IRRIGATION 123 



flow to get the necessary capacity and to keep the canal 

 clean, or, as usually stated, cause it to " scour.' ' 



The construction of a canal is an important matter. In 

 the early stages of irrigation practice in the United States, 

 most canals were made with earth embankment, but the 

 increase in the value of irrigation water has led to the intro- 

 duction of methods to prevent waste from the canals by 

 seepage. It is estimated that 47 per cent of the irrigation 

 water now used in the United States is wasted in this way, 

 and in some cases the losses run as high as 85 per cent. 



Some irrigation canals are ranked among the world's 

 greatest engineering achievements. The Cavour Canal in 

 Europe cost $20,000,000; its waterway is 66 feet wide and 12 

 feet deep, and it crosses the drainage lines of several rivers. 

 It passes under the Sesia River in a masonry siphon 820 feet 

 long. There are some large canals in Egypt and India. 

 Among them may be mentioned the Chenab Canal, which is 

 250 feet wide at the bottom and carries 11 feet of water. 

 The main canal is 400 miles long, and has 1200 miles of 

 tributary canals. It cost $10,000,000, and it is said to irri- 

 gate 2,645,000 acres of land. There are no canals in the 

 United States that will compare with it. The Bear River 

 canal in Utah cost $1,000,000 and waters approximately 

 100,000 acres. The Modesto-Turlock canal system of 

 California is designed to water 275,000 acres, and cost about 

 $3,000,000. 



Reservoirs. Reservoirs, either natural or artificial, 

 obtained by the damming or storage of water in natural 

 watercourses, are often made the source of supply of water 

 for irrigation purposes, inasmuch as water which would 

 ordinarily be wasted is held in storage until needed. In 

 some localities reservoirs are quite necessary, as streams 

 furnish the minimum amount of water during the time when 



