WATER SUPPLIES FOR IRRIGATION. 



CHAPTER V. STORAGE RESERVOIRS AND DAM SITES. 



BY F. C. FINKLE, C. E. 



MANY of the large and important irri- 

 gation systems of the world derive 

 their supply of water from storage or im- 

 pounding reservoirs. 



A storage reservoir is the artificial lake 

 bed or basin formed by closing the outlet 

 or outlets to a valley or canyon. After the 

 outlets are closed by means of artificial 

 barriers called dams, water entering the 

 valley is retained in the basin and accu- 

 mulates, thus forming an artificial lake 

 from which water can be drawn as desired. 

 The object of storage reservoirs therefore 

 becomes apparent, the water entering the 

 basin at times of the year when it is not 

 needed for irrigation, being conserved to 

 be drawn off and used at other times when 

 irrigation is practiced. 



A storage reservoir for developing a 

 water supply for irrigation purposes is ex- 

 pedient and useful only in regions where 

 such a large proportion of the precipita- 

 tion takes place during the proportion of 

 the year, when no irrigation is practiced, 

 as to render the natural water supply in- 

 adequate for the purpose of irrigating 

 when irrigation is necessary. 



There are certain requirements making 

 a storage reservoir practicable, or at times 

 even possible. These we will now enum- 

 erate and proceed to discuss in the natural 

 order. They are as follows : 



(1.) A suitable valley or basin. 



(2.) A favorable dam site. 



(3.) An adequate water-shed. 



(4.) Proximity to irrigable lands. 



It is always necessary that these re- 

 quirements be all combined in such a de- 

 gree as not to have the failure of an enter- 

 prise due to the absence of either one of 

 them. At the same time it is not to be 

 expected that they are all to be found 

 present in a perfect state, nor even in a 

 relative state of perfection for that matter. 



The first and most necessary essential 

 to constitute a valley or basin suitable for 

 storage purposes is sufficient area. As 

 will be seen later on the cost of building 

 dams is always great so that it is necessary 

 to have a considerable area which can be 



flooded in order to make an undertaking 

 of water storage profitable. 



Of course no fixed area can be stated as 

 a minimum for the reason that the figure 

 must correspond to the amount of invest- 

 ment required for dams. Extreme cases 

 are sometimes encountered when small 

 tracts of only 300 or 400 acres can be 

 profitably utilized as storage reservoirs. 

 This is only the case, however, when the 

 other requirements are developed in such 

 a marked degree as to render the cost of 

 constructing a dam very small and the 

 average depth of water nearly if not quite 

 equal to the height of the dam. The 

 locality also tends to influence the ques- 

 tion of whether a reservoir site of limited 

 area can be profitably improved or not. 

 The value of water varies so much in dif- 

 ferent localities that an enterprise, which 

 would prove profitable in one place, might 

 possess no value at all in another locality. 



Next to sufficient area the most impor- 

 tant requirement in a reservoir site is that 

 the slope or pitch of the land included in 

 it be light, uniform and gradual. Abrupt 

 descent toward the dam site or steep side 

 slopes from the middle of a valley toward 

 either side often render it unfit for a reser- 

 voir site although it be of very large area. 

 In order to make the flooding of a consid- 

 erable area possible, where the slope is 

 abrupt, a very high dam must be con- 

 structed, and the cost of dams increases so 

 rapidly, in proportion to their height, as to 

 condemn such propositions from a financial 

 point of view. 



The value of water for irrigation pur- 

 poses in the particular locality in question 

 is also an important factor to be carefully 

 considered as well in this connection as 

 has already been suggested in connection 

 with the question of area. In localities 

 where water is very valuable a grade of 

 seventy- five feet per mile in the reservoir 

 site may not be objectionable, while in 

 other localities where water is cheap a 

 grade of twenty-five feet per mile may be 

 sufficient to condemn a storage project. 



Should the soil in a proposed reservoir 



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