The Amazon River flows eastward toward the Atlantic, but researchers know that wasn't always the case, notes the Latin Times. Long ago, the region's water moved in the opposite direction. Just what was it, though, that caused the reversal? Earlier findings linked the shift to the movement of molten rock deep underground, but a new study contests that. A geophysicist at Brazil's University of Sao Paolo used expansive computer modeling to come to a different conclusion: It's all about erosion related to the rising Andes Mountains as they grew millions of years ago, Science reports.
Victor Sacek's model, laid out in Earth and Planetary Science Letters, shows how the region's terrain might have developed over 40 million years, and its results match real-life geological findings. Here's what it suggests: The downward push of the Andes Mountains created a trough that resulted in lakes. Afterward, sediment poured into these lakes and created higher terrain. That led, 10 million years ago, to ground that sloped from the mountains to the Atlantic—and drove the water in that direction. (Meanwhile, scientists think they've figured out where the river actually starts.)