The Amazon is not just a river. It is roughly 20 percent of all the freshwater that enters the oceans of the planet. At its mouth, near Belém in northeastern Brazil, the Amazon discharges an average of about 209,000 cubic metres of water per second into the Atlantic — more than the combined discharge of the next seven largest rivers in the world. The river is so massive at its mouth that fresh water remains detectable on the ocean surface several hundred kilometres out to sea. The drainage basin is even more staggering: 7 million square kilometres, roughly the size of Australia, covering nearly half the continent of South America and parts of nine countries (Brazil holds about 60 percent, followed by Peru, Colombia, and smaller portions of Bolivia, Ecuador, Venezuela, Guyana, Suriname, and French Guiana). The basin contains the largest rainforest on Earth, an estimated 10 percent of all known species, and some 30 million people, including hundreds of indigenous groups whose languages have been spoken on the river since before the Roman Empire.
How Long Is It?
The Amazon's length is contested. The conventional figure is around 6,400 kilometres, which would make it shorter than the Nile (6,650 km). But several surveys — particularly a 2007 Brazilian National Institute for Space Research expedition that traced the river's headwaters in the Peruvian Andes — have argued for a length of 6,800 or even 7,000 kilometres, which would make it the longest river in the world. The dispute depends on which tributary is considered the headwater (the Apurímac, the Mantaro, or the Marañón all have claims) and on how the multi-channel mouth is delimited. The question is unlikely to be definitively settled because the river's mouth shifts seasonally and the source rivers themselves change in form. In practice, the Amazon and the Nile are roughly tied, and the rankings depend on how you measure.
The Black and White Waters
One of the most striking visual features of the Amazon basin is the difference between its "white-water" and "black-water" tributaries. The Solimões (the upstream name for the Amazon in Brazil) carries enormous loads of sediment from the Andes and appears milky-tan in colour. The Rio Negro, draining the dense forest areas of northern Amazonia, is clear but stained dark brown by decomposed leaf material — essentially a very large, very clean tea. When the two rivers meet near Manaus, they flow side by side without mixing for about six kilometres, the dark Negro water and the pale Solimões water maintaining a sharp visual boundary that has become a tourist attraction. The two waters eventually mix as turbulence increases downstream, but the meeting of waters is one of the most-photographed natural features in the basin.
The Rainforest
The Amazon rainforest covers approximately 5.5 million square kilometres of the basin. It is the largest contiguous tropical rainforest in the world and contains an estimated 390 billion individual trees representing 16,000 species. The biodiversity is extraordinary at every taxonomic level: one hectare of Amazon rainforest can contain 300+ tree species (more than all of Europe), 2,000+ insect species, and hundreds of bird species. The forest plays a major role in global climate by absorbing carbon dioxide and producing oxygen, and by recycling rainfall through evapotranspiration in a feedback loop that has been called "the river in the sky."
The forest is being lost at a substantial rate. The peak years of deforestation in the 1990s and early 2000s saw losses of roughly 27,000 km² per year, primarily for cattle ranching and soybean cultivation. Deforestation rates dropped significantly between 2004 and 2012 under the first Lula government's environmental policies, then rose again, peaking under the Bolsonaro administration's 2019-2022 period at about 13,000 km² per year. The second Lula government has reduced rates again. Cumulative deforestation through 2024 has cleared roughly 20 percent of the original forest, with substantially higher rates in the southeastern "arc of deforestation" where most agricultural conversion is concentrated. Whether the remaining forest will hold up to combined pressures of deforestation, climate change, and fire is among the most consequential environmental questions of the early 21st century.
The Tipping Point
Climate scientists have warned for over a decade that the Amazon rainforest may be approaching a "tipping point" beyond which forest loss becomes self-reinforcing. The mechanism involves the forest's role in recycling rainfall: the rainforest evaporates so much water that it generates much of its own rain. If deforestation reduces the forest's evaporation enough, the basin's rainfall declines, which dries out the remaining forest, which then dies back further, in a feedback loop that ends with much of the basin converting from rainforest to a degraded savanna or scrub. Various models place the tipping point at somewhere between 20 and 40 percent of original forest lost. The 2023-2024 drought period in much of the basin — which produced unprecedented low water levels on the Amazon main stem and tributaries — has revived these concerns sharply. Whether the tipping point has been crossed, is about to be, or remains comfortably distant is a matter of active scientific dispute.
Manaus and the River Cities
The major city of the Amazon basin is Manaus, in the central rainforest at the confluence of the Negro and Solimões. Manaus has about 2 million inhabitants. It is connected to the rest of Brazil mainly by river and air; the road connections through the surrounding forest are limited and frequently impassable. The city had a brief golden age in the late 19th century during the rubber boom, when Brazilian rubber from the Amazon was the dominant source of latex for the industrialising world. The Teatro Amazonas, an opera house built in Manaus in the 1890s with European materials and acoustics, is the most visible relic of that period. The rubber economy collapsed in the early 20th century when British smuggling of rubber tree seeds out of Brazil allowed the establishment of rubber plantations in Malaya and Ceylon. Manaus did not recover economically until the 1960s, when the Brazilian government established it as a free-trade zone for electronics manufacturing.
Other significant river cities include Belém at the mouth (about 1.5 million people), Iquitos in Peru (the largest city in the world inaccessible by road, with about 470,000 residents reachable only by river or air), Santarém in central Brazil (300,000), and Leticia/Tabatinga at the Brazil-Colombia-Peru triple border.
The People
The Amazon basin contains hundreds of distinct indigenous groups, ranging from extensively documented communities like the Yanomami (whose Brazilian reserve covers about 96,000 square kilometres on the Venezuelan border) to dozens of small groups in the more remote interior. A small number of indigenous groups remain "uncontacted" — that is, without sustained interaction with the surrounding state — and are the subject of specific Brazilian and Peruvian protection policies. Estimates of pre-contact populations in the basin range widely; some recent archaeological work argues that the basin supported many millions of people in complex agricultural societies before the demographic collapse following European contact in the 16th century. The remains of large earthworks, raised fields, and tree-crop orchards across the basin support this view, though the question is still actively debated.
The Mouth
The Amazon enters the Atlantic through a complex multi-channel delta around the city of Belém. The mouth is so wide — over 320 kilometres from north shore to south — that it contains an island, Marajó, larger than Switzerland. The river's discharge is so powerful that it pushes freshwater hundreds of kilometres into the open Atlantic and creates a measurable plume of low-salinity water visible from satellites. The river also produces a particularly notable tidal phenomenon called the pororoca — a tidal bore wave that runs upstream against the current at the highest tides, reaching heights of 4 metres and travelling for several hundred kilometres into the river. Surfers occasionally ride the pororoca for record-breaking distances.
The Long View
The Amazon is the largest natural feature on Earth's continental surfaces. Its future will be a substantial part of the global climate story over the next several decades. The deforestation pressure, the tipping-point risk, the changes in rainfall patterns, the political volatility of the basin states (particularly Brazil, with its alternating left- and right-leaning governments and their different environmental policies), and the question of how the indigenous and rural communities of the basin will adapt to or resist the encroaching agricultural and extractive economies are all converging on a few critical decades. Whether the river that carries 20 percent of the world's freshwater and the forest that surrounds it can survive in anything like their current form is one of the more important questions of our time, and it has no easy answer.