Fig. 69—A stream of the intermittent type in the coastal desert of Peru. Depth of water in the Puira River at Puira, 1905. (Bol. de Minas del Perú, 1906, No. 45, p. 2.)
Fig. 70—A stream of the perennial type in the coastal desert of Peru. Depth of water in the Chira River at Sullana, 1905. Data from May to September are approximate. (Bol. de Minas del Perú, 1906, No. 45, p. 2.)
These three regions contain all the fertile coastal valleys of Peru. The larger ones are impressive—with cities, railways, ports, and land in a high state of cultivation. But they are after all only a few hundred square miles in extent. They contain less than a quarter of the people. The whole Pacific slope from the crest of the Cordillera has about 15,000 square miles (38,850 sq. km.), and of this only three per cent is irrigated valley land, as shown in [66] . Moreover, only a small additional amount may be irrigated, perhaps one half of one per cent. Even this amount may be added not only by a better use of the water but also by the diversion of streams and lakes from the Atlantic to the Pacific. Figs. 67 and 68 represent such a project, in which it is proposed to carry the water of Lake Choclococha through a canal and tunnel under the continental divide and so to the head of the Ica Valley. A little irrigation can be and is carried on by the use of well water, but this will never be an important source because of the great depth to the ground water, and the fact that it, too, depends ultimately upon the limited rains.
The inequality of opportunity in the various valleys of the coastal region depends in large part also upon inequality of river discharge. This is dependent chiefly upon the sources of the streams, whether in snowy peaks of the main Cordillera with fairly constant run-off, or in the western spurs where summer rains bring periodic high water. A third type has high water during the time of greatest snow melting, combined with summer rains, and to this class belongs the Majes Valley with its sources in the snow-cap of Coropuna. The other two types are illustrated by the accompanying diagrams for Puira and Chira, the former intermittent in flow, the latter fairly constant.[20]