In June 1542, a Spanish expedition led by Francisco de Orellana was floating down an unknown river in an unknown continent, starving and exhausted. On one particular stretch, they were attacked by indigenous warriors among whom, according to the chronicle kept by Gaspar de Carvajal, were women who fought alongside the men and appeared to lead them. Orellana's men, who had grown up on Greek myth, thought immediately of the Amazons of classical lore. The name stuck. Rio Amazonas, they called it, and the label traveled back to Europe with the survivors. The etymology of the word itself is older and stranger than the Greek association. Some scholars trace it to an Iranian root meaning warriors or those fighting together. Others connect it to the Tupi word amassona, meaning boat-destroyer. Whatever the true origin, the name has become one of the most recognizable river names on Earth.

For millions of years, long before humans existed, the Amazon basin drained the opposite direction - from east to west, into what is now the Pacific. Then the Andes rose. The uplift of the mountain chain along South America's Pacific edge blocked the old outflow, and the rivers that had run toward the sunset had to find somewhere else to go. Over geological time they reversed course and began flowing east, cutting through the continent toward the Atlantic, a process that created the drainage system we see today. The Andes are still rising, and the river is still responding. Sediment from the young mountains feeds the Amazon's muddy color and the floodplains that support its enormous productivity. The fact that we call it an Atlantic river is, in geological time, a relatively recent development.

Archaeologists now believe that, when Orellana floated downriver in 1541, the Amazon basin was home to more than three million people. Some regions, like the island of Marajo at the river's mouth, supported complex chiefdoms of perhaps 100,000 residents with social stratification, agriculture, and monumental earthworks. The indigenous inhabitants managed the forest actively, using fire to improve soil and cultivating fruit trees in ways that have left their mark to this day. Much of the Amazon's famously dark, fertile soil - terra preta de indio, Indian dark earth - is the direct result of these cultivation practices. European diseases and colonial violence then collapsed these populations so thoroughly that by the 19th century, the Amazon appeared to European explorers to be a pristine wilderness empty of human history. It was not. It was a forest that had grown over the bones of civilizations whose very existence had been forgotten.

The Amazon rises on the Cordillera Rumi Cruz in Peru, flows north as the Mantaro, joins other headwater streams to become the Ucayali, meets the Maranon near Iquitos, and only then does it start being called the Amazon in most of South America. Brazilians call it the Solimoes until it meets the Rio Negro at Manaus, where the famous Meeting of the Waters - the dark Negro running alongside the sandy-brown Solimoes without mixing for several kilometers - creates one of the most photographed geographic features in the country. Downstream of Manaus, the river widens into something that becomes hard to describe as a river at all. At points it is 48 kilometers across. The annual flood can raise water levels 9 meters and inundate vast floodplains of varzea forest. At its mouth, the river braids into channels that cradle the island of Marajo, an island larger than Switzerland. The flow is so massive that the influence of the Amazon's fresh water can be detected 320 kilometers offshore in the Atlantic.

The Amazon's ecological status as the single largest river on Earth has never translated into peace. Commercial fishing, oil exploration, hydroelectric dams, gold mining, and cattle ranching have all pushed into the basin at different times. Indigenous groups like the Urarina, the Huaorani, the Yanomami, and dozens of others continue to live along the tributaries, often in conflict with extractive industries that want the land beneath them. The arapaima, one of the world's largest freshwater fish, has been in these waters for at least 23 million years and is now farmed and fished in ways that stress its populations. Pink river dolphins navigate tributaries that lose half their volume each dry season. Twenty percent of the planet's freshwater flow passes through this single system, which means the decisions South American countries make about their forests and rivers have consequences measured not only in local communities but in global climate patterns. The Amazon is still the same river Orellana saw, but what lives along it is changing faster now than at any point since the forest reclaimed its lost cities.