When the Amazon River Flowed Backwards

The Amazon is, to put it simply, huge. In some places, the river’s width can be measured as up to 11 km- during the dry season. During the wet season, when the river floods, it can stretch to 40 km. That’s nearly 25 miles across. And, while some argue that Africa’s River Nile is actually longer (measuring the exact length of rivers can be a little tricky, but the Nile is generally regarded as the first-place winner), none can dispute that when it comes to area drained, the Amazon is truly the world’s largest river. The Amazon River basin covers around 40% of the entire continent of South Africa. The Nile’s, by comparison, covers a measly 10.3% of Africa. As if all those numbers weren’t impressive enough, the Amazon is thought to account for as much as 20% of all freshwater emissions into oceans. And though we won’t get into all those creatures swimming through its depths- including but not limited to freshwater dolphins, 200-lb catfish, green anacondas (which can reach lengths of 30 feet and literally eat jaguars for breakfast), and bull sharks- it’s safe to say that this is one river we will not be swimming in anytime soon. All in all, the Amazon is an impressive, terrifying, and in some cases downright alien part of our own Planet Earth.

amazon river basin

Amazon River Basin – courtesy of NASA Earth Observatory

Also, it used to flow backwards.

Really. Thanks to an entirely accidental discovery back in 2006, geologists found that, for a brief period of time (“brief” translating to “millions and millions of years” in geology-speak), the Amazon’s waters moved westward. Today, of course, they move eastward, depositing all those gallons of water in the Atlantic Ocean. So what happened?

Changing a river- especially a river like the Amazon- is no easy feat. In fact, scientists believe that what triggered the shift was nothing less than continental divide. When South America began pulling away from Africa, the east coast of South America may have begun to lift upwards, causing the river to change directions. And though continental drift took its own sweet time going forward, scientists say that the Amazon’s own changes were rather abrupt (presumably in comparison, as otherwise it raises the question of exactly how abrupt 1/5 of the world’s fresh water changing direction can be).

However, the Amazon’s return to our “normal” course didn’t happen quite so fast. What most likely happened was that the Purus Arch (a low ridge that, at one point, divided the South American continent pretty much in half) eventually split the river into two parts; one flowing west to the Pacific, the other east to the Atlantic. Then came the Andes Mountains, rising skyward on the continent’s westward edge. The mountain range reversed the Amazon’s flow once again, and over time, the water eroded a path through the Purus Arch and rejoined the other half. Eventually, also thanks to the Andes, the westward basin it had once occupied filled in with sediment, as giant water basins no longer filled with water are apt to do.

Though this news is around eight-years old , it’s worth mentioning that models and theories are still being made about the specifics of these events. In other words; we don’t know a lot of things about the Amazon, and in most cases we don’t know what to look for, because we don’t know what we don’t know. How many other discoveries about our Earth are still beneath the surface, waiting to be found?


Amazon 57.53278W 2.71207S“. Licensed under Public domain via Wikimedia Commons.

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