80 THE IRRIGATION AGE. 



8,000 second feet or more, can be safely piloted through the country 

 for many miles. 



But, in most cases in the arid states of America, the canals do not 

 traverse great level plains, but wind about the sloping sides of low 

 hills, or through rolling prairies, or are dug out of the sides of steep 

 bluffs and benches. Here they mnst take whatever course a hill side 

 may allow of, since they must follow its contour. In these circum- 

 stances, a large canal is obviously unsafe. The sidelong nature of the 

 ground in which it has to be excavated, limits its width very strictly; 

 and the frequent sharp curves und even bends, it takes as it winds 

 around a hillside, limit its velocity and depth. As however in such 

 cases, there is plenty of fall or grade in the country, irrigation re- 

 quirements are easily met by having a series of small canals at differ- 

 ent levels. A marked feature in such country is that advantage can 

 be taken to turn into reservoirs, valleys and depressions traversed by 

 placing embankments across their natural outlets, and filling them up 

 at times when the water is not in demand for irrigation. 



An important feature that largely affects the efficient working of 

 a canal, depends on this difference of construction imposed on us by 

 the physical nature of the country traversed, whether we can have 

 one large canal with a single head at a favorable point in the river, or 

 'whether we must have a series of canals with separate heads at differ- 

 ent points in the river. That feature is tti6-weir across the river be- 

 low a canal head. In the former case we dan afford to spend a large 

 sum on a perfect weir that will hold up the whole, or as much of the 

 Driver supply -as ifre need, in times of low water, and yet be perfectly 

 safe when passing floods and torrents in times of high supply. By 

 holding up the supply, we can feed the canal with topwater, tolerably 

 free from the heavier sandy sediment; and we form a large settling 

 basin in the river above the weir. This can be easily scoured out 

 from time to time by opening the undersluices of the weir, when the 

 river bed is allowed to resume its original natural slope or grade. It 

 is always advisable to close the canal head at such times to prevent 

 the silt laden water entering and silting up theihead channel. If the 

 "canal becomes much silted, and at the time the demand for water for 

 'irrigation is great, so that it is not advisable to lose water by running 

 it through the -escapes to scour out and wash away the silt, it is often 

 possible to force a supply for a time over the silted bed, by raising 

 the supply level at the canal head by means of the weir. 



But in the latter case of a series of small canals, it would not pay 

 to provide each head with such a costly weir; and in consequence such 

 canals must work less efficiently. On the American rivers where so 

 often the water is all fully appropriated among different canals, and 

 the supply is insufficient to meet the demands, the great point is to 



