Geology, Geomorphology, and Drainage

Geology, Geomorphology, and Drainage

In contrast to the Andes, which rose to elevations of nearly 7,000 meters in a relatively recent epoch and inverted the Amazon's direction of flow from westward to eastward, Brazil's geological formation is very old. Precambrian crystalline shields cover 36 percent of the territory, especially its central area. The principal mountain ranges average elevations just under 2,000 meters. The Serra do Mar Range hugs the Atlantic coast, and the Serra do Espinhaço Range, the largest in area, extends through the south-central part of the country (see fig. 3). The highest mountains are in the Tumucumaque, Pacaraima, and Imeri ranges, among others, which traverse the northern border with the Guianas and Venezuela.

In addition to mountain ranges (about 0.5 percent of the country is above 1,200 meters), Brazil's Central Highlands include a vast central plateau (Planalto Central). The plateau's uneven terrain has an average elevation of 1,000 meters. The rest of the territory is made up primarily of sedimentary basins, the largest of which is drained by the Amazon and its tributaries. Of the total territory, 41 percent averages less than 200 meters in elevation. The coastal zone is noted for thousands of kilometers of tropical beaches interspersed with mangroves, lagoons, and dunes, as well as numerous coral reefs.

Brazil has one of the world's most extensive river systems, with eight major drainage basins, all of which drain into the Atlantic Ocean. Two of these basins--the Amazon and Tocantins-Araguaia--account for more than half the total drainage area. The largest river system in Brazil is the Amazon, which originates in the Andes and receives tributaries from a basin that covers 45.7 percent of the country, principally the north and west. The main Amazon river system is the Amazonas-Solimões-Ucayali axis (the 6,762 kilometer-long Ucayali is a Peruvian tributary), flowing from west to east. Through the Amazon Basin flows one-fifth of the world's fresh water. A total of 3,615 kilometers of the Amazon are in Brazilian territory. Over this distance, the waters decline only about 100 meters. The major tributaries on the southern side are, from west to east, the Javari, Juruá, Purus (all three of which flow into the western section of the Amazon called the Solimões), Madeira, Tapajós, Xingu, and Tocantins. On the northern side, the largest tributaries are the Branco, Japurá, Jari, and Negro. The above-mentioned tributaries carry more water than the Mississippi (its discharge is less than one-tenth that of the Amazon). The Amazon and some of its tributaries, called "white" rivers, bear rich sediments and hydrobiological elements. The black-white and clear rivers--such as the Negro, Tapajós, and Xingu--have clear (greenish) or dark water with few nutrients and little sediment.

The major river system in the Northeast is the São Francisco, which flows 1,609 kilometers northeast from the south-central region. Its basin covers 7.6 percent of the national territory. Only 277 kilometers of the lower river are navigable for oceangoing ships. The Paraná system covers 14.5 percent of the country. The Paraná flows south into the Río de la Plata Basin, reaching the Atlantic between Argentina and Uruguay. The headwaters of the Paraguai, the Paraná's major eastern tributary, constitute the Pantanal, the largest contiguous wetlands in the world, covering as much as 230,000 square kilometers.

Below their descent from the highlands, many of the tributaries of the Amazon are navigable. Upstream, they generally have rapids or waterfalls, and boats and barges also must face sandbars, trees, and other obstacles. Nevertheless, the Amazon is navigable by oceangoing vessels as far as 3,885 kilometers upstream, reaching Iquitos in Peru. The Amazon river system was the principal means of access until new roads became more important in the 1970s. The São Francisco was also used for transportation in the past. Dams and locks in the Paraná system have made it an important artery for interstate and international trade in the 1990s.

The various river systems descending from the shields have endowed Brazil with vast hydroelectric potential, estimated at 129,046 megawatts (MW), of which 30,065 MW were in operation or under construction in 1991. The largest hydroelectric projects are Itaipu, in Paraná, with 12,600 MW; Tucuruí, in Pará, with 7,746 MW; and Paulo Afonso, in Bahia, with 3,986 MW.

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