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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 method- 

 ology by v/hich 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 mun- 

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

 of development were included in the projections. These projected depletions, 

 and the types of development projected, provide a basis for determining the 

 level of impact that would occur if these levels of development were carried 

 through. 



IMPACTS ON THE RESERVOIR FISHERY 



Through the State Water Planning Model (see Report No. 1 of this series), 

 the depleted streamflows for the Tongue River Basin which would result from 

 implementation of three alternative levels of energy or irrigation use were 

 calculated. For the low projected level of development for either type of 

 use, the storage capacity of the existing Tongue River Reservoir was increased 

 from 69,000 af to 125,000 af. The intermediate and high levels of development 

 required the High Tongue River Dam, which would have an active capacity of 

 320,000 af and would provide a firm annual yield of 134,000 af. 



The impacts of development on the reservoir fishery were evaluated. 

 Sport fishes in the reservoir, which spawn during April and May, depend on 

 stable or rising water levels during this period. A decreasing water level 

 would result in a loss of available spawning habitat and would desiccate in- 

 cubatina fish eaas. StorT.cc patterns projected were judged detrimental if 

 they required reservoir level decreases in April and May. 



For the low level of energy development, only one year out of 29 resulted 

 in poor reproductive potential. The intermediate level showed six years with 

 poor spawning potential. With the high projection, storage patterns resulted 

 in poor reproductive potential in 10 years out of 29, an average of one year 

 out of three with the possibility of reduced spawning. Even with poor repro- 

 duction every third year, a good fishery should result from the remaining 

 years' reproduction. 



Storage patterns associated with the three levels of irrigation develop- 

 ment shov/ed no serious problems. Impacts on the fishery of the Tongue River 

 Reservoir would be minimal. 



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