river on Earth. Britannica puts it this way: about one-fifth of all the water that runs off Earth's surface is carried by the Amazon, and its flood-stage discharge at the mouth is four times that of the Congo and more than ten times that of the Mississippi.
• Its basin covers about 2.7 million square miles (7 million sq km), the largest drainage basin in the world, nearly twice the size of the Congo's.
• The main river has no bridges across it anywhere along its length. Crossings are made by boat and ferry.

Introduction

Search for the “Amazon river in America” and you will find it, but not where a United States reader might first guess. The Amazon is in America in the older, continental sense of the word: it flows across South America, the southern continent of the Americas. It does not pass through the United States, Mexico, or anywhere in North America. By the time its water reaches the Atlantic Ocean it has crossed northern South America from the high Andes of Peru to the coast of Brazil, a journey of some 4,000 miles.

What makes the Amazon worth a long explanation is not its address but its scale. By the volume of water it carries, it is not merely the largest river in South America but the largest river on Earth, and by a wide margin. Its drainage basin is the biggest in the world. Its length is argued about precisely because it is close enough to the Nile to contest the title of longest river on Earth. This article sets out the verified facts: where the river is, how long it is and why the measurement is disputed, where it starts and ends, how much water it moves, the tributaries that build it, and the strange places where its waters meet without mixing.

Where the Amazon Actually Is

The Amazon belongs to the northern half of South America. Its headwaters rise in the Andes Mountains of southern Peru, remarkably close to the Pacific coast: Britannica notes that the river’s westernmost source is within 100 miles (160 km) of the Pacific Ocean. From there the river does the opposite of what that geography suggests. Instead of taking the short route west to the Pacific, it turns inland and crosses the entire continent, flowing generally eastward, roughly along the equator, until it empties into the Atlantic Ocean on the northeastern coast of Brazil.

That one decision, uphill water draining east across a continent, is what makes the Amazon enormous. A river that crosses a continent collects a continent’s worth of rain. The main stem passes through three countries. It is formed in Peru, brushes Colombia along a short stretch where the three countries meet, and then spends most of its length in Brazil. Roughly two-thirds of the Amazon’s main stream lies within Brazil, and the river reaches the sea entirely on Brazilian coastline.

The name itself changes along the way, which confuses map readers. In Peru, the upper main stream down to its meeting with the Ucayali River is called the Marañón, and from there to the Brazilian border it is called the Amazonas. In Brazil, the stretch from the border to the confluence with the Rio Negro is called the Solimões. Only from the Rio Negro downstream to the Atlantic does Brazil call it the Amazonas. It is one continuous river under all of these names. The river got its European name in 1541, when the Spanish explorer Francisco de Orellana traveled its length and reported battles with female warriors, whom he likened to the Amazons of Greek mythology.

The Length Question: Amazon or Nile?

Ask how long the Amazon is and the honest answer has two parts. The figure Britannica stands behind is “at least 4,000 miles (6,400 km),” measured from the headwaters of the Ucayali-Apurímac river system in southern Peru. That puts it slightly shorter than the Nile, which is conventionally listed at about 4,130 miles (6,650 km). On those numbers, the Amazon is the second-longest river in the world.

But the contest is real, and it is not settled. Reported measurements of the Amazon range from about 3,899 miles (6,275 km) to about 4,365 miles (7,025 km), depending on which source stream is treated as the true headwater and which of the river’s several mouths is treated as its end. A 2007 expedition, measuring from a more distant Peruvian source, reported an Amazon length of roughly 4,225 miles (6,800 km), which would put it ahead of the Nile on the figures that expedition used. A 2014 study then identified the headwaters of the Mantaro River, on the Cordillera Rumi Cruz in Peru, as the Amazon basin’s most distant source, displacing the Apurímac headwaters on Nevado Mismi, which had held that status for nearly a century.

