The most desirable mininum flow recommejidations derived 



from the trout-flow data ranged from 64 to 80% of the mean 



flows and fell within the range of flows that Tennant describes 



as optimum (60-100% of the mean flow of record) . 



Evidence presented in this section suggests that an abso- 

 lute minimum instream flow recommendation based on a fixed 

 percentage of the mean flow of record may be valid for the 

 trout rivers of southwest Montana. The percentages derived in 

 this study fell within the approximate range of 31 to 51% which 

 is considerably higher than the minimum of 10% recommended by 

 Tennant. The percentage selected as an absolute minimum recom- 

 mendation appears to depend on the channel morphology with the 

 wider, shallower rivers such as the Madison requiring a higher ' 

 percentage of the mean flow. The more typical rivers of the 

 study area (Beaverhead, Gallatin and Big Hole rivers) required 

 an absolute minimum instream flow equal to about 33% of the 

 mean. 



IFG Increment al Method - ,.-■•' 



The optimum instream flows derived from the IFG incremen- 

 tal method are compared to those values derived from the 

 standing crop and flow data in Table 20. The actual optimums 

 for the five reaches could not be derived from these data. 

 What is available for comparison are the most desirable mini- 

 mum flow recommendations listed in Table 15. The actual 

 optimxjms should either equal or exceed the most desirable 

 minimums. The IFG predicted optimums that equal or exceed 

 the most desirable minimums are judged acceptable as optimum 

 flow recommendations. 



Thirteen comparisons are available for the five reaches. 

 In 6 of the 13 comparisons, the IFG predicted optimums ex- 

 ceeded the most desirable minim\ams derived from the standing 

 crop and flow data. In the remaining seven comparisons, the 

 IFG predicted optimums were less than the most desirable mini- 

 mums and in six of these seven the IFG predicted optimums __'^.. 

 were even less than the absolute minimum flow recommendations 

 listed in Table 15. The IFG optimum flow recommendations were ' 

 acceptable in only 6 (46%) of the 13 comparisons. 



The IFG optimum recommendations for brown and rainbow 

 trout were acceptable in the Beaverhead and Big Hole reaches 

 and unacceptable in the remaining three reaches , for an 

 acceptability rate on a reach basis of only 40%. 





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65 



