What Is the Amazon River? The World's Largest River Explained

The Amazon: A River Like No Other

The Amazon River is, by almost any measure, the world's greatest river. It carries approximately 20% of all freshwater that flows into the world's oceans — more than the next seven largest rivers combined. Its drainage basin (the area of land draining into the river) covers 7 million square kilometers across nine countries, encompassing most of the Amazon rainforest. At its mouth near Marajó Island in Brazil, the river discharges so much freshwater into the Atlantic Ocean that the ocean remains fresh for hundreds of kilometers offshore.

The Amazon is approximately 6,400 km long, though which river is "longest" (the Amazon or the Nile) depends on how source rivers are counted — a debate that remains geographically contested. What is not contested: the Amazon moves more water by volume than any other river on Earth.

Source and Flow

The Amazon originates in the Andes of Peru — its most distant source is the Apacheta cliff in the Arequipa region at approximately 5,170 meters elevation. Tributaries from the Andes carry sediment-rich "white water" (actually pale brown, laden with Andean sediments) into the main river system. Other tributaries carry "black water" — dark, acidic, low-sediment rivers draining ancient weathered soils of the Brazilian and Guiana Shields.

The Amazon flows east across the broad, flat Amazon Basin — a vast sedimentary depression between the Andes and the Atlantic coast. The gradient is almost imperceptibly gentle: 3,700 km from the Peruvian border to the Atlantic, the river drops only about 60 meters in elevation. This creates an extraordinarily slow-moving river with massive seasonal flooding — during the wet season, the flooded forest (várzea) can extend 50 km on either side of the main channel.

The Amazon Basin's Biodiversity

The Amazon basin contains the greatest concentration of biodiversity on Earth. The numbers are staggering:

• ~40,000 plant species (including ~16,000 tree species — more tree species in one hectare of Amazon forest than in all of North America)
• ~3,000 freshwater fish species — more than in the entire Atlantic Ocean. The river's fish include the piranha (largely scavengers, not the movie monsters), the arapaima (one of the world's largest freshwater fish, up to 3 meters), the giant river otter, river dolphins (the pink boto), and electric eels
• ~1,300 bird species (about 15% of all bird species on Earth)
• ~430 mammals, ~1,000 amphibians, ~400 reptiles
• Insects: estimated in the millions of species, most still undescribed by science

The diversity exists because the Amazon basin has been a stable, warm, wet environment for millions of years — allowing evolution to generate extraordinary specialization and diversity without the mass extinctions that affected other regions during ice ages and other climate shifts.

The River's Role in Climate

The Amazon is not merely a river — it is a climate system. The Amazon rainforest generates roughly half its own rainfall through transpiration: trees absorb water through roots and release it through leaves as water vapor, which condenses and falls again as rain. Scientists call these moisture-recycling pathways "flying rivers" — atmospheric rivers of water vapor carrying as much water as the Amazon River itself.

The Amazon also stores enormous quantities of carbon — an estimated 150–200 billion tons of carbon in its trees and soils. When forest is burned or cleared, this carbon is released as CO₂, contributing to climate change. Conversely, intact Amazon forest absorbs CO₂, acting as a carbon sink (though recent research suggests parts of the degraded Amazon may now be net carbon emitters).

Threats to the Amazon

The Amazon faces severe and accelerating threats:

• Deforestation: Brazil has lost approximately 20% of its original Amazon forest. Agricultural expansion (primarily beef cattle and soy), illegal logging, and mining drive deforestation. Peak deforestation in the 2000s was reduced by policy changes, but rates have surged again in recent years.
• Fire: Most Amazon fires are intentionally set to clear land. Smoke from Amazon fires is visible from space; record fire years (2019, 2020) generated international alarm.
• Tipping point risk: Scientists warn that deforestation combined with climate change could push the Amazon past a "tipping point" — at which the forest can no longer generate enough rainfall to sustain itself, leading to a self-reinforcing transition to savanna ("dieback"). Estimates suggest this tipping point may be as close as 20–25% deforestation (the Amazon is approximately 20% deforested now).
• Indigenous peoples: Approximately 400 indigenous peoples live in the Amazon basin, many with little contact with the outside world. Deforestation and development threaten their territories, cultures, and lives — and indigenous-managed territories show significantly lower deforestation rates than other areas.