THE TORRENTS OF SWITZERLAND. 49 



the streams are larger, and where the erosive action can not be modi- 

 fied by enlarging the channel, as is the case in many gorges, it is 

 necessary to make an artificial bed for the stream and at times to 

 supplement this by masonry dams. The dams are not permanent 

 in their effects, for as soon as the basin immediately above the dam 

 has been filled with deposits and the original slope of the stream 

 has re-established itself, the products of the erosion pass over the 

 dam. They prove, however, of great utility at times of large freshets 

 by causing deposits which are subsequently carried down in much 

 smaller quantities. This prevents the disasters that would be caused 

 by sudden enormous deposits when the streams are swollen. 



The prospect for many a narrow valley would be a sad one in- 

 deed if means had not been taken to prevent the lateral erosions so 

 common with the mountain torrents and so productive of land- 

 slips. Where the stream has provided itself with a stony bed on 

 which to roll, it often tries to do damage by leaving the stones and 

 attacking the earthy banks. It is in these cases that the danger has 

 proved so insidious; for until disasters actually occurred, in many 

 instances, the undermining effect of the water was not suspected, 

 being entirely hidden from view. 



The most economical way of combating such cases is by mak- 

 ing use of those points that by their formation arrest the erosive 

 action. This is done by re-enforcing them in such manner as to pro- 

 duce a series of natural steps. The upward march of the excavating 

 action of the water is stopped at least partially and temporarily at 

 each step. Such a method is of great advantage when it is necessary 

 to delay a more costly correction for financial reasons. 



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. An- 

 other plan which appears very contradictory of what has just been 

 said is also employed. This consists in preventing, by means of hori- 

 zontal trenches, the water from flowing off on the surface in the re- 

 gions above the timber line. The water is forced to percolate through 

 the soil and so reaches the wooded portions, where it would per- 

 meate anyhow, much more gradually than otherwise. The soil thus 



