The system begins as snowmelt and rain in the Peruvian Andes, gathering into the Maranon and the Apurimac, with the highest ground in the whole watershed standing at Yerupaja, 6,635 meters up. From there the water has about 6,400 kilometers to travel before it reaches the Atlantic, and almost all of that distance is nearly flat. Whether the Amazon or the Nile is the longer river is still argued by scientists who cannot agree where either one properly begins. On volume there is no argument at all. The Amazon system carries roughly a fifth of all the fresh water that the world's rivers deliver to the oceans, a discharge so large that mariners once reported drinking fresh water from the sea while still out of sight of land.

Rainforest soils are famously poor. Heat and rain strip nutrients out of the ground almost as fast as they arrive, which is why so much of the basin's living matter is held in the trees rather than the earth beneath them. Some of what replaces the losses comes from a very long way off: around ten million tonnes of Saharan dust cross the Atlantic each year and settle over the basin, carrying phosphorus mined from an African lakebed to South American roots. Under the canopy, the competition for light is just as inventive. Orchids and bromeliads skip the ground floor entirely, anchoring on branches and trunks as epiphytes, drawing nothing from their hosts but height.

About 2,500 fish species are known from the basin, and researchers estimate more than a thousand more remain undescribed. Roughly 45 percent of the known species live nowhere else. Two orders dominate: characins account for some 43 percent of species, catfishes another 39 percent. The extremes are absurd in both directions, from catfish that migrate thousands of kilometers to breed down to sleeper gobies barely a centimeter long. Some Amazonian fish barely bother with water at all. The splash tetra lays its eggs on leaves above the surface and keeps them wet by flicking water upward; the South American lungfish waits out the dry season sealed in a mucous cocoon underground. Above the water, more than 1,400 mammal species and around 1,500 bird species share the basin. In Peru's Manu National Park, surveys have recorded more than 1,300 butterfly species. All of Europe has about 473.

Amazonians do not divide the year into hot and cold. They divide it into high water and low. When the rivers rise, they push out into the surrounding forest, submerging trunks to head height and turning woodland into a flooded canopy that fish swim through, feeding on fruit and seed. When the water falls, it leaves fresh silt on beaches and islands, and families plant rice, beans and corn directly into ground that needs no fertilizer because the flood delivered it. Macaws and parrots come down to exposed clay banks in the western basin nearly every day that it is not raining, eating the mineral clay by the hundreds. Almost everything alive here runs on that annual breath, which is precisely why damming and diverting the tributaries changes so much more than the water.

The old story held that the Amazon was too infertile to support large societies before Europeans arrived. Archaeology has taken that story apart. Patches of terra preta, dark anthropogenic soil built up over centuries of composting and low-temperature burning, turn up across the basin and still grow crops today. More than 400 distinct peoples have lived here, speaking hundreds of languages, most of them now critically endangered; an estimated hundred groups remain uncontacted by choice. Most of the region's population is concentrated in a few river cities such as Iquitos in Peru, Manaus and Belem in Brazil, with settlements scattered thinly in between. COICA, the basin-wide indigenous confederation, coordinates across all nine political territories, because rivers do not stop at borders and neither do the pressures on them.

Amazon vegetation holds roughly 56.8 billion tonnes of carbon above ground, and the forest has long been counted as one of the planet's great carbon sinks. That accounting has begun to fail. Over the two decades to 2021, the Brazilian Amazon released about 13 percent more carbon dioxide than it absorbed, the arithmetic flipped by clearing for cattle pasture and soy. The forest also makes its own weather: somewhere between half and four-fifths of the water that falls here is recycled back into the air by the trees themselves and rains again further inland. Cut enough of them and the basin loses the ability to hold on to its own rain. The river reversed once already, when the mountains rose. This time the change would come from the ground up.