The length of the Amazon

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The debate over the location of the true source of the Amazon and over the river’s precise length sharpened during the second half of the 20th century, as technological advances made it possible to explore deeper into the extremely remote locations of the Amazon’s headstreams and to more accurately measure stream lengths. Beginning in the 1950s, explorers of the region cited various mountains in Peru as possible sources, but they did so without taking precise measurements or applying hydrological research. An expedition in 1971, sponsored by the National Geographic Society, pinpointed Carruhasanta Creek, which runs off the north slope of Mount Mismi in southern Peru, as the source of the river. This location became widely accepted in the scientific community and remained so until the mid-1990s—although a Polish expedition in 1983 contended that the source of the river was actually another stream, nearby Apacheta Creek. (The Carruhasanta and Apacheta streams form the Lloqueta River, an extension of the Apurímac.)

With the introduction of Global Positioning System (GPS) technology in the 1990s, researchers again attempted to navigate the entire length of the Amazon. The American geographer Andrew Johnston of the Smithsonian Institution’s Air and Space Museum in Washington, D.C., employed GPS gear to explore the various Andean rivers that flow into the Amazon. Using the definition of the river’s source as being the farthest point from which water could flow into the ocean and where that water flows year-round (thereby eliminating those rivers that freeze in winter), he concluded that the source was Carruhasanta Creek on Mount Mismi.

By the early 21st century, advanced satellite-imagery technology was allowing researchers to match the river’s dimensions even more precisely. In 2007 an expedition that included members of Brazil’s National Institute for Space Research and other organizations traveled to the region of Carruhasanta and Apacheta creeks in an attempt to determine which of the two was the “true” source of the Amazon. Their data revealed that Apacheta was 6 miles (10 km) longer than Carruhasanta and carries water year-round, and they concluded that Apacheta Creek was indeed the source of the Amazon River. The team then proceeded to measure the river’s length. As part of this process, they had to determine from which of the Amazon’s three main outlets to the sea to begin the measurement—the Northern or Southern channels, which flow north of Marajó Island, or Breves Channel, which flows southward around the western edge of the island to join the Pará River estuary along the southern coast of the island. They chose to use the southern channel and estuary, since that constituted the longest distance from the source of the river to the ocean (at Marajó Bay); according to their calculations, the southern outlet lengthened the river by 219 miles (353 km). Their final measurement for the length of the Amazon—from Apacheta Creek to the mouth of Marajó Bay—was about 4,345 miles (6,992 km).

This team of researchers, using the same technology and methodology, then measured the length of the Nile River, which they determined to be about 4,258 miles (6,853 km); that value was some 125 miles (200 km) longer than previous calculations for the Nile but nearly 90 miles (145 km) shorter than the length the group gave for the Amazon. These measurements infer that the Amazon may be recognized as the world’s longest river, supplanting the Nile. However, a river like the Amazon has a highly complex and variable streambed—made more so by seasonal climatic factors—which complicates the process of obtaining an accurate measurement. Thus, the final length of the river remains open to interpretation and continued debate.

Landforms and drainage patterns

The Amazon basin is a great structural depression, a subsidence trough that has been filling with immense quantities of sediment of Cenozoic age (i.e., dating from about the past 65 million years). This depression, which flares out to its greatest dimension in the Amazon’s upper reaches, lies between two old and relatively low crystalline plateaus, the rugged Guiana Highlands to the north and the lower Brazilian Highlands (lying somewhat farther from the main river) to the south. The Amazon basin was occupied by a great freshwater sea during the Pliocene Epoch (5.3 to 2.6 million years ago). Sometime during the Pleistocene Epoch (about 2,600,000 to 11,700 years ago) an outlet to the Atlantic was established, and the great river and its tributaries became deeply entrenched in the former Pliocene seafloor.

The modern Amazon and its tributaries occupy a vast system of drowned valleys that have been filled with alluvium. With the rise in sea level that followed the melting of the Pleistocene glaciers, the steep-sided canyons that had been eroded into the Pliocene surface during the period of lower sea levels were gradually flooded. In the upper part of the basin—in eastern Colombia, Ecuador, Peru, and Bolivia—more-recent outwash from the Andes has covered many of the older surfaces.