A river runs backward. Erosion and other processes taking place at Earth’s surface help explain why large portions of the Amazon River (watershed depicted in lighter colors) reversed course.

Image: Jesse Allen/NASA (using SRTM data courtesy of Global Land Cover Facility/U. MD); River data: WWF HydroSHEDS Project

A river runs backward. Erosion and other processes taking place at Earth’s surface help explain why large portions of the Amazon River (watershed depicted in lighter colors) reversed course.

Why the Amazon flows backward

Sid is a freelance science journalist.

Millions of years ago, rivers flowing westward across what is now northern Brazil reversed their course to flow toward the Atlantic, and the mighty Amazon was born. A previous study suggested that the about-face was triggered by gradual changes in the flow of hot, viscous rock deep beneath the South American continent. But new computer models hint that the U-turn resulted from more familiar geological processes taking place at Earth’s surface—in particular, the persistent erosion, movement, and deposition of sediment wearing away from the growing Andes.

The Andes mountains lie just inland of the western coast of South America. The central portion of that mountain range began growing about 65 million years ago, and the northern Andes started rising a few million years later, says Victor Sacek, a geophysicist at the University of São Paulo in Brazil. Yet, field studies suggest that the Amazon River, which today carries sediment-laden water from the Andes across the continent to the Atlantic Ocean, didn’t exist in its current form until about 10 million years ago. Before then, rainfall across much of what is now the Amazon Basin drained westward into massive lakes that formed along the eastern rim of the Andes and then flowed north via rivers into the Caribbean. The geological processes that caused ancient drainage patterns to shift to their modern configurations have been hotly debated.

The lakes east of the Andes formed in a long trough created when the immense weight of that growing mountain chain pressed Earth’s crust downward, Sacek says. But for some reason, the terrain beneath the trough slowly gained elevation over millions of years, and those lakes gradually gave way to a long-lived region of wetlands covering an area the size of Egypt or larger. Later, after the landscape rose even farther, the wetlands disappeared altogether. Previously, scientists proposed that changes in the circulation of molten material in Earth’s mantle—the slow-flowing material that lies between our planet’s core and its crust—pushed the terrain east of the Andes upward, thereby changing drainage patterns.

But new research pins the blame on something more mundane: erosion. Sacek developed a computer model that includes interactions between growth of the Andes, the flexing of Earth’s crust in the region, and climate. (For instance, as the mountains rise, they intercept more moist airflow and receive more rainfall, which in turn boosts the rate of erosion.) The model simulates the evolution of South American terrain during the past 40 million years—a period that commenced after the birth of the central Andes but before the eastern flank of those mountains began to rise, Sacek notes.

Results of the simulation reproduce much of the evidence seen in the geological record, Sacek reports online ahead of print in Earth and Planetary Science Letters. Initially, the lakes form east of the Andes because the mountains press Earth’s crust downward to form a trough faster than sediment can fill it. Then the sinking of the terrain slows down, and accumulation of sediment spilling off the Andes catches up, gradually filling in the lakes and building the landscape higher. Eventually, the terrain just east of the mountain chain becomes higher than that in the eastern realm of the Amazon Basin, a shift that provides a downhill slope extending all the way from the Andes to the Atlantic beginning about 10 million years ago.

“Erosion and sedimentation are powerful forces,” says Jean Braun, a geophysicist at Joseph Fourier University in Grenoble, France. Sacek’s model shows that these processes explain the geological record seen in northern South America, “and they do so with the right timing,” he adds. They also suggest that the amount of sediment carried to the mouth of the Amazon each year and then dumped offshore should increase over time—something actually seen in sediment cores drilled from that area. “That’s a nice bit of prediction by the model,” Braun says.

The gradually increasing rate of sediment accumulation possibly stems from the long time needed for the material to hopscotch its way across the continent, being dumped in one spot and then remobilized by erosion later, says Carina Hoorn, a geologist at the University of Amsterdam. Or, she suggests, the increase may stem from a geologically recent boost in erosion in the Andes triggered by a series of ice ages that commenced about 2.4 million years ago.

One thing Sacek’s model doesn’t do a good job of predicting, he admits, is the size, shape, and persistence of the large area of wetlands that formed in what is now the central Amazon Basin between 10.5 million and 16 million years ago. But it’s possible, he notes, that changes in mantle circulation beneath the region did play a minor role in the evolution of the terrain. Sacek will try to incorporate such processes into future versions of his terrain simulation, to see if they better explain how the landscape evolved.

Such changes in mantle flow are “difficult to quantify and even more difficult to discern [in the real world],” Braun says. But by combining the modest effects of such changes with those triggered by surface processes such as erosion, “you might end up with something that works.”

Posted in Earth, Physics