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Scientists Figure out Why the Amazon River Flows Backwards


Photo credit: ESA. The shape of the Amazon Basin mapped here using satellite altimeters is a relatively recent feature of the Earth.

The Amazon used to flow from east to west, just opposite to the present direction. The fact that the largest river on Earth reversed its direction is not a trivial thing, so the geologists have been trying to find out the cause for some time.

Dr Victor Sacek from the University of Sao Paulo in his article published in Earth and Planetary Science Letters has demonstrated that it is erosion that greatly attributes to such magnificent shift.

As the mighty Andes are located at the western end of South America, it appears logical that the rivers in the continent flow east. Compared with other rivers on the Earth, the water discharged by the Amazon is five times bigger that any of them. The Orinoco and the Rio de la Plata flow the same way, thus dwarfing any river in North America or Europe in the process.

But at the time of 10 million years before, most of the part where the Amazon basin is located now was drained by a river that went westwards into a giant lake at the bottom of the northern Andes. Then the water flowed north to the Caribbean Sea from there. At that moment, the Isthmus of Panama was not formed yet, so this water was swept away in the west direction into the Pacific.

Geologists had speculated that tilting an entire continent could lead to the changes in convection within the Earth’s mantle; thus possibly caused by the break-up of Africa and South America.

In addition, Sacek also demonstrated that as the South American plate rode over the Nazca Plate, so the rise of the Andes could give explanation to the process on the proper timescale. In his model Sacek clearly illustrated the fact that in the process of the mountains rising, they intercepted more rain-bearing clouds, which gave rise to much more erosion as a result.

At the beginning, the rise of the Andes created a trough to the east, which turned to be the paleolake into which the westward-flowing Amazon emptied. However, as times went by, such sinking slowed down and erosion accelerated up, resulting in the replacement of the lake by a series of wetlands known as the Pebas. Such gorgeous Pebas marshlands would have become an ecosystem unlike anything we know now, but in the end, sediment accumulation led to raising the area to the point where the rainfall in the region was brought back the other way.

Sacek’s model helped prove the observation that sediment which was deposited at the Amazon’s mouth has increased during the period when it flowed eastward. In the beginning, when the sources of the Amazon were relatively flat, most sediment was dumped to its mouth, only being remobilized eons later.

In summarizing his work, Sacek confessed that his present model was unable to make a full reproduction of the evolution of the Pebas system both in the spacial and temporal way with geological data collected so far, so more work should be done for future research in this regard.

Source: Science

Journal reference: Sacek, Victor. “Drainage reversal of the Amazon River due to the coupling of surface and lithospheric processes.” Earth and Planetary Science Letters 401 (2014): 301-312.