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Amazon (river) (Portuguese and Spanish Amazonas), river in northern South America, largely in Brazil, ranked as the largest in the world in terms of watershed area, number of tributaries, and volume of water discharged. Measuring 6,400 km (4,000 mi) from source to mouth, it is second in length only to the Nile among the rivers of the world. With its hundreds of tributaries, the Amazon drains a territory of more than 7 million sq km (2.7 million sq mi), roughly half of which is in Brazil; the rest is in Peru, Ecuador, Bolivia, and Venezuela. It is estimated that the Amazon discharges between 34 million and 121 million liters (9 million and 32 million gallons) of water per second and deposits a daily average of 3 million tons of sediment near its mouth. The annual outflow from the river accounts for one-fifth of all the fresh water that drains into the oceans of the world. The outpouring of water and sediment is so vast that the salt content and color of the Atlantic Ocean are altered for a distance of about 320 km (about 200 mi) from the mouth of the river.
The major headstreams of the Amazon are the Ucayali and Marañón rivers, both of which rise in the permanent snows and glaciers of the high Andes Mountains. Feeding the Ucayali is the Apurimac River, the Amazon's most distant source. The headwaters of the Apurimac are located at a glacial meltwater of Nevado Mismi, an Andean peak in southern Peru. The Ucayali and Marañón follow parallel courses north before joining near Nauta, Peru. From this confluence the main trunk of the Amazon flows in a generally eastern direction to the Atlantic Ocean. The Amazon enters the Atlantic through a broad estuary, roughly estimated at 240 km (about 150 mi) in width. Here delta deposits have formed a maze of islands that separate the river into branches. The mouth of the main stream is 80 km (50 mi) wide. This branch, known as the Pará, is separated from a smaller branch by Marajó Island, which has an area of more than 36,000 sq km (14,000 sq mi). During new and full moon a tidal bore, or wave front from the ocean, sweeps some 650 km (more than 400 mi) upstream at speeds in excess of 65 km/h (40 mph). This phenomenon can cause waves 5 m (16 ft) or more in height along the banks of the river.
The Amazon watershed includes the largest and wettest tropical plain in the world. Heavy rains drench much of the densely forested lowland region throughout the year but especially between January and June. Seasonal variation in rainfall is reflected by the width, rate of flow, and discharge volume of the river. During the months of maximum precipitation, broad areas traversed by the Amazon are subject to severe floods. In Brazil the width of the river ranges between 1.6 and 10 km (1 and 6 mi) at low stage but expands to 48 km (30 mi) or more during the annual floods; the rate of flow ranges between 2.4 and 8 km/h (1.5 and 5 mph); and the crest of the water at flood time often rises 15 m (50 ft) above the norm. To drain the vast mass of water, the Amazon has carved a deep bed in the plain through which it flows. In one sector near Óbidos, Brazil, the bed is more than 91 m (300 ft) below the average surface level of its water.
Because of its vastness, annual floods, and navigability, the Amazon is often called the Ocean River. The total number of its tributaries is as yet uncounted, but more than 200 are in Brazil alone. Seventeen of the largest known tributaries are more than 1,600 km (1,000 mi) in length. The Amazon proper is navigable to ocean liners of virtually any tonnage for two-thirds of its course. Transatlantic ships call regularly at Manaus, nearly 1,600 km (1,000 mi) upstream; and smaller ships can reach Iquitos, Peru, 3,700 km (2,300 mi) from the river's mouth, the farthest point from sea of any port serving ocean traffic. River steamers of more modest tonnage can navigate on more than 100 of the larger tributaries.
The delta region of the Amazon may have been seen by the Spanish explorer Vicente Yáñez Pinzón in 1500, but exploration did not begin until 1541 and 1542, when an expedition led by Francisco de Orellana started down the Napo River, in what is now Ecuador, and reached the Atlantic Ocean. Pedro Teixeira undertook the first upstream voyage. Between October 1637 and August 1638 he ascended the Amazon to the source of the Napo River and crossed the Andes to Quito, Ecuador. Later, he returned by the same route. In modern times the river has been explored by many scientific expeditions, including that led by former American president Theodore Roosevelt in 1914 and others sponsored by the National Geographic Society and the government of Brazil.
Some authorities believe that the river 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 Native American 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 stream valleys, Native American tribes hostile to strangers continue to live much as they did before the arrival of the Europeans. Most commerce is narrowly confined to the navigable sectors of the river system, although the development of highways has improved access and goods transportation into the area. The economy continues to be dominated by primitive agriculture, hunting and fishing, and the gathering of various forest products. Commercial farming, tourism, and industry play minor but growing roles in the region, but mining and lumbering, the principal economic activities, are increasingly important.
Large-scale development of the Amazon began in the 1980s. Such development cleared large sections of its natural forest cover. Although the forest quickly covers over cleared areas, the regrowth is not as diverse as the original cover. This decline in diversity affects both animal life and the human population, as animals and plants on which they depend become rare or extinct and available land becomes scarce. After the area gained the attention of international conservation groups, the Brazilian government made efforts to monitor development and protect the Amazon's forest resources. However, the environmental effects of development and deforestation continue to be difficult issues as the region is developed.
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