HYDROGRAPHY. The Andes system is the source of most of the larger streams of South America. Through nearly its whole extent, wherever the system comprises more than a sin gle main range, the westernmost of these ranges separates the drainage to the Atlantic from that to the Pacific. In Ecuador, however, no fewer than seven of the ten high valleys between the ranges are drained westward, and in southern Chile, as has been seen, glaciers have eroded their sources back across the whole range to the Argentina plains. The western streams are short, and owing to the light rainfall on most of the western slope, have small volume. Hence their cutting power is slight. On the other hand, the streams to the east are long, with great drainage basins, and, except in Argentina, are supplied with abundant precipitation by the trade winds. Hence they are powerful streams of large volume, and have eroded their courses far up into the mountains.
The Andes of Colombia are drained northward to the Caribbean Sea by the Magdalena, Cauca, and Atrato rivers, and eastward to the same body of water by the Orinoco, and to the Atlantic by the Negro and Yapur5. great branches of the Amazon. The system in Ecuador, Peru, and most of Bolivia is drained eastward by count less tributaries of the Amazon, among which are the Napo, Maranon, Ucayali, Beni, and Ma more. Of these, the Maranon heads between the
ranges far to the south, near the Knot of Cerro de Pasco, flowing northwest within the mountain system for 400 miles before breaking through the eastern range into the Amazon basin. The Huallaga, Mantaro, Apurimac, and Urubamba, tributaries of the Maranon, also head betWeen the ranges, cutting gorges through the eastern range. In Bolivia and northwest Argentina is a great region, 800 miles in length. lying between the ranges, with an average altitude of 13,000 feet, which has no drainage to either ocean. In this region is the great Lake Titicaca, which drains by the Rio Desaguadero to Lake Poopo, where the drainage of this semi-desert region is collected. This lake in earlier times drained to the Amazon, but by shrinkage in volume its outlet has been closed, and now it discharges only by evaporation. The eastern slope of the Andes in southern Bolivia and northern Argentina is drained to the Plata, while farther south short er streams, the Rio Colorado, the Negro. Chubut, and the Deseado, and the Arroyos Bayo and Salado, and other smaller streams, carry the drainage directly to the Atlantic.