Peru is divided into three contrasting topographical regions: the coast (costa), the highlands (sierra), and the eastern rainforests (selva). The coastline is a narrow ribbon of desert plain from 16 to 160 km (10 to 100 mi) broad. It is scored by 50 rivers, which water some 40 oases. Only a few of these rivers, which have their source in the Andean snowbanks, reach the sea in all seasons. Although the coastal region constitutes only 12% of the national territory, it contains the ports and chief cities of Peru. Inland, the low costa rises through the steep wastes of the high costa (760–2,000 m/2,500–6,500 ft), then ascends abruptly to the western cordillera (Cordillera Occidental) of the Andes, which, with its ridge of towering peaks, runs parallel to the coast and forms the Peruvian continental divide. The less regular Cordillera Central and Cordillera Oriental merge in central Peru with the Cordillera Occidental. They branch off to the southeast, meeting a transverse range that becomes a crescent of peaks forming the drainage basin of the 8,288 sq km (3,200 sq mi) Lake Titicaca, the highest large navigable lake in the world (about 3,810 m/12,500 ft high), which is bisected by the Peruvian-Bolivian border. Of the 10 Peruvian peaks that rise above 5,800 m (19,000 ft), Huascarán, 6,768 m (22,205 ft), is the highest.
The intermontane basins, deep-gashed canyons, and high treeless plateaus (punas) of the Andes form the sierra and constitute 27% of the country's surface. The most important rivers draining the Andes on the Atlantic watershed, such as the Marañón, Huallaga, and Ucayali, flow north or south and eventually east to form the Amazon Basin. The selva covers 61% of Peru and consists of the low selva (the Amazon rainforest) and the high selva, a steeply sloping transition zone about 100–160 km (60–100 mi) wide between the sierra and the rain forest.