1HL IRRIGATION AGE. 



107 



can be conveniently taken into a pipe or flume and carried to a ditch 

 below, where firm ground is reached. Of course the water can often 

 be taken out by an underground pipe from a low point in the dam, if 

 the grade to the point of use is sharp enough. 



Holding water by a submerged dam has advantages. It employs 

 a subterranean reservoir and largely reduces loss by evaporation, 

 which is so great in surface storage. It is also a reservoir presuma- 

 bly with tight sides, else the prospect would not have shown sufficient 

 water to impound. It is a safe dam, because it offers no resistance to 

 freshets. If it lacks elevation for outflow, it serves as a fine cistern 





FIG. ''.Submerged dam in dry creek bed- 



for pumping and conserves water for that purpose. A point of excel- 

 lence in a submerged dam is its perfect bedding and construction so as 

 to be water tight. Tnis is not always attained, especially when the 

 attempt is made to intercept a wide valley stream without definite 

 banks, and some submerged dams are not as efficient as calculated 

 upon. In work on a small scale it is not advised to try any experi- 

 ment with such a source. For the farm supply the underground stream 

 should be comparatively small and well confined on both sides. 



DEVELOPMENT OF SPRINGS. 



The opening up of springs is often a very satisfactory means of 

 obtaining a farm supply of irrigation water. Their development some- 

 times consists in the excavation of a reservoir in a piece of springy or 

 marshy ground, or in laying underdrains to take their flow and con- 

 necting them with a more convenient reservoir site at a distance. 

 Sometimes a spring whose flow can not be recovered from the area of 

 boggy ground below it can be opened up and its waters readily direct- 

 ed to n single channel, or to a pipe leading to a reservoir (figs 7 and 8). 

 By this means waste land, which is both useless and treacherous, is 



