More on Amazon longer than Nile

Scientific American

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.