64 IRRIGATION FARMING. 



shed. If the quantity and value of the land to be watered 

 and the capacity of the reservoir are great as compuivu 

 with the available water to be stored, it may be ad visi- 

 ble to build a reservoir of sufficient capacity to contain 

 much more than the minimum 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 constructed a 

 Lrivat degree of engineering skill is required. The charac- 

 ter of the construction of the dam will differ in every 

 case with the nature of the foundation and the availa- 

 bility of structural material. Some of the greatest reser- 

 voir dams ever built have been constructed 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 selected must be such as will 

 store the greatest amount of water w r ith the greatest 

 economy of construction. It is manifest that inasmuch 

 as reservoirs cannot be excavated within reasonable con- 

 ditions 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 some which can 

 be controlled by tapping below the level of the natural 

 discharge. Such reservoirs will be most economical as 

 outlets, only they will have to be constructed and guarded 

 by bulkheads, and the natural evaporation surface will not 

 be enlarged. 



Reservoir sites may be divided into two great classes : 

 Natural lakes or depressions, and reservoir sites on 

 drainage lines. Such sites have two important adyan- 



the dams are not endangered by the enormous 

 iloods that are bound to occur on streams, and an oppor- 

 tunity is afforded for disposing of the rock and silt from 

 the storm waters stored before they reach the iVMT\<>ir-. 



In tin- location of small individual ponds no great 

 engineering skill is required and the construction is at 



