Which is the longest river in South America?

The Amazon, and by volume it is not close to anything - it discharges more water than the next seven largest rivers on Earth combined, roughly a fifth of all river water reaching the ocean. Its outflow freshens the Atlantic for well over a hundred kilometres offshore, and sailors used to fill barrels with drinkable water out of sight of land.

It used to run the other way. Before the Andes rose, the continent drained westward into the Pacific; the mountains dammed it, creating vast inland lakes and wetlands, until the water eventually broke east and cut through to the Atlantic. The river is younger than the mountains that turned it round.

It also fertilises another continent. Roughly 27 million tonnes of dust blow across the Atlantic from the Sahara each year, and a substantial share falls on the Amazon basin - carrying phosphorus that the rainforest's heavily leached soils are desperately short of. A large part of what makes the Bodele Depression in Chad so dusty is the dried bed of an ancient lake, so the rainforest is being fed by the remains of a lake that no longer exists.