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 “much” and “little” with reference to rainfall are here purely relative terms.

The rainy season of Copiapé extends from May to August, with occasional rains as early as April and as late as October; but the rain is limited to individual showers: the average num- ber of rainy days a year in the period 1888-1913 was between three and four. The heaviest individual rainfalls were 36.58 mm, (1.4 inches) on August 13 and 14, 1888; 54.4 mm, (2.1 inches) on July 10, 1902; 42 mm. (1.6 inches) on August 21, 1900. Such heavy rainfalls result in flood. Floods, too, orig- inate in the still heavier storms of the cordillera.

Floods are not unknown in the nitrate region, as we noted on an earlier page, but they are of short duration and quite infre- quent even though the greatest of them are known to have covered vast stretches of the nitrate pampa. Such floods are of unusual occurrence because they require unusual snowfall in the mountains combined with very rapid melting, and these two circumstances are rarely brought about in the same sea- son. South of the nitrate desert, as in the mountain region east of Copiapé, not only the rains but the snows are much more frequent, hence also the chance of foods. | Here the combina- tion of rain and melting snow may greatly augment the stream flow. Whereas only about one cubic meter a second was flow- ing in the natural channel of the Copiap6é River when I saw it in July, 1913, it has been known to increase to 100 cubic me- ters a second, as in 1888 when 1.4 inches of rain fell at one time. Added to this we have a very interesting physiographic condition which heightens the floods and increases the risks of the valley dwellers who depend upon a mountain stream to furnish life to their fields and gardens—the great convergence of headwater streams in the mountains, A drainage map of almost any desert region shows a system curiously disorgan- ized and without plan save as a few radial lines of streams focus upon a given desert basin (Fig. 18). At intervals one basin may be found draining into another, and at still rarer