Ancient Amazon River reversed course due to soil erosion

Science : Millions of years ago, the Amazon River reversed course and has been flowing eastward toward the Atlantic Ocean ever since. A previous study had suggested that the reversal was driven by a changing topography due to the movement of hot mantle material from Earth’s interior to its surface. Now Victor Sacek of the University of São Paulo in Brazil says it is more likely that erosion and sedimentation were the driving forces. More than 10 million years ago, rainfall across what is now the Amazon basin drained westward into lakes that formed along the eastern edge of the Andes, on the west coast of South America, and flowed north into the Caribbean Sea. To try to figure out what made the water flow reverse direction, Sacek ran computer models simulating the evolution of the South American terrain. He determined that sediment eroding off the Andes gradually filled in the lakes and built the landscape up such that it created a downhill slope extending from the Andes to the Atlantic. Neverthess, Sacek does not entirely rule out deep geologic processes; even though their overall effects are probably modest, he says, he plans to include them in future versions of his simulation to try to achieve a more accurate picture of how the landscape evolved.