Explorers pinpoint source of the Amazon.

The mighty Amazon River, a tropical torrent that carries about 20 percent of the fresh water that flows into the world's oceans, starts out as a trickling snow-fed stream.

That's what a five-nation team of explorers funded by the National Geographic Society found when they used the Global Positioning System (GPS) to survey five remote rivers in a 100-square-kilometer area in southern Peru last July.

After setting up a GPS base station and camp at an elevation of about 15,500 feet, small teams braved harsh terrain at even higher altitudes to map the Andean headwaters of the world's largest river. The 22-person expedition found that the source of the Amazon is a small stream that flows from the upper slopes of Nevado Mismi, an 18,363-foot-high peak. National Geographic announced the results of the GPS survey last month.

The source of a river can be defined in several ways, says Andrew Johnston, a geographer at the National Air and Space Museum in Washington, D.C., and a member of the expedition. For example, it could be considered the point that is most distant from the mouth of the river and where water flows year-round. Or it might be defined as the furthest point from which water could possibly flow to the river's mouth; this point is typically located on a continental divide. Locations on Nevado Mismi fit each of these definitions, Johnston says.

The snow-fed stream, which flows into the Apurimac River, was identified as the source by a National Geographic team in 1971, but that designation fell into question when recent expeditions found another contender about 7 km away. Johnston, who was responsible for the recent GPS surveys, says that the contender is indeed longer than the stream from Nevado Mismi. However, the distance from the Amazon's mouth to the continental divide near the challenger is shorter, and the water doesn't flow year-round in the contender's upper reaches.

Though geographers have now pinpointed the source of the Amazon, problems at the other extreme make determining the length of the river a difficult problem. Says Johnston, "The mouth of the Amazon is so indistinct and has so many islands that you can't really tell where it ends."