In the competition between the world's largest rivers only the Nile is considered to be more than 4,000 miles (6,435 kilometers) long and only the Amazon discharges nearly four times as much water as any other river. But proud Brazilians have launched an expedition to prove once and for all that the Amazon is, in fact, longer than the Nile.

Guido Gelli, director of science at the Brazilian Institute of Geography and Statistics, argues that there is no question the Amazon is longer, based on a 2000 expedition that identified the farthest headwaters of this river-sea as emanating from Nevado Mismi in the Peruvian Andes. That expedition used global positioning systems (GPS) to verify that the Amazon starts as a trickle from an Andean glacier that becomes the Ucayali River and is 620 miles (1,000 kilometers) south of the previously verified source—the Marañón River.

Now a joint Brazilian-Peruvian team has reached the Ucayali headwaters and confirmed that it is indeed the farthest point in the Amazon drainage basin. "We have been there and found out the coordinates," Gelli says. "We can confirm it is really longer than it used to be said, and longer than the Nile." Including the new headwaters and at least an additional 90 miles (150 kilometers) at the mouth makes the Amazon 4,256 miles (6,850 kilometers) long, at least 100 miles longer than the Nile.

A second expedition set for September—the beginning of the dry season—will confirm whether water flows year-round, but such measurements are inherently subjective. "We take the longest, straightest tributary," explains Jennifer Runyon, a researcher at the U.S. Geological Survey's (USGS) Board on Geographic Names. "We look at the [drainage] map, identify the longest one and we go with it. It may not have as much water in it. Or even be what the local people think of it."

In the case of the Mississippi, for example, the USGS considers the headwaters to be Lake Itasca in Minnesota—the straightest flow. Yet, if its longest tributary is taken into account—the Jefferson and Missouri rivers—the Mississippi becomes three times as long (though still not as long as either the Amazon or Nile).

Such subjective definitions make it impossible to definitively judge whether the Amazon or Nile is the world's longest river. But new technology, such as satellite mapping, does allow scientists to study such river systems in their entirety.

"This is the beginning of a long series of scientific research, performed systematically, to understand the Amazon River as a whole," Oton Barros, a researcher at Brazil's National Institute for Space Research (INPE), said. And that is probably more important than a place in the record book.