Which is the longest river in South America?

The Amazon drains a basin that covers roughly a third of the continent, gathering water from the Andes in the west and pushing it across the whole width of Brazil to the Atlantic. Its length depends on which Andean headstream you accept as the true source, and rival surveys have put the figure slightly above or below that of the Nile, but within South America there is no serious competitor. What makes the Amazon extraordinary is less its length than its volume: it discharges more freshwater into the ocean than the next several largest rivers combined, enough to noticeably freshen the Atlantic far offshore. The basin's flatness is part of the story too, since the river falls only a few dozen metres over its final couple of thousand kilometres, which is why ocean-going ships can reach deep into the interior and why seasonal floods spread out across enormous areas of seasonally inundated forest.

A natural channel called the Casiquiare links the Orinoco basin to the Amazon basin, a rare case of one great river system leaking into another.