THE IREIGA T10N AGE. 113 



may be constructed which will serve to elevate the water to the 

 required level. This is a practice very frequently adopted in irri- 

 gating low bottoms, but is applicable only where the land is but 

 slightly elevated above the stream; as where its elevation is very 

 considerable the height of dam thus required would, in most cases 

 involve an expense prohibiting its building. 



Frequently the lands to be reclaimed occupy positions remote 

 from the streams whose waters are to accomplish their irrigation, 

 and are elevated several hundred feet above it. In such cases neither 

 pumping nor damming the stream would be feasible, on account of 

 the expense involved. If the stream had but a slight fall, land so 

 situated could not be irrigated from it at all. One of the charac- 

 teristics of the streams of the arid regiou, however, as noted above, 

 is the excessive declivity of their slopes. There are few whose fall 

 is less than 4 or 5 feet per mile and 40 feet is not unusual, especially 

 in the case of the smaller streams in the vicinity of the mountains. 

 When, therefore, it is desired to irrigate a body of land which 

 occupies a position of great elevation above the stream selected as its 

 source of supply, the elevation of the latter does not necessarily 

 prohibit the enterprise, because, while at all contagious points along 

 the stream the water is much below the land, the excessive fall 

 characteristic of these water courses permits of the selection of some 

 point farther upstream, whose elevation exceeds that of the land 

 whose irrigation is desired, and from this point its waters may be 

 conducted by means of a gravity canal to the lands to be reclaimed. 

 This point of diversion may be, and frequently is, many miles up the 

 river from the lands to be watered. 



The lands irrigated usually lie between the ditch or canal and the 

 stream furnishing the supply, and below the former. The water 

 is drawn off by letting it out of the artificial conduit and permitting it 

 to run over the lands below. 



It is evident, however, that under some conditions irrigation can 

 not be thus accomplished. If, for example, the irrigating of a body 

 of elevated lands were contemplated from a stream whose rate of fall 

 does not exceed that required by the canal through which the waters 

 are to be conveyed to the land, the latter could not be covered. 

 It would also be impracticable to irrigate an elevated body of land 

 from a stream whose fall exceeded but slightly that required for the 

 canal through which the water is to be conveyed to the land. If a 

 canal whose slope must be 6 inches per mile is designed for the irri- 

 gation of land lying 150 feet above a stream whose slope is 1 foot per 

 mile, its point of diversion would have to be located 300 miles up the 

 stream from the lands to be irrigated, and the enterprise would 

 consequently be impracticable. For these reasons much of the land 



