The experience gained since a connected system was begun in the management of the water courses has been of incalculable value, and many have been the landslips arrested and prevented by seeking their causes in the hidden erosive action of a small stream. In still another manner does the water threaten the mountain sides, and that is by permeating the soil (which is thus rendered much heavier), until it reaches a bed of rock or other layer that it can not penetrate. There it forms a layer of slippery mud on which the soil above slides bodily down. The method pursued in such cases is sometimes that of a ditch dug to the impermeable layer, sometimes that of a drain. In both cases there may or may not be small feeding ditches. Another plan which appears very contradictory of what has just been said is also employed. This consists in preventing, by means of horizontal trenches, the water from flowing off on the surface in the regions above the timber line. The water is forced to percolate through the soil and so reaches the wooded portions, where it would permeate anyhow, much more gradually than otherwise. The soil thus escapes the enormous increase of pressure due to the sudden absorption of great quantities of water, and consequently is less in danger of leaving its fastenings. One of the most difficult of the problems that these torrents give rise to is that of their control where they suddenly enter a valley, and where the slope is consequently greatly decreased. The decrease of current entailed causes the deposit of stones and material at the mouth of the gorge, and the water then spreads itself over the valley. This occurs more or less regularly with certain torrents that are usually dry and where it is impracticable to prevent the erosions above. It then becomes necessary to build a stone canal from the mouth of the gorge to the principal water course of the valley. As this must be built on the alluvion (which presents the surface of a cone), it is often higher than the rest of the valley, and one may find other small canals for the draining of the valley passing under the larger one and meeting the principal stream below. A similar action to that of the torrent on entering the valley is that of a stream with a rapid current emptying into one whose current is slower. Here the deposits will at times force the smaller stream to seek another channel, and it frequently occurs that the correction moves the mouth of the stream a considerable distance.