Port. Amazonasämäzōˈnəs, world's second longest river, c.3,900 mi (6,280 km) long, formed by the junction in N Peru's Andes Mts. of two major headstreams, the Ucayali and the shorter Marañón. It flows across N Brazil before entering the Atlantic Ocean near Belém.

The Amazon carries more water than any other river in the world. The drainage basin is enormous (c.2,500,000 sq mi/6,475,000 sq km; c.35% of South America), gathering waters from both hemispheres and covering not only most of N Brazil but also parts of Bolivia, Peru, Ecuador, Colombia, and Venezuela. For most of its course the river has an average depth of c.150 ft (50 m). The gradient of the river is very low: Manaus, c.1,000 mi (1,610 km) upstream, is only c.100 ft (30 m) higher than Belém and is an ocean port; ships with a draft of 14 ft (4 m) can reach Iquitos, Peru, c.2,300 mi (3,700 km) from the sea. Peru, Ecuador, and Colombia have international shipping rights on the Amazon. In the lowlands stretching east from the Andes is the largest rain forest (selva) in the world—a wet, green land rich in plant life. The tropical climate is tempered by the heavy rainfall (exceeding 150 in./381 cm annually in parts of the upper and lower regions) and by high relative humidity; the average temperature at Santarém, 400 mi (644 km) upriver, is 78 degrees Fahrenheit (26 degrees Celsius).

Course

Geologically, the Amazon basin is a sediment-filled structural depression between crystalline highlands of Brazil and Guiana. The riverbed (1–8 mi/1.6–12.9 km wide) is in a wide floodplain that is up to 30 mi (48 km) wide. For much of its course, the Amazon wanders in a maze of brownish channels amid countless islands, but is unobstructed by falls.

Its headstreams, however, arise cold and clear in the heights of the Andes. They descend northward before turning east to join and form the Amazon (which is, however, occasionally called the Solimões from the Brazilian border to the junction with the Rio Negro). Of the Amazon's more than 500 tributaries, the chief ones are the Negro, Japurá (Caquetá), Putumayo (Içá), and Napo, which enter from the north; and the Javari, Juruá, Purús, Madeira, Tapajós, and Xingú rivers, which enter from the south. The Casiquiare River, a natural canal, links the Amazon basin (through the Rio Negro) with the Orinoco basin.

Below the Xingú the river reaches its delta, with many islands formed by alluvial deposit and submergence of the land. Around the largest of these, Marajó, the river splits into two large streams. The northern stream is the principal outlet and threads its way around many islands. The southern channel, called the Pará River, receives the Tocantins River and has the important port of Belém. The awesome tidal bore (up to 12 ft/3.7 m high) of the Amazon is called pororoca; it travels c.500 mi (800 km) upstream. The river's immense silt-laden discharge is visible far out to sea.

Exploration and Development

The Amazon was probably first seen by Europeans in 1500 when the Spanish commander Vicente Yáñez Pinzón explored the lower part. Real exploration of the river came with the voyage of the Spanish explorer Francisco de Orellana down from the Napo in 1540–41; his fanciful stories of female warriors gave the river its name. Not long afterward (1559) the Spanish conquistador Pedro de Ursúa led an expedition down from the Marañón River. In 1637–38 the Portuguese explorer Pedro Teixeira led the voyage upstream that definitively opened the Amazon to world knowledge. The river continued to be of enormous importance to explorers and naturalists, among them Charles Darwin and Louis Agassiz.

There is archaeological evidence of clustered, densely populated pre-Colombian settlements in parts of the Amazon basin, but at the time of the early European explorations these settlements had already been wiped out, probably by smallpox and other diseases, The valley was largely left to its sparse remaining indigenous inhabitants (mostly groups of the Guaraní-Tupi linguistic stock and of meager material culture) until the mid-19th cent., when steamship service was regularly established on the river and when some settlements were made. In the late 19th and early 20th cent., the brief wild-rubber boom on the upper Amazon attracted settlers from Brazil's northeastern states, and in the 1930s Japanese immigrants began developing jute and pepper plantations. Until recently the area remained largely unpopulated, yielding small quantities of forest products (rubber, timber, vegetable oils, Brazil nuts, and medicinal plants) and cacao. Extensive road networks are now opening these lands to colonization, although agricultural success has been limited by adverse climate, poor soils, and great market distances.

The establishment of a health service (chiefly by launch) in World War II was followed by the creation of a UNESCO research institute in 1948, and several developmental programs, both governmental and private, were set up in Brazil to foster the valley's development. In the 1960s the Amazon region began experiencing increased economic development brought on by tax incentives and construction of the Trans-Amazon Highway, the Belém-Brasília Highway, and two rail lines. Near Manaus and Amapá, factories make use of ample oil and manganese resources. In addition, a port at the Brazilian city of Macapá was connected by rail in the 1950s to the inland stores of manganese.

The Brazilian government implemented a "poles of development" policy in 1974 to plan for population increase. Since 1985 the Carajás project, centered in W Maranhão, has seen the development of major iron ore deposits, the construction of a new railroad, and the initiation of forest clearance, land colonization, cattle ranching, large-scale farming, and urban development on an unprecedented scale. This policy has had mixed results, leading to environmental damage and to the disruption of the original inhabitants' lives. Large sections of the rain forest have been destroyed in recent years, threatening rare species of plants and contributing to the increase in the atmosphere's carbon dioxide and the consequent impact on global warming.

Bibliography

See C. R. Marham, ed., Expeditions into the Valley of the Amazon (1859); R. Furneaux, The Amazon (1969); J. R. Holland, The Amazon (1972); B. Weinstein, Amazon Rubber Boom, 1850–1920 (1983); B. Kelly and M. London, Amazon (1985); J. T. Medina, The Discovery of the Amazon (2d ed. 1988).

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