Amazon River Once Flowed Other Way, Study Says

Sean Markey
for National Geographic News
October 25, 2006

The entire Amazon Basin, the world's largest river drainage system, once flowed in the opposite direction—a find that could pave the way to new oil and gas exploration, scientists say.

Geologists in the United States and Brazil say the discovery that the river previously flowed east-to-west was accidental. The team was studying how swiftly sediment travels in the Amazon Basin from its headwaters in the Andes Mountains of Peru to the Atlantic Ocean.

Using airplanes and riverboats, the team traversed nearly 80 percent of the Amazon Basin to collect geologic samples (map of Brazil).

"All the current indicators in the ancient sediments"—including ripple marks and telltale mineral traces—"showed that the current, the river flow, was from the east to the west," said study author Drew Coleman, a geologist at the University of North Carolina at Chapel Hill.

Previous studies have shown that some segments of the Amazon Basin once flowed opposite their present course.

But the new study is the first to suggest that the entire drainage system flowed from the Atlantic to the Pacific for a period of time. (Related: "Amazon Logging Twice as Heavy as Thought, Images Show" [October 20, 2005].)

Researchers presented the study today at the annual meeting of the American Geological Society in Philadelphia, Pennsylvania.

Continental Breakup

The Amazon was flowing west-to-east, as it does now, when the shift occurred. That reversal is "almost certainly tied" to the breakup of the South American and African continents, which began about 130 million years ago, Coleman said.

"When that happened, the east coast [of South America] was uplifted—and the whole river flowed the other way," he said.

When the Andes Mountains started growing at about the end of the Cretaceous period (around 65 million years ago), the geologic tide begin to shift again in favor of the Amazon's current west-to-east course.

Precisely when the basin made these dramatic shifts remains unknown, Coleman says, but the changes were abrupt.

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