Originating within 100 miles (160 km) of the Pacific Ocean at its westernmost source high in the Peruvian Andes, the Amazon flows almost 4,000 miles (6,400 km) across northern Brazil to its mouth in the Atlantic Ocean on the northeastern coast of Brazil. Its length is second only to that of the Nile. It is estimated that about 20 percent of all the water that runs off the Earth's surface is carried by the Amazon. The flood-stage discharge at the river's mouth is about 6,360,000 cubic feet (180,000 cubic m) per second, which is more than 10 times that of North America's Mississippi River and four times that of the Congo (Zaire) River in Africa. The discharge is so great that it turns the ocean's water from salty to brackish for more than 100 miles offshore.
There are more than 1,000 known tributaries of the Amazon, rising in the Guiana Highlands, the Brazilian Highlands, and (principally) the Andes. Seven of these are longer than 1,000 miles (1,600 km); one, the Madeira, flowing northeastward from Bolivia, is more than 2,000 miles (3,200 km) long. Navigable throughout the year, the Amazon can accommodate large freighters as far inland as Manaus, 1,000 miles upriver from the Atlantic. The Amazon flows over a gently sloping terrain and is characterized by oxbow lakes, abandoned channels, and other marks of a meandering river. Floods, by depositing fertile silt as they recede, annually rejuvenate an area of some 25,000 square miles (65,000 square km), or nearly double that of The Netherlands, and the river's total drainage basin, about 2,700,000 square miles (7,050,000 square km), is nearly twice as large as the area drained by any other river in the world. The Amazon and its tributaries may be described as a vast sea of freshwater, supporting millions of fish, including catfish, electric eels, and piranhas.
The tribal cultures of South America are so various that they cannot be adequately summarized in a brief space. The mosaic is baffling in its complexity: the cultures have interpenetrated one another as a result of constant migratory movements and through intertribal relations, leading to the obliteration of formerly significant differences, and to new cultural systems made up of elements of heterogeneous origin. Hundreds of languages, in very irregular geographic distribution, with innumerable dialects, are or have been spoken in the tropical area of South America. Thus, only the broadest generalizations can be made; one can mention certain cultural manifestations that are present in a great number of groups, even though varying in their actual expression, and illustrate them with specific examples--but always with the qualification that in a neighbouring tribe or group a distinctly contrasting idea or institution may exist.
The innumerable native peoples differ in their patterns of adaptation to their natural environment. Whether they live in the rain forest, in the gallery forests lining the rivers, in the arid savannas, or in the swamps, however, they share a common cultural background; they often combine fishing and hunting with rudimentary farming. Most are relatively sedentary, but some are nomadic or semi-nomadic. Greater differences are sometimes found among neighbouring groups living in the same forest than between some forest and savanna peoples. And some tribes, when migrating to open areas, maintain to a great extent the forest characteristics of their culture.
On the banks of the great rivers but also in zones between the forest and the savanna live tribes who gain their subsistence from farming and fishing. Hunters and gatherers, almost all of whom also practice some farming, have settled near the heads of rivers, in open land, or in gallery forests.
Tribes speaking related languages are scattered over a large part of the continent. The tribes of the Arawak and the Carib linguistic families are most numerous in the Guianas (French Guiana, Guyana, Suriname, and the adjacent regions of Venezuela and Brazil) as well as in other parts of the northern Amazon, but the former have representatives as far south as the Chaco and the latter as far south as the upper Xingu. The Tupí tribes extend to the south of the Amazon valley. The Ge family includes groups most of which are located in the semi-arid lands of central Brazil. In the extreme northwest of Brazil and in the jungles of eastern Peru and Bolivia live the Pano tribes. The Jívaro of Ecuador are famous headhunters. They cut off the enemy's head, separate the soft part from the skull, and, with the help of hot sand, reduce it to the size of a fist without altering the physiognomy. They attribute great magical power to these trophies, or tsantsa.
The overwhelmingly dominant feature of the Amazon basin is the tropical rain forest, or selva. From the air the Amazon forest appears to stretch unbroken to the horizon like a tufted green carpet. Closer inspection reveals its bewildering complexity and prodigious variety of trees; as many as 100 arboreal species have been counted on a single acre of forest with hardly any one of them occurring more than once. The Amazon forest has a strikingly layered structure. The sun-loving giants of the uppermost story, the canopy, soar to as much as 120 feet above the ground; occasional individual trees, known as emergents, rise beyond the canopy, frequently attaining heights of 200 feet. Their straight, whitish trunks are splotched with lichens and fungus. A characteristic of these giant trees is the buttresses, or basal enlargements of their trunks, which presumably help stabilize the top-heavy trees during infrequent heavy winds. Further characteristics of the canopy trees are their narrow, downward-pointing "drip-tip" leaves that easily shed water and their cauliflory (the production of flowers directly from the trunks rather than from the branches). Flowers are inconspicuous. Among the canopy species, which capture most of the sunlight and conduct most of the photosynthesis, prominent members include the rubber tree (Hevea brasiliensis), the silk-cotton (Ceiba pentandra), the Brazil nut (Bertholletia excelsa), the sapucaia (Lecythis), and the sucupira (Bowdichia). Many creatures, including monkeys and sloths, spend their entire lives in this sunlit canopy. Below it are found two or three levels of shade-tolerant trees, including many species of palms, such as Mauritia, Orbignya, and Euterpe. Myrtles, laurels, bignonias, figs, Spanish cedars, mahogany, and rosewoods are also common. They support a myriad of epiphytes (plants living on other plants)--such as orchids, bromeliads, and cacti--as well as ferns and mosses. The entire system is laced together by a bewildering network of woody ropelike vines known as lianas.
In addition to the rain forests of the terra firme, there are two types of inundated rain forests, várzea and igapó, which constitute about 3 percent of the total Amazonian rain forest. Várzea forests can be found in the silt- and nutrient-rich floodplains of whitewater rivers such as the Madeira and the Amazon, with their ever-changing mosaic of lakes, marshes, sandbars, abandoned channels, and natural levees. They are generally not as high, diverse, or old as those of the terra firme, being subject to periodic destruction by floods and human manipulation. (The várzea and its flood-free margins are the principal rain-forest habitat of human beings.) Wild cane (Gynerium) and aquatic herbs and grasses, as well as fast-growing pioneer tree species such as Cecropia, Ficus, and Erythrina, are conspicuous.
Igapó forests grow along the sandy floodplains of blackwater rivers such as the Negro, the Tapajós, and the Trombetas. Because human settlement is limited in these plains, there may be undisturbed, seasonally flooded forests that stand in water for up to half the year, the water reaching heights of up to 40 feet. A canoe can often be paddled between the trunks of trees adapted to such an aquatic environment. The lowland rain forest on the Andean fringe grades into a discontinuous, tangled montane or cloud forest of misshapen trees cloaked with mosses, lichens, and bromeliads. Here one encounters the cinchona or fever-bark tree, once exploited for its antimalarial agent quinine. At still higher elevations is found the grass and shrub growth of the cold puna and páramo regions.
Along the drier, southern margin of the Amazon basin, high forest gives way to the cerrado (savanna and scrub) and caatinga (heath forest). The latter is characteristic of parts of the Mato Grosso Plateau, where taller forest is restricted to the stream courses and swales (marshy depressions) that dissect the upland surface. On the sandy soils of the lower Negro and the Branco drainage areas and locally in Amapá, grassy savannas dotted with stunted trees replace the high forest.
