96 DYKES. 



lo walk and saw running by the city a very high bank, higher than the 

 top of the houses. At one place, there was a stone stair. Going up 

 this, I found a wide, gravelly stream-bed, quite dry on top, with the 

 inside banks built of stone. 



This was the torrent. The dyke walls had been raised from time 

 to time to counter the channel raised from debris deposits. 



The dyke system is not so simple as some people think it is. 

 Surely it is a cheaper thing to prevent your torrent. 



Dykes are also used to check and diffuse flood water. This is the 

 Egyptian system. Diffusion and division of flood current requires en- 

 gineering skill not easy to flnd. 



Diversion of torrent channels from localities where they do damage 

 to others where they do less, is a method of treatment that, under fa- 

 voring circumstances, may be effective. 



An interesting work of this kind exists at the Lake of Thun. in 

 Switzerland. A torrent had been created there which was doing great 

 and frequent damage. It was beyond any ordinary method of control. 

 The plan of diversion was devised and carried out at a large cost, 

 which has proved to be a great economy. The torrent was diverted to 

 an artificial channel on a steep grade, through a hill to the Lake of 

 Thun. The water in the lake at the precipitous point, where the torrent 

 was delivered, was 200 feet deep. In a few years the torrent filled in 

 an extensive area, and has made a delta for itself, where a short time 

 before was deep water. 



An important and valuable plan of torrent palliation for us is that 

 of artificial rainfall storage reservoirs. A torrent that has once gained 

 volume is almost impossible to deal with on this plan. The reasons 

 are: First, that there can be no efiicient method of flood diversion 

 from a mountain flood in the mountains. If there were, your catchment 

 basin, or reservoir, would soon fill with torrent detritus. Second, that 

 a dam in the canyon of the torrent would soon fill with sand and 

 stones, and might break and deliver an accumulation of flood water. 

 Several dams in creek and river courses have burst, with damage and 

 loss of life. Arizona has a number of such cases. The Johnstown dis- 

 aster is one of the most notable cases in this country, both from the loss 

 of property and of life. It was proposed by the miners to build a dam 

 across the outlet of the Yuba to the plain, by tremendous blasts 

 in the rock at the outlet gorge of the river. The object was 

 to control the hydraulic mine slickens, which is artificial torrent 

 action. The trouble with the plan was that the dam might 



