Amazon River, (Portuguese and Spanish: Amazonas), one of the great rivers of the world and the chief river of South America. With a length of about 4,000 miles (6,400 km), it is the second longest river in the world, exceeded only by the Nile. The Amazon's flow is by far the world's largest. On average, the river discharges some 4,200,000 cubic feet (119,000 m 3) per second into the Atlantic Ocean—a volume of water roughly seven times as great as that discharged by the Mississippi River. The Amazon is also one of the world's deepest rivers. Downstream from Manaus there are numerous places with depths of more than 300 feet (90 m).
The Amazon begins as the Huarco, a small stream high in the Andes of southern Peru, about 100 miles (160 km) from the Pacific Ocean. From there it flows northward, progressively becoming the Toro, Santiago, Ene, Tambo, Apurímac, and Ucayali. The Amazon proper begins at the junction of the Ucayali and the Marañón, south of Iquitos, Peru. From Iquitos the Amazon flows eastward through the Amazon Basin of Brazil, reaching the Atlantic Ocean through a wide estuary at the Equator. Most of the water enters the sea through several branches north of Marajó Island; some of the flow goes south of the island via the Pará River past the port city of Belém.
After leaving the Andes, the Amazon becomes increasingly wide and follows a gentle gradient, or slope, to the sea. In many areas there are numerous bordering lakes, marshy islands, and swamps. Although it carries silt in suspension, the Amazon's water, in a chemical sense, is amazingly pure, in places approaching the quality of distilled water. Unlike most rivers that flow into the ocean, the Amazon has not built a sizable delta. Instead, the river's enormous discharge moves far out to sea, where its silt is intercepted and carried northward by strong ocean currents. Tides occur in the estuary and a tidal bore, a wall of water that moves rapidly upstream, sometimes forms.
Thousands of rivers flow, directly or indirectly, into the Amazon, draining parts of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, and Venezuela. The longest tributary is the 2,100-mile (3,380-km) Madeira River; other major tributaries include the Negro, Tapajós and Xingu. Many tributaries have falls and rapids that impede navigation. The Amazon river system is joined to the Orinoco system by the Casiquiare River in Venezuela.
The Amazon Basin covers an area of about 2,720,000 square miles (7,040,000 km 2)—almost 40 per cent of the total area of South America.
One of the world's largest forested tracts covers the Amazon Basin. Growing there is a profusion of different species of trees in various kinds of forests. Most widespread are the tropical rain forests, where trees reach heights of up to 200 feet (60 m) and form a dense canopy overhead. Because little sunlight reaches the forest floor, there is scant underbrush. There are also savanna grasslands in the basin.
The forests and rivers of the Amazon Basin teem with a wide variety of animal life. Carnivores such as pumas, jaguars, and ocelots prey on deer, monkeys, rodents and other animals. Reptiles include crocodiles, caimans, and such snakes as boa constrictors and anacondas. Hundreds of kinds of brightly colored birds and myriad species of insects fill the forests. The rivers abound with many species of fish, including piranhas and pirarucus.
Large-scale development of the Amazon Basin began in the early 1970's. Roads were built and vast tracts of forests were cleared for the development of mining, farming, ranching, and lumbering, and for settlements. The clearing of forests scarred the land and threatened the habitats of many species of plants and animals. The destruction of the Amazon rain forest environment reached its height during the mid-1980's.
The river is navigable as far as Iquitos, Peru, some 2,300 miles (3,700 km) from its mouth. Oceangoing ships can reach Manaus, Brazil. The chief cities on the river include Iquitos, Manaus, and the Brazilian cities of Macapá and Santarém. Belém, the chief port, is on the Pará River. Elsewhere there are small riverside towns and settlements and aboriginal Indian communities.
The Spanish, arriving in the early 16th century, were the first Europeans to see the Amazon. Francisco de Orellana was the first white man to sail down the river. He is credited with naming it, for the legendary female warriors, after supposedly coming in contact with warlike women along the river. A few missions were established here, but serious settlement did not occur until the rubber boom of the 19th century. When Brazil lost its monopoly on rubber to Asian plantations, the industry declined rapidly.