1899. 



AMERICAN FORESTRY ASSOCIATION. 



45 



ten times as great as now. The ditches already 

 built cover four times the area on which crops 

 are being cultivated and matured by the aid of 

 the natural flow of the streams alone. The 

 best that can be done with the three-fourths 

 now left uncultivated is to water it once during 

 the flood season, so increasing the pasturage 

 or possibly raising one crop of alfalfa instead 

 of the three or four crops which could be pro- 

 duced with an ample water supply. Many 

 of the ditches already built have to remain 

 idle and empty during more than half of the 

 irrigation season, and the value of the greater 

 part of the land under them is but little more 

 than it would bring for pasturage alone. The 

 period of greatest need in irrigation extends 

 from the middle of June to the middle of July, 

 while the demand during the last half of July 

 is often as great as during the first half of June. 

 There is no profit in planting crops which can- 

 not be matured ; hence the limit of the area 

 which can be cultivated by the natural flow 

 alone is not fixed by the flood discharge of May 

 and June, but by the short supply of July. 



Briefly stated, the future of irrigation 

 development will largely depend upon 

 the storage of flood waters that are now 

 permitted to flow unused to the sea. 

 How this end is to be achieved is a 

 problem on the solution of which many 

 minds are at work. That it will be 

 solved, no one acquainted with the ac- 

 tivity and ambition of the Western people 

 doubts for a moment. That various 

 measures will be adopted is probable. 

 One of these is certain to be the re- 

 habilitation, preservation and extension 

 of forest protective areas. Even if the 

 construction of a storage reservoir sys- 

 tem would answer for all the purposes of 

 water conservation (which it could not), 

 the maintenance of protective forests 

 would yet be an imperative necessity for 

 the adequate protection and the most 

 economical operation of irrigation works. 

 The rapid confluence of storm waters is 

 not the only evil that follows forest de- 

 struction in a mountainous region. 

 Falling water washes loose particles of 

 soil and gravel and small fragments of 

 rock. The carrying power of flowing 

 water increases as the sixth power of its 

 velocity.* A torrent which has its source 

 in the timber stripped area of a steep 

 mountain side often attains, after heavy 

 rains, a power that not only moves sand 



*LeConte's Geology, page 18. 



and gravel and small fragments, but 

 transports boulders and other rock 

 masses. These are deposited as the 

 force of the current is checked, perhaps 

 choking the bed of a stream and causing 

 by its overflow the ruin of the lower 

 lands on either side. As the slope of 

 the receiving stream decreases the coarse 

 gravel and fragments are deposited. 

 The sand and silt are still carried in sus- 

 pension, to settle finally on the bottom 

 of some irrigation canal, so limiting its 

 capacity by making it shallower, or to 

 be deposited on the bed of a reservoir, 

 the available storage capacity of which 

 is thus decreased. 



The cleaning of the sandy sediment 

 from the bottom of canals is reckoned 

 upon annually as an expensive but neces- 

 sary item in the operation of many irri- 

 gation systems. In eastern Colorado 

 the engineers on the Amity Canal re- 

 ported that in one instance a small reser- 

 voir of two or three acres in extent was 

 filled with a deposit of thirty feet of silt 

 and sand in the single season of 1895. 

 The construction of scouring sluices in 

 canal systems and settling basins in con- 

 nection with reservoirs may seem to re- 

 duce these difficulties to a minimum. 

 Such expedients, however, are wasteful 

 of either water or money, and results 

 obtained are not always satisfactory. 

 Even under the best conditions there 

 will always be more or less erosion and 

 movement of soil and detritus by flood 

 waters ; and a considerable deposit of 

 the finer particles in the slower currents 

 and still waters of irrigation works must 

 always be taken into account. How to 

 reduce these evils to the minimum is a 

 question of the very greatest importance 

 to the irrigator. Here then is one of the 

 great offices of the protective mountain 

 forests. To break the force of rapidly 

 descending waters and hold a part of 

 them in check to feed the springs and 

 brooks after the season of flood is gone 

 is indeed a useful function; but one 

 scarcely less useful is that of binding of 

 soil on slopes and, to a great extent, de- 

 priving the flood of its haimful power. 



On page 42 The Forester presents a 



