Amazon River

SOME AUTHORITIES believe that the river in BRAZIL and neighboring countries was named after the Amazons, women warriors of Greek mythology, who were thought to reside in the region; other scholars insist that the name is derived from the local native word amassona, meaning “boat destroyer.” Despite centuries of effort to overcome the dominance of nature, people have made little impact on the Amazon and most of its vast drainage basin. No bridge spans the river. Except near its mouth, the Amazon watershed constitutes one of the most thinly populated regions in the world. Much of the territory drained by the river system has never been thoroughly explored. One may fly for hours over the tropical forests that cover much of the river’s FLOODPLAIN and see no sign of human settlement. In many valleys, native tribes remain hostile to strangers, continuing to live much as they did before the arrival of the Europeans.

The Amazon Basin is a vast structural depression covering approximately 2,722,000 square mi (7,049,800 square km), an area nearly as large as the continental portion of the UNITED STATES. It is twice as large as the world’s next largest drainage BASIN (the CONGO RIVER), and supports the world’s largest rainforest. Geologically, two large stable masses of Precambrian rock, the Guyana Shield or Highlands to the north and the Central Brazilian Shield or Plateau to the south, bound the Amazon River and basin. Some 15 million years ago, before the ANDES MOUNTAINS were formed, the Amazon River flowed westward into the PACIFIC OCEAN. With the uplift of the Andes Mountains, however, the course of the river was eventually blocked to the west. As the river system backed up, the environment of the Amazon basin changed and numerous freshwater lakes began to form an inland sea. Ultimately, the gradient and volume of water was sufficient to push the flow to the east toward the ATLANTIC OCEAN where it empties today.

Most sources list the Amazon as 4,080 mi long (6,580 km), making it second to the NILE in length. However, there is some debate about the length of the Amazon versus Nile depending on the actual starting point of the source. According to recent discoveries, the Amazon is 4,195 miles (6,712 km) long if the course follows the Apurimac branch (rather than the Urubamba branch) of the Ucayali to a point 17,200 ft (5,242 m) above sea level, making it 50 mi (80 km) longer than the Nile. The Ucayali and Maranon rivers, both of which rise in the permanent snows and glaciers of the high Andes Mountains, flow along roughly parallel courses to the north before joining near Iquitos, Peru. Beyond Iquitos the river turns abruptly eastward, flowing along a very gentle gradient (approximately 1.25 in or 3.2 cm per mile) more or less paralleling the equator as it meanders over lowland plains. At Manaus, approximately 1,000 mi (1,610 km) upstream from the coast, the elevation is only 100 ft (30 m) higher than Belem, which is an ocean port.