Neither side is being dishonest. River length is not a fixed property like a coastline surveyed once and for all. It depends on definitions. Pick a different Andean stream as the source, or a different channel around Marajó Island as the mouth, and the total moves by hundreds of miles. What is not in dispute is the ranking by the measure that matters most for a river: water carried. On discharge, the Amazon has no rival at all, as the next sections show.

From the Andes to the Atlantic: The River’s Course

The Amazon begins as meltwater and mountain streams high in the Peruvian Andes. The Apurímac River, rising on Nevado Mismi, was long considered the most distant source; the Mantaro headwaters identified in 2014 are the current challenger. Either way, the young river descends the eastern slopes of the Andes through deep gorges, gathering the Marañón and Ucayali branches, which join in the Peruvian lowlands near the city of Iquitos to form the main stem as most of the world recognizes it.

From Iquitos the river is already huge, and it still has thousands of miles to run. It flows east past the triple frontier where Peru, Colombia, and Brazil meet, then deep into Brazil. At Manaus, the largest city on the river, the dark Rio Negro joins from the north in the famous Meeting of the Waters described below. Below Manaus the river, now called the Amazonas in Brazil, continues east through a broadening floodplain, past the mouths of the great southern tributaries, and finally splits around Marajó Island into channels that reach the Atlantic. Marajó, the island sitting in the river’s mouth, is roughly the size of Switzerland, which gives some sense of the width of water involved. Britannica counts three outlets to the Atlantic: two on the northern side of Marajó and one to its south.

The river also rises and falls dramatically with the seasons. Each year, rains across the basin swell the Amazon by many meters, flooding the low forests along its banks. Those seasonally flooded forests, called várzeas, are enriched by the silt the water leaves behind, while the higher ground that never floods, the terra firme, carries the dense upland rainforest.

A Basin the Size of a Continent’s Heart

A river is the product of its basin, the land that drains into it, and the Amazon basin is the largest on Earth: about 2.7 million square miles (7 million sq km), according to Britannica, nearly twice the area drained by the Congo River, the world’s other great equatorial system. The World Wildlife Fund describes the wider Amazon region as covering about 40 percent of South America and spanning eight countries plus the territory of French Guiana. The river basin proper, on Britannica’s account, includes the greater part of Brazil and Peru, significant parts of Colombia, Ecuador, and Bolivia, and a small area of Venezuela.

This is why lists of “countries the Amazon flows through” and “countries in the Amazon basin” give different answers. The main river itself flows through Peru, Colombia, and Brazil. The basin, the web of tributaries feeding it, reaches into at least six countries. Rain that falls on Andean slopes in Bolivia or on highlands in Venezuela can end up in the same channel that passes Manaus.

Inside that basin sits the Amazon rainforest, which follows the river system across the lowlands. The rainforest represents about half of all the rainforest remaining on Earth, and more than two-thirds of the basin is covered by it. The river and the forest are one system: the forest exists because the basin is drenched by equatorial rain, and the river is huge because a forested continent’s worth of water drains through it.

The Greatest Flow on Earth

Length can be argued about. Discharge cannot. The Amazon’s average flow is commonly cited at about 209,000 cubic meters of water per second (roughly 7.4 million cubic feet per second), more than the next seven largest rivers on Earth combined. No other river is close.

Britannica’s way of framing it is equally striking. About one-fifth of all the water that runs off Earth’s surface is carried by the Amazon. At flood stage, the discharge at the river’s mouth is four times that of the Congo, the world’s second-largest river by flow, and more than ten times the amount carried by the Mississippi River. The Amazon enters Brazil carrying only about a fifth of the flow it will finally discharge into the Atlantic, yet even at that border point it already has a greater flow than any other river in the world manages anywhere.

All that fresh water does not stop being noticeable at the shoreline. The outflow is so immense that it dilutes the saltiness of the Atlantic for more than 100 miles (160 km) from shore. Early Portuguese sailors who encountered drinkable water far out at sea called the outflow the Rio Mar, the River Sea, and the name survives as a description of the phenomenon: a plume of river water, carrying sediment and drifting debris, extending deep into the ocean.

