5 o POPULAR SCIENCE MONTHLY. 



escapes the enormous increase of pressure due to the sudden absorp- 

 tion 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 val- 

 ley 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 dis- 

 tance. 



The manner in which the streams in the valleys are made to aid 

 in their own correction is most interesting. Whereas in the moun- 

 tains it is usually desirable to decrease the erosive action of the 

 water, in the valleys the contrary is the case, as the deposits in the 

 lowlands are as dangerous to life and property as was their abstraction 

 above. The great desideratum to be attained is to have the mountain 

 streams arrive in the valleys in a purely liquid condition, and to give 

 the valley streams the power to carry to the lakes any material they 

 may be so unfortunate as to have taken in charge. To accomplish 

 this latter purpose, the sinuosities of the streams are often reduced 

 to straight lines, an increase of slope being thus secured. The new 

 channels are made of a cross-section to enable the water to carry on 

 its alluvion and silt. Where great freshets occur it is necessary to 

 guarantee the artificial beds against the enormous increase of the 

 water's destructive action. 



The usual plan is that of having the cross-section of the stream 

 with a deep depression in the center. This depression is of di- 

 mensions to insure a proper flow under ordinary conditions. When 

 the stream becomes swollen it overflows the borders of this depres- 

 sion and spreads over a much larger area until the banks proper are 

 encountered. This sudden increase of cross-section reduces the 

 velocity of the water and consequently its destructive power. When 



