Ecuador is characterized by three distinct regions: the coast; the highlands, or Sierra; and the eastern interior lowlands, or Oriente. The coast, except for a hilly area west of Guayaquil, is a low alluvial plain from 32 to 185 km (20 to 115 mi) wide, comprising about one-quarter of the national territory. It extends from sea level to the base of the Cordillera Real of the Andes, at an elevation of about 460 m (1,500 ft). The Guayas in the southwest and the Esmeraldas in the northwest form the principal river systems and serve as important arteries of transportation in their respective regions.
The highlands constitute another fourth of the country. This region is formed by two parallel ranges of the Andes, from 110 to 290 km (70 to 180 mi) wide, and the intervening narrow central plateau, nearly 640 km (400 mi) long. This inter-Andean plateau is divided into 10 basins at altitudes from 2,400 to 2,900 m (7,800 to 9,500 ft), some draining east and some west. The Andes are studded with massive snow-capped volcanoes, the highest of which are Chimborazo, 6,267 m (20,561 ft); Cotopaxi, 5,897 m (19,347 ft), the world's third-highest active volcano; Cayambe, 5,790 m (18,996 ft); Antisana, 5,705 m (18,717 ft); Altar, 5,320 m (17,454 ft); Iliniza, 5,266 m (17,277 ft); Sangay, 5,230 m (17,159 ft); and Tungurahua, 5,016 m (16,457 ft).
The Oriente, forming part of the upper Amazon Basin, begins at the base of the Andes at about 1,200 m (4,000 ft). The land at first drops quickly and is segmented by rushing torrents escaping from the cold highlands. At about 260 m (850 ft), the forests become almost level, and the streams suddenly widen into sluggish, meandering rivers as they begin their journey down the Amazon system to the Atlantic.