RKSKRVOIRS AND PONDS. 87 



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 construdled 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 impor- 

 tant advantages — the dams are not endangered by the 

 enormous floods that are bound to occur on streams, 

 and an opportunity is afforded for disposing of the 

 rock and silt from the storm waters stored before they 

 reach the reservoirs. 



In the location of small individual ponds no great 

 engineering skill is required, and the construdlion is 

 at once a very simple and easy task, especially where 

 only an earth excavation is required, on flat land or in 

 a draw. If a place can be found from which the water 

 will naturally run in several dire(5lions, all the better, 

 because more land may then be reached at less cost. 

 Where there is a good clay subsoil, not porous, and 

 the soil above has in it considerable admixture of clay, 

 a first-class reservoir may be construdled out of the 

 soil. 



In treating the construdlion of reservoirs we shall 

 endeavor to take up the subje(5ts separately, so that the 

 reader may not be confounded as to instrucftions that 

 may apply to a work of lesser importance than that in- 

 tended. Large reservoirs are a menace too often to 

 public safety and mark the danger-line in irrigating 

 works, so that no serious mistake should be made in 

 building them. 



