The rise of the Andes reversed the direction of the Amazon River’s flow from the Pacific to the Atlantic.
Rivers flow from high country down to lakes, seas, and oceans. Their path may meander and change, but the start and end points are pretty well set. Except when they’re not. We know of at least two rivers that have changed direction. One of those was natural, the other was artificial. Today I’m writing about the natural one.
The Amazon River is so old that it was flowing when South America and Africa were the same continent, Gondwana. Or rather, a “proto-Amazon” was flowing – it began in Africa, flowed across South America, and emptied into the ocean to the west. Gondwana broke up, leaving the South American portion of the Amazon stranded. Tectonic shifts messed with the river further: the rise of the Andes blocked off its westward exit, and a rising mountain range in the middle of South America split the Amazon in two.
At this point, the eastern section of the Amazon began flowing east, and the western section continued flowing west. Except the rising Andes got in the way, so the western section was blocked from reaching the ocean. It built up as an enormous lake, also receiving waters from the newly-minted Andes.
The lake built up so much that it eroded down that range that had split the river. Suddenly the entire Amazon was flowing east, from the Andes down to the Atlantic Ocean. Its flow was reversed, and that’s the Amazon we know today.