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THE NATURALLY OCCURRING PATTERN OF LOW FLOWS 



Because the aquatic biota of the Tongue River has developed in response 

 to both high and low flows, the frequency and magnitude of naturally occur- 

 ring low flows was examined. Shown in table 19 is a summary of historic 

 flows and low-flow occurrences of the Tongue River near its mouth for the 

 years 1931-75. Although this report refers to naturally occurring low flows, 

 the term is misleading because 1) flows of the Tongue River are regulated 

 by Tongue River Dam, and 2) the gaging station is located downstream from 

 the lowermost irrigation diversion. 



When comparing naturally occurring low flows in the Tongue with those 

 that would occur after the projected levels of development, it is necessary 

 to distinguish between basin outflows at the mouth of the river and 

 instream flows occurring immediately below the Tongue River Dam. Because 

 both the agricultural and industrial development projected would rely on 

 increased storage, more water would be released during low-flow periods 

 for downstream use. Hence, at least for the portion of the river just 

 below the dam, instream flows after development would probably be high and 

 more constant through the year (since release for industrial use would not 

 vary) than are existing flows. However, those additional releases of water, 

 plus much of the existing flow, would be diverted and consumed along the 

 course of the river; at the river's mouth, flows after development would be 

 much lower than are naturally occurring low flows, resulting in the severe 

 impacts described in tables 20 and 21 as opposed to the lesser, naturally 

 occuring impacts described in table 19. 



Naturally occurring low flows also result in high TDS concentrations. 

 Four months (October-January) commonly show levels greater than 700 mg/1 . 

 Two months, December and January, occasionally reach 1000 mg/1. However, 

 these concentrations do not occur at critical fish life-history stages, and 

 little impact occurs. 



IMPACTS OF INCREASED FUTURE USE 



PROJECTIONS OF FUTURE USE 



In order to adequately and uniformly assess the potential effects of 

 water withdrawals on the many aspects of the present study, it was necessary 

 to make projections of specific levels of future withdrawals. The methodology 

 by which this was done is explained in report No. 1 in this series, in which 

 also the three projected levels of development, low, intermediate, and high, 

 are explained in more detail. Summarized in appendix A, these three future 

 levels of development were formulated for energy, irrigation, and municipal 

 water use. Annual water depletions associated with the future levels of 



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