86 IRRIGATION FARMING. 



plain a dam was built, generally to retain the waters 

 and prevent their sudden invasion of the plain before 

 they were required. 



Location of Reservoirs. — In the sele<5lion of 

 reservoir sites regard must be had to several considera- 

 tions — the area and character of land to be irrigated 

 and its distance from the proposed reservoir ; the area 

 of the watershed, the drainage from which is to fill it ; 

 and both the maximum and minimum annual rainfall 

 of the watershed. If the quantity and value of the 

 land to be watered and the capacity of the rCvServoir are 

 great as compared with the available water to be 

 stored, it may be advisable to build a reservoir of suf- 

 ficient capacity to contain much more than the mini- 

 mum annual run-off, so that the discharge of wet years 

 may be saved for use in time of drouth. If storage 

 reservoirs are to be construdled, a great degree of en- 

 gineering skill is required. The character of the con- 

 struc5lion of the dam will differ in every case with the 

 nature of the foundation and the availability of struc- 

 tural material. Some of the greatest reservoir dams 

 ever built have been constru<5led for purely irrigation 

 purposes, and have required more skill in their design 

 than have any built for city water-supply or other 

 hydraulic uses. The basin seledled must be such as 

 will store the greatest amount of water with the 

 greatest economy of constru(5lion. It is manifest that 

 inasmuch as reservoirs cannot be excavated within rea- 

 sonable conditions of cost, they must be natural basins. 

 In many cases these will be existing lakes, and while 

 many such will require dams at their outlets in order 

 to regulate by gates the outflow of the water, there are 



