The Cordillera Occidental is a chain of dormant volcanoes and solfataras, volcanic vents emitting sulfurous gases. Bolivia's highest peak, the snowcapped Sajama (6,542 meters), is located here. The entire cordillera is of volcanic origin and an extension of the volcanic region found in southern Peru. Most of the northern part of this range has an elevation of about 4,000 meters; the southern part is somewhat lower. Rainfall, although scanty everywhere, is greater in the northern half, where the land is covered with scrub vegetation. The southern area receives almost no precipitation, and the landscape consists mostly of barren rocks. All of the Cordillera Occidental region is sparsely populated, and the south is virtually uninhabited.
The Altiplano, the high plateau between the two cordilleras, comprises four major basins formed by mountainous spurs that jut eastward from the Cordillera Occidental about halfway to the Cordillera Oriental. Along the Altiplano's eastern side is a continuous flat area, which has served as Bolivia's principal north-south transportation corridor since colonial times. The entire Altiplano was originally a deep rift between the cordilleras that gradually filled with highly porous sedimentary debris washed down from the peaks. This sedimentary origin explains its gradual slope from north to south; greater rainfall in the north has washed a larger quantity of debris onto the platform floor.
The most prominent feature of the Altiplano is the large lake at its northern end, Lake Titicaca. At 3,810 meters above sea level, it is the highest navigable body of water in the world. With a surface area of 9,064 square kilometers, it is larger than Puerto Rico and is South America's largest lake. Lake Titicaca is also deep, about 370 meters at its maximum, but with an average depth of 215 meters; its volume of water is large enough to maintain a constant temperature of 10° C. The lake actually moderates the climate for a considerable distance around it, making crops of corn and wheat possible in sheltered areas.
Lake Titicaca drains southward through the slow-moving, reedfilled Desaguadero River to Lake Poopó. In contrast to the freshwater Lake Titicaca, Lake Poopó is salty and shallow, with depths seldom more than four meters. Because it is totally dependent on seasonal rainfall and the overflow from Lake Titicaca, Lake Poopó's size varies considerably. Several times in the twentieth century, it nearly dried up when rainfall was low or the Desaguadero River silted. In years of heavy rainfall, however, Lake Poopó has overflowed to the west, filling the Coipasa Saltpan with shallow water.
Rainfall in the Altiplano decreases toward the south, and the scrub vegetation grows more sparse, eventually giving way to barren rocks and dry red clay. The land contains several salt flats, the dried remnants of ancient lakes. The largest of these is the Uyuni Saltpan, which covers over 9,000 square kilometers. The salt is more than five meters deep in the center of this flat. In the dry season, the lake bed can be traversed by heavy trucks. Near the Argentine border, the floor of the Altiplano rises again, creating hills and volcanoes that span the gap between the eastern and western cordilleras of the Andes.
The much older Cordillera Oriental enters Bolivia on the north side of Lake Titicaca, extends southeastward to approximately 17 south latitude, then broadens and stretches south to the Argentine border. The northernmost part of the Cordillera Oriental, the Cordillera Real, is an impressive snow-capped series of granite mountains. Many of these peaks exceed 6,000 meters, and two--Illimani (6,322 meters), which overlooks the city of La Paz, and Illampu (6,424 meters)--have large glaciers on their upper slopes. South of 17 south latitude, the range changes character. Called the Cordillera Central here, the land is actually a large block of the earth's crust that has been lifted and tilted eastward. The western edge of this block rises in a series of steep cliffs from the Altiplano. The backbone of the cordillera is a high, rolling plain, with elevations from 4,200 to 4,400 meters, interspersed with irregularly spaced high peaks. Too high to be exploited for large-scale commercial grazing, this area takes its name from the predominant vegetation type, the puna.
Data as of December 1989