The Amazon once flowed west to the Pacific until the Andes rose 15 million years ago. Sediment analysis in the Atlantic shows it reversed direction completely.
European Space Agency, CC BY-SA 3.0 igo, via Wikimedia Commons
A River in Reverse
Geological studies confirm that for millions of years, the Amazon River flowed in the opposite direction—from east to west. During the Cretaceous Period, between 65 and 145 million years ago, the tectonic plates of South America and Africa split apart. This separation created a highland area in what is now northeastern South America. With the Andes Mountains yet to be formed, this eastern highland topography directed the continent's drainage westward, possibly into the Pacific Ocean or as part of a proto-Congo river system when the continents were joined.
The primary evidence for this westward flow comes from the analysis of ancient river sediments. Geologists discovered tiny, durable mineral crystals called zircons in sedimentary deposits near the Amazon's center. These zircon grains have ages of around 1.3 to 2.1 billion years old, matching the ancient rocks of the highlands in eastern South America. If the river had always flowed east, the sediments in its central basin would have come from the much younger rocks of the Andes. The presence of these ancient eastern zircons far to the west is clear proof of the river's original, backwards course.
The Andean Wall and a Great Lake
The Amazon's westward flow ended because of one of the planet's significant geological events: the rise of the Andes. Starting around 15 million years ago, the collision of the South American and Nazca tectonic plates began forcing the land upward, creating the mountain range that now lines the continent's western edge. This immense barrier blocked the river's path to the Pacific.
Unable to flow west, the river's water began to pool in the vast, low-lying basin between the new Andes and the ancient eastern highlands. This blockage formed a massive system of wetlands and lakes. Known as the Pebas Mega-Wetland System, this body of water covered more than 1 million square kilometers of western Amazonia, spanning parts of modern-day Brazil, Peru, Colombia, and Ecuador. For millions of years, this large freshwater system harbored unique life, including giant caimans over ten meters long and turtles with shells 3.5 meters in diameter. Many species of stingray found today in the Amazon's freshwater are most closely related to Pacific Ocean species, remnants of this period of isolation.
The reversal happened between five and ten million years ago. As the Andes continued to rise, they shed vast amounts of sediment, which gradually filled the Pebas lake system. Eventually, the accumulated water and sediment overtopped the lower-lying eastern highlands. This breach carved a new path, establishing the modern, eastward-flowing Amazon River that empties into the Atlantic Ocean. The entire transcontinental river as we know it today took its present shape around 2.4 million years ago.
💡Fun Facts
The Pebas Mega-Wetland System, which formed when the Andes blocked the river, existed for roughly 13 million years.
Over 20 species of freshwater stingray in the Amazon are descended from marine ancestors that adapted to the vast inland lake system.
Today, the Amazon discharges so much sediment into the Atlantic that the brownish river water can be detected 100 kilometers offshore.
Some of the zircon crystals used to prove the river's westward flow are over 2 billion years old, originating from eroded mountains in the east.
24/7; the river is a natural feature, with tour availability varying by operator and season.
Admission
Access to the river is free; guided tours and boat trips from Manaus have associated costs.
Accessibility
The river and surrounding rainforest are primarily accessible by boat. Terrain on excursions can be uneven and challenging. Accessibility for people with mobility issues is limited.
