Bolivia has three geographic zones: the Andean highlands in the southwest, running north to south; the moist slopes and valleys on the eastern side of the Andes, called the Yungas and Valles; and the eastern tropical lowland plains, or Oriente. In Bolivia, the Andes, divided into two chains, attain their greatest width, about 640 km (400 mi), and constitute about one-third of the country. Between the Cordillera Occidental, forming the border with Chile and cutting Bolivia off from the Pacific, and the complex knots of the Cordillera Oriental lies a broad sedimentary plateau about 4,000 m (13,000 ft) above sea level, called the Altiplano, which contains about 28% of Bolivia's land area and more than half of its population. In the north of this plateau, astride the border with Peru, lies Lake Titicaca, 222 km (138 mi) long and 113 km (70 mi) wide; with its surface at an altitude of 3,805 m (12,484 ft), it is the highest navigable lake in the world. The lake is drained to the south by the 322-km (200-mi) Desaguadero River, which empties into shallow, salty Lake Poopó. Farther south are arid salt flats. The Cordillera Oriental has high habitable basins and valleys collectively referred to as the Puna. Bolivia's most majestic mountains are in the northern part of the Cordillera Oriental around Lake Titicaca, where the mountain sector is capped with snow; the highest of these is Ancohuma (6,550 m/21,489 ft). Illimani and Illampu, both rising more than 6,400 m (21,000 ft), overlook the city of La Paz, which is protected from cold winds by its position in the spectacular gorge formed by the headwaters of the La Paz River. The three important valleys of this region, Cochabamba, Sucre, and Tarija, are from 1,830 to 3,050 m (6,000 to 10,000 ft) in altitude.
Bolivia's important rivers descend across the Yungas and Valles into the low tropical plains of the Oriente, which comprises three-fifths of the land but has only about one-fifth of the population. The Guaporé, the Mamoré, the Beni, and the Madre de Dios rivers cross the often-flooded northern savanna and tropical forests, all converging in the northeast to form the Madeira, which flows into Brazil. The plains become drier in the southeast, forming Bolivia's scrub-covered Chaco. Crossing the Chaco to the southeast, the Pilcomayo River leaves Bolivia to form the border between Paraguay and Argentina.