In most cases however, the _valle_ has no running water. What reaches it from the lateral _quebradas_ is lost in the alluvial beds accumulated at the point where the _quebrada_ enters the _valle_. In order to make use of it the cultivated areas are grouped on the cone of deposition; at least, that is the position in the great majority of the oases. A _costa_ is a line of separate oases with their backs to the same slope. When the _valle_ is narrow, the _costas_ on either side of the sterile depression face each other, like two parallel roads. The water of the _quebrada_ 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 disappear altogether. They then built a stone dam at the outlet of the _quebrada_, 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.