Amazon

(also Amazonas), a river in South America. It is the world’s largest river in terms of the volume of water carried, the size of the basin, and the length of the river system. The aboriginal Indians call the Amazon the Paraná-tinga (White River) and Paraná guazu (Great River). The area of the basin is 7.18 million sq km; the length from the main source, the Marañón River, is 6,400 km; and the length from another source, the Ucayali River, is more than 7,000 km. A large portion of the basin is located in Brazil, where the part of the Amazon between its confluence with the Ucayali and the mouth of the Rio Negro is called the Solimóes. The southwestern and western regions are located in Bolivia, Peru, Ecuador, and Colombia. The boundaries of the basin are the eastern slopes of the Andes, the southern slopes of the Guiana Highlands, the entire northern and central Brazilian highlands, and the Amazon basin, one of the largest lowland territories in the world.

The basic course of the Amazon is located between the equator and 5° S lat.—that is, in the region with the most abundant and even precipitation (1,500–3,000 mm per year). The main source, the Marañón River, rises in the eastern slopes of the western Cordilleras in Peru at an elevation of 4,840 m above sea level, flows in the mountains parallel to the ocean coast in a deep depression, and then turns east, cutting through the Andes to form 27 so-called pongos (deep, rocky, narrow gorges with nearly vertical walls). Beyond this the Marañón leaves the mountains and enters the Amazon lowlands and, joining the Ucayali River which comes in on the right, becomes the Amazon. Over the entire remaining distance, the Amazon flows across a plain which is frequently swampy and is covered by humid equatorial forests. The channel of the Amazon is bordered by low banks which descend to the river in three broad stages. The upper stage is terra firma, or the noninundated bank formed by the original valley wall, and it is 50 m or more in elevation. Below this stage stretches the floodplain. The middle stage, or várzea, is a part of the floodplain, which is inundated during major floods of the Amazon. The lower stage, the igapó, or swamp, is covered by water from the usual overflowing of the river. Below the confluence of the Rio Negro, the width of the floodplain reaches 80–100 km, narrowing somewhat only near the towns of Obidos and Santarém. The surface of the floodplain is cut by the channels of the numerous branches and streams and is dotted with lakes and old river beds. Low channel banks stretch along the shore. Some 350 km from the ocean, the Amazon forms a delta which is one of the largest in the world, encompassing an area of about 100,000 sq km. A major portion of the drainage travels along the northeastern channels, and a portion flows along the eastern channel of the Pará. Between this and the main channels lies one of the world’s largest river islands, Marajó.

The Amazon is fed by numerous tributaries, about 20 of which are large rivers some 1,500 to 3,500 km long. The most significant tributaries on the right are the Ucayali, the Juruá, the Purus, the Madeira, the Tapajós, the Xingu, and the Tocantins (which flows into the Pará). On the left are the Napo, the Içá, the Yapurá, and the Rio Negro. These tributaries rapidly increase the amount of water in the Amazon. Past the confluence with the Ucayali, the river’s width is about 2 km; in the middle of its course it reaches 5 km; in the lower part it is 15 to 20 km; and right before the mouth it is 80 to 150 km. The river’s depth is approximately 70 m in the middle reaches, 135 m near the town of Óbidos, and 15 to 45 m at the mouth. The Amazon’s tributaries differ not only in their sizes and amounts of water but also in the color of the water. The waters of the Rio Negro are basically dark, and the Rio Branco is a milky color. There are rivers with yellow, gray, greenish, and even reddish casts to the water. There is no other river in the world which has such an abundance of colored waters as the Amazon.

The Amazon has a complex and unique regime. It is in full flood all year. The right tributaries, with their basins lying in the southern hemisphere, and the left tributaries, with their drainage basins in the northern hemisphere, flood at different seasons because the rainfall occurs in different seasons. The right tributaries flood from October through March, which is the summer in the southern hemisphere, while the left tributaries flood from April through October, which is the summer in the northern hemisphere. In this way the seasonal fluctuations in the drainage are smoothed out. The southern tributaries carry more water; from May through July they cause the greatest rises in the water level and the highest floods on the Amazon. In August through September, their water levels are low. The maximum water discharges in the Amazon are 300,000 cubic meters per second (cu m per sec) and more. In the Atlantic Ocean, as far as 300 km offshore, a yellowish tint can be seen in the water. With low water levels, the discharge drops to 70,000–80,000 cu m per sec. The mean discharge is about 175,000 cu m per sec, with an annual drainage of about 5,520 cu km. The Amazon is responsible for 15 to 17 percent of the total annual drainage of all the world’s rivers. Each year, the Amazon carries an average of more than 1 billion tons of solid material out of its basin. In the lower reaches, the river’s regime is substantially affected by the tides, which extend 1,400 km up the river. At the mouth, the tides are accompanied by the so-called pororocas (“roaring waters”)—breaking waves sometimes as high as 4 or 5 m which rush up the river at high speed and with a great roar, inundating and destroying the banks. In one of the local Indian dialects the pororoca is called an amazunu. (Some geographers derive the name of the river itself from this.)

The plant and animal world of the Amazon is very rich. In the lakes and channels of the Amazon is found the Queen Victoria waterlily of the Nymphaeaceae family, the leaves of which reach enormous sizes. Among the mammals are the manatee (in the river’s mouth) and the river dolphins— Inia geoffroyensis and others. There are numerous fishes (approximately 2,000 species, or one-third of all the freshwater fauna in the world). The arapaima (approximately 4 m long) is of economic importance, and the piranha is characteristically a predator. Rays, eels, snakes, and crocodiles are also found.

With its tributaries, the Amazon forms the world’s greatest system of inland waterways, with a total length of more than 25,000 km. The main channel of the Amazon is navigable over a distance of 4,300 km (up to Pongo de Man-seriche gorge). Large ocean-going vessels go as far as Ma-naus, 1,690 km from the mouth. The main ports in the Amazon basin are Belém (Pará), Santarém, Óbidos, Manaus, and Iquitos. The Amazon possesses enormous power potential, estimated at nearly 280 million kilowatts; however, the hydropower utilization of the river is insignificant.

The mouth of the Amazon was discovered in 1500 by the Spaniard Vincente Yáñez Pinzon, who called the river Rio Santa Maria de la Mar Dulce or “The River of Saint Mary of the Fresh Sea.” (Scores of kilometers away from shore the water was still fresh.) The first voyage up the river was made in 1541 by the Spanish conquistador F. de Orellana, and the first scientific voyage was made by the French scientist La Condamine in 1743 and 1744. A survey of the region of the Amazon’s sources was mainly carried out by a German-Peruvian expedition in 1955.