The origin of the world's largest river—by volume—has been surprisingly hard to pin down. Explorers and scientists have argued over where to locate the start of the Amazon River since at least the mid-1600s, with no fewer than five rivers in southwestern Peru given the honor over the years.

Now the authors of a study published in the journal Area say they've located the mighty river's true source: the Mantaro River in southwestern Peru. If they're right, their discovery would add 47 to 57 miles (75 to 92 kilometers) to the length of the Amazon, currently measured at about 4,000 miles (6,437 kilometers) by the U.S. Geological Survey.

Using six different methods of measurement—including GPS tracking data and satellite images—professional kayaker James Contos and his team, funded in part by a grant from the National Geographic Society, determined that the Mantaro River is about 10 percent longer than the Apurímac River, which has been considered the Amazon's source since 1971.

One reason the Mantaro may have been overlooked, say Contos and his anthropologist co-author Nicholas Tripcevich of the University of California, Berkeley: A twisting bend, or kink, in the river's lower half makes it look much shorter than it really is.

But whether geographers accept the claim of Mantaro as the Amazon's true origin depends on which definition of a river's source they choose to apply: Is it the farthest point upstream that provides the largest volume of water, or is it the most distant point up the longest tributary in the river's drainage basin?

The current internationally accepted definition is the most distant point of a river's longest tributary that flows continuously. In their study, Contos and Tripcevich argue that the length of a tributary should trump whether it flows year-round.

Too Many Beginnings?

Throughout the 1700s, the Marañón River in northern Peru held the distinction of being the Amazon's source, says Andrew Johnston, a geographer with the Smithsonian's National Air and Space Museum in Washington, D.C. That's because the Marañón held the largest volume of flowing water compared to other tributaries flowing into the Amazon. The Ucayali River—a lower extension of the Apurímac River—took over as the designated source for a while because it was considered the longest tributary flowing into the Amazon.

Then, in 1971, a National Geographic expedition led by Loren McIntyre identified the snow-capped peak of Mismi (map) as the headwaters of the Apurímac River and the ultimate source of the Amazon. A follow-up National Geographic expedition in 2000 that included Johnston, further confirmed Lake Ticlla Cocha at the base of Mismi as the headwaters and the Apurímac as the longest upstream extension of the Amazon River.

A Question of Flow

The trouble with Contos and Tripcevich's claim of overturning this long-held view, say some geographers, is that the Mantaro River runs dry for about five months of the year, when the Tablachaca dam, built in 1974, diverts its flow around its kink, leaving the loop devoid of water.

Continuously flowing water is important in determining the true source of a river, says Juan Valdés, the geographer at National Geographic. "You can have the longest tributary, but if it doesn't have continuous flow, then it's a moot point." Mantaro does not have water flowing through its channels year-round, says Valdés.

But the study's authors propose that short-term changes in flow or man-made alterations to a river's natural course should not be a deal-breaker in determining the source of a river. The most distant source should be defined as the farthest point "from which a drop of rain will make its way to the river's mouth"—regardless of whether it flows continuously. By that measure, they assert, the Mantaro is the most distant source. The Smithsonian's Johnston, even though he was on the expedition that confirmed the primacy of the Apurímac, thinks the new study provides a fresh perspective. "Assuming the measurements stand up to scrutiny, yes, I think the Mantaro could be considered a new source of the Amazon," he says, but not "the source."

When water flows through the Mantaro, the river "probably has a greater flow distance than any other Amazon tributary," Johnston says. But when the Mantaro is dry, "the Apurímac is once again the place where water flows the greatest distance into the Amazon."

It may be more straightforward to trace the source of other rivers, Johnston notes, but "the Amazon, by far the largest river in the world, is not so simple." Scientists need to look at all three of its proposed source tributaries—the Marañón, the Apurímac, and the Mantaro—he says, to truly understand the mighty Amazon.