10 BULLETIN 1026, U. S. DEPARTMENT OF AGRICULTUEE. 



season instead of reservoir water to provide for an irrigation after 

 canal supplies fail. 



The total amount of seepage return to the river, exclusive of that 

 coming through regular channels, is about 110,000 acre-feet annually. 

 During the winter months only the return above the intake of the 

 Cache la Poudre Reservoir may be used, and this is estimated to be 

 al)out 8,000 acre-feet. During the summer months canals on the 

 lower reaches of the river depend to a considerable extent on this 

 return flow to satisfy their rights, and it is estimated that during 

 these months 51,000 acre- feet are available for their use. Combined, 

 the total available flow for the year is 59,000 acre-feet. 



To summarize, the water supply of the valley includes a normal 

 run-off of 340,000 acre-feet in the river and its tributaries, 35,000 

 acre-feet of foreign water, 5,000 acre-feet pumped from wells, and 

 available seepage to the amount of 84,000 acre-feet, a total supply 

 of 464,000 acre-feet. This supply in the course of time will be in- 

 creased slightly by pumping and by a greater return of seepage, but 

 any material increase seems improbable. 



With the exception of very short periods during high floods, the 

 entire available flow of the stream is taken on rights which have 

 • been in existence for years. There is an occasional surplus subject 

 to storage, but because of the uncertainty attaching to it and the 

 probable high cost of developing it, the feasibility of such a suj^ply 

 is highly questionable. The demand on the river in June the month 

 of maximum flow, is close to 120,000 acre-feet,* which is equivalent 

 to an average flow of approximately 2,100 second-feet. From Table 

 2 it appears that the river has failed to reach that discharge in 18 

 of the 33 years covered by the records. To produce a surplus of 

 20,000 to 25,000 acre-feet a discharge of at least 2,500 second-feet 

 would be required during June, and such a discharge occurred in 

 only 9 of the 33 years covered by the records. From this it is clear 

 than any further storage projects would have to depend on a surplus 

 which would be of considerable size only about once in 3 or 4 years. 

 The amount available annually for use from such a supply would 

 be very small and very costly. 



SEEPAGE RETURN. 



One of the questions of particular interest now in many irrigated 

 valleys is that of seepage return to streams. It is a well-known fact 

 that, with the extension of irrigation, this return has so increased 

 that canals on the lower reaches of the rivers, which once suffered 

 seriously because their rights would not be satisfied, are now plenti- 

 fully supplied with water. In the Cache la Poudre Valley the 

 effect of the seepage return is very marked. Late in the season it 

 is often the case that the river is dry in several places, j^et a number of 



* See Tables 7 and 8. 



