Page:Isaiah Bowman - Desert Trails of Atacama (1924).pdf/131



The floods of a desert basin that is self-contained are limited in size and in devastating effect because the watershed is lim- ited. In a desert country there are as many separate floods as there are separate basins. When the basins coalesce, however, it is the sum of all the floods that reaches the main valley. Not merely this, the run-off in such cases is much more rapid because there is a succession of channels down to the main through-flowing stream rather than a series of bordering allu- vial flats into which all the floods may readily sink.

It is precisely this condition which is encountered in the Andes in the region of Copiapó (cf. pp. 47-48). The rainfall of the high mountain zone is sufficient to bring about a normal organization of stream courses on both sides of the Andes, and instead of the interior basins of northwestern Argentina, south- western Bolivia, and northern Chile we have here wide-branch- ing tributaries and streams that flow through to the sea (Fig. 86). The relation of such an organized drainage system to floods is not merely of technical interest; it is immensely im- portant to the people who live in the valley below.

Naturally a more constant stream like the Copiapó River calls into being a larger settlement, and in general we find that streams and settlements in desert regions are proportionate in size to each other. This means that if damage is done be- cause of the great floods that come down the Copiapó valley, or any valley so situated in relation to the snows and rains of a high mountain belt, it will be on a far larger scale than in the ravines on the drier western mountain border and interior basins of northern Chile.

It is the fate of desert communities that they should be devastated by the same agent to whose gentler operations they look with such delight. To take a specific example, on May 21, 1905, snowstorms raged in the cordillera, and the Copiapó River rose “enormously,” doing no end of damage throughout the whole middle and lower valley, cutting the rail- road below Copiapó, and sweeping away a bridge. Alfalfa fields were filled with mud and clay, tracks and roads were