The Tributaries That Build It

No single stream could supply that flow. The Amazon is fed by more than a thousand tributaries, and some of them are giants in their own right. Two of the ten largest rivers in the world by discharge are tributaries of the Amazon.

The Madeira River, joining from the southwest out of Bolivia and Brazil, is the Amazon’s largest tributary, itself longer than 2,000 miles (about 3,250 km) and carrying an enormous share of the main river’s sediment. The Rio Negro, joining at Manaus, is the largest left-bank tributary and one of the biggest rivers on Earth by flow, famous for its dark, tea-colored water, stained by organic acids from the flooded forests it drains. From the south come the Xingu and the Tapajós, clear-water rivers flowing off the ancient Brazilian highlands. In the Peruvian headwaters, the Marañón and the Ucayali are effectively the river’s two founding branches.

Each tributary type contributes something different. Andean rivers like the Marañón arrive loaded with pale sediment eroded from the young mountains, which is why the main stem runs milky brown. Rivers from the old, weathered highlands run clear or dark. The mixture, arriving from a basin spanning a continent, is what makes the main stem’s flow so relentless through both wet and dry seasons: somewhere in a basin that size, it is always raining.

Where the Waters Meet Without Mixing

Just outside Manaus, the Amazon system performs its most famous trick. The Rio Negro, its water darkened almost to black by dissolved organic matter, flows into the pale, sand-colored Solimões, and the two waters refuse to blend. For about 3.5 miles (6 km), they run side by side in the same channel, a dark half and a light half with a visible line between them, before finally mixing into the uniform brown of the lower Amazon.

The explanation is physics, not mystery. The two rivers arrive at different temperatures, different speeds, and different densities. The Rio Negro is warmer and much slower, draining flat, flooded forest; the Solimões comes faster and cooler off the Andean foothills, heavy with sediment. Water masses that differ in temperature and density do not merge instantly, especially when one is flowing several times faster than the other, so the boundary between them persists for miles before turbulence finally folds them together. The Meeting of the Waters, Encontro das Águas in Portuguese, is one of the clearest places on Earth to watch the fact that a river is not one water but many.

A Great River With No Bridges

Here is a fact that surprises most readers: no bridge crosses the Amazon River itself, anywhere along its roughly 4,000 miles. Towns and cities on its banks, including Manaus, are reached by boat, ferry, and air. The gap is not an engineering failure. For most of its length the river is bordered by floodplain that is underwater for months each year, so a bridge would have to span not just a channel several miles wide but, in the wet season, a flooded valley far wider still. Roads on either side are few, and river transport has always done the work.

The nearest thing to an exception proves the rule. The bridge opened near Manaus in 2011, often mentioned in this context, crosses the Rio Negro. It connects Manaus to the far bank of a tributary. The main stem of the Amazon remains, from Andes to Atlantic, an unbridged river.

Life in and Around the River

The Amazon system is one of the richest freshwater habitats on Earth. Britannica records about 2,500 fish species identified within the system, with many more still unidentified. Among them are the pirarucu, one of the world’s largest freshwater fish, and various giant catfish. The famous piranha is real but less fearsome than its reputation: it generally feeds on other fish. The river is also home to caimans, river turtles, manatees, and the pink river dolphin, while its banks support the capybara, the largest rodent in the world.

The wider basin raises the stakes further. Conservation organizations such as the World Wildlife Fund commonly estimate that the Amazon region is home to about one in ten of all species known to science, a figure quoted as an estimate rather than a census, but consistent with the region’s measured richness: tens of thousands of plant species and new species still being described every year. The river, its flooded forests, and its rainforest uplands together form the largest single reserve of biological resources on the planet, which is how Britannica describes the rainforest itself.

Frequently Asked Questions

Is the Amazon River in the United States?

