420 MOUJSTTAIlSr LAKES. 



Mountain Lakes. 



Other inconveniences of a very serious character have often 

 resulted from the natural wearing down, or, much more frequent- 

 ly, the imprudent destruction, of the barriers which confine moun- 

 tain lakes. In their natural condition, such basins serve both to 

 receive and retain the rocks and other detritus brought down bj 

 the torrents which empty into them, and to check the impetus of 

 the rushing waters by bringing them to a temporary pause ; but 

 if the outlets are lowered so as to drain the reservoirs, the tor- 

 rents continue their rapid flow through the ancient bed of the 

 basins, and carry down with them the sand and gravel with which 

 they are charged, instead of depositing their burden as before in 

 the stiU waters of the lakes. 



It is a common opinion in America that the river meadows, 

 bottoms, or intervales, as they are popularly called, are generally 

 the beds of ancient lakes which have burst their barriers and left 

 running currents in their place. It was shown by Dr. Dwight, 

 many years ago, that this is very far from being universally true ; 

 but there is no doubt that mountain lakes were of much more 

 frequent occurrence in primitive than in modern geography, and 

 there are many chains of such still existing in regions where man 

 has yet httle disturbed the original features of the earth. In the 



lakes, to obtain an increased supply of water for irrigation and as a mechan- 

 ical power, but as it was not proposed to depress the surface below the lowest 

 natural low-water level, there seems to have been little ground for the fears 

 expressed. 



See, for important observations on the character and probable results of 

 these projects, Tagliasecchi, Notizie, etc., dei Ganali dell' Alta Lombardia, 

 Milano, 1871. 



Jacini says : " A large proportion of the water of the lakes, instead of dis- 

 charging itself by the Ticino, the Adda, the Oglio, the Mincio, filters through 

 the silicious strata which underlie the hills, and follows subterranean channels 

 to the plain, where it collects iu the fontanili, and being thence conducted 

 into the canals of irrigation, becomes a source of great fertility." — La Pro- 

 prietd Fondiaria, etc. , p. 144. 



The quantity of water escaping from the lakes by infiltration depends much 

 on the hydrostatic pressure on the bottom and the walls of the lake-basins, and 

 consequently the depression of the lake surface, diminishing this pressure, 

 would diminish the infiltration. Hence it is possible that the lowering of the 

 level of these lakes would manifest itself in a decreased supply of water for 

 the springs, fontanili and wells of Lombardy. 



