42 THfi OASES OF THE NORTH-WEST 



oases with their backs to the same slope. When the 

 valh is narrow, the costas on either side of the sterile 

 depression face each other, like two parallel roads. The 

 water of the quehrada is never sufficiently abundant to 

 irrigate the whole of the cone of the torrent. In order 

 to create an oasis there, they have selected the most 

 easily cultivable zone, which is usually the foot of 

 the cone, where the deposits are finer and more fertile, 

 retain the moisture better, and require less watering. 

 The summit of the cone is composed of coarse stones, 

 the first to be dropped by the torrent as it loses its 

 strength. These are bad lands, where the water is 

 wasted. 



To meet the occasional drought and the danger of 

 sudden floods in this fluvial zone, which is entirely the 

 domain of the torrent, there is need of constant care 

 and ingenuity. At Colalao del Valle the cultivated 

 fields are five or six miles from the summit of the cone. 

 After a number of successive years of drought the 

 stream of water which reached them on the flanks 

 of the cone lost half its volume and threatened to dis- 

 appear altogether. They then built a stone dam at 

 the outlet of the quehrada, and the water accumulates 

 behind this during the night. At three o'clock in the 

 morning the sluices are opened, and the stream, having 

 thus nursed its strength, reaches the fields down below 

 about seven o'clock. Then the sun and the wind rise, 

 just at the time when the reservoir is empty, and by 

 the middle of the day the stream ceases, and irrigation 

 is suspended. At Andalgala, above which rises the 

 glittering crest of Aconcagua, the waters of the melting 

 snows which feed the torrent have not time to be 

 " decanted " before they reach the valley. They come 

 down laden with mud and sand. Above the points 

 where the irrigation-channels begin the people make, 

 in the bed of the torrent, a dam of branches of trees 

 which filters the water. It is swept away by every 

 flood that occurs, and is at once restored. 