No. The Amazon River is in South America. It rises in the Andes of Peru and flows east across Peru, along a short stretch of Colombia’s border, and through Brazil to the Atlantic Ocean. The word “America” in the common search phrase refers to the Americas, the continents, not to the United States.

Which countries does the Amazon River flow through?

The main river flows through three countries: Peru, Colombia (briefly, at the triple frontier), and Brazil, which holds roughly two-thirds of its length. Its drainage basin is wider, covering the greater part of Brazil and Peru and significant parts of Colombia, Ecuador, and Bolivia, plus a small area of Venezuela.

Is the Amazon the longest river in the world?

It is either first or second, and the ranking is honestly unsettled. Britannica gives the Amazon at least 4,000 miles (6,400 km), slightly shorter than the Nile’s conventional 4,130 miles (6,650 km). Other measurements, using a more distant Peruvian source stream, put the Amazon ahead. The disagreement is about which source and which mouth to count, not about the river’s size, and by volume of water the Amazon is the world’s largest river by a wide margin.

How much water does the Amazon River carry?

Its average discharge is commonly cited at about 209,000 cubic meters per second, more than the next seven largest rivers combined. Britannica notes that about one-fifth of all the water running off Earth’s surface passes through the Amazon, and that its flood-stage discharge at the mouth is four times the Congo’s and more than ten times the Mississippi’s.

Where does the Amazon River start and end?

It starts in the Andes Mountains of southern Peru, in the Apurímac and Mantaro headwater systems within about 100 miles of the Pacific Ocean. It ends on the Atlantic coast of northeastern Brazil, where it reaches the sea through channels around Marajó Island.

Why are there no bridges over the Amazon?

Because for most of its length a bridge would have to cross not only a channel miles wide but a seasonally flooded valley far wider than that, in a region where boats and ferries have always done the work of roads. The one major bridge near the river, at Manaus, crosses the Rio Negro tributary, not the Amazon itself.

Conclusion

The Amazon River is the giant of South America: at least 4,000 miles long, fed by a basin of about 7 million square kilometers, and carrying about a fifth of all the river water on Earth to the Atlantic. Its location is the first thing to get right. It belongs to the southern continent of the Americas, rising almost within sight of the Pacific in Peru and choosing instead to cross a continent to reach the Atlantic through Brazil. The argument over whether it beats the Nile in length will probably never be fully settled, because it depends on definitions. The argument over which river rules the world’s waters was settled long ago, and it is not close.

Sources & references

• Encyclopaedia Britannica (britannica.com), "Amazon River": the river's location in northern South America; length of at least 4,000 miles (6,400 km) from the Ucayali-Apurímac headwaters and the debate over its length and source; the basin of about 2.7 million square miles (7 million sq km), nearly twice the Congo's, and the countries it includes; about one-fifth of Earth's surface runoff carried by the Amazon; flood-stage discharge four times the Congo's and more than ten times the Mississippi's; the outflow diluting ocean salinity more than 100 miles from shore; the Marañón, Amazonas, and Solimões naming sequence; the three Atlantic outlets around Marajó Island; about 2,500 identified fish species and the river's notable wildlife.
• Encyclopaedia Britannica (britannica.com), "Amazon River" FAQ and river references: source within 100 miles of the Pacific Ocean; Francisco de Orellana's 1541 journey and the origin of the river's name; the rainforest as about half of Earth's remaining rainforest and the largest single reserve of biological resources.
• World Wildlife Fund (worldwildlife.org), Amazon facts: the Amazon region covering about 40 percent of South America and spanning eight countries and French Guiana; the commonly cited estimate that the region contains one in ten known species on Earth, with 40,000 plant species and 3,000 freshwater fish species across the region.
• Springer Nature, Encyclopedia of Hydrological Sciences reference entry, "Amazon river": mean discharge on the order of 220,000 cubic meters per second, five times that of the Congo and ten times that of the Mississippi, and a drainage area of about 6.9 million sq km, as institutional reference ranges for the figures used in this article.