208 



IBEIGATION. 



supply for irrigation will be derived from streams. The 

 water, in most cases, will be taken directly from the 

 stream at its regular level, by means of a main supply 

 canal, into which it is diverted by the ordinary flow, or 

 by means of wing drains placed in the stream ; or else 

 the level of the stream must be raised by a dam, and the 

 flow diverted at a higher level than the usual one. It is 

 always advisable, in fact necessary when profit is the main 

 purpose, to choose such a location for the commencement 

 of the canal as shall give the greatest possible head of 

 water. The cost of a few miles of canal may be in- 

 significant, as compared with the value of several thousand 

 acres of land that may be brought under irrigation, by 

 adding a foot to the head of the supply. But a dam may 

 often be constructed at a much less cost than would be 



Fig. 99. LONGITUDINAL WING DAM. 



necessary to carry a large canal a distance of even a 

 thousand feet. If the level needs to be raised but a few 

 feet, a wing dam may be constructed. This should be 

 placed where the level of the stream falls sufficiently, 

 and should be carried up the stream, at a convenient 

 distance from the bank, as far as may be necessary to 

 raise the water to the hight required. 



The manner of constructing the wing dam will vary 

 according to the character of the stream, the nature of 

 the river bed, and the materials to be most conveniently 

 procured. The typical form of the dam is shown at fig. 

 99, in which the structure is seen projected up the 



