About 10 million years ago, the Amazon changed course from flowing into the Pacific Ocean.
The river's redirection was caused by the formation of the Andes, whose uplift created huge lakes in their eastern foothills. Over time, these lakes were filled with sediment that washed down from the slopes of the Andes with rainfall. Huge amounts of sediment gradually filled the lakes and continued to flow, raising the ground level. In short, as the surface of the western part of the continent rose above the eastern part, the Amazon River began to flow in the opposite direction.
