Non- field Method 



The study results suggest that minimum flow recommendations 

 based on a fixed percentage of the mean flow of record may be 

 valid for the trout rivers of southwest Montana. The percentage 

 required appears to depend on the channel morphology with the 

 shallower, wider rivers requiring a greater percentage of the 

 mean. The more typical rivers of the study area required an 

 absolute minimum flow equal to about 3 3% of the mean. A mini- 

 mum flow of 10% of the mean as recommended by the "Tennant or 

 Montana" method was totally inadequate in this study. 



The discrepancy between the minimum flow recommendations 

 derived from the trout-flow data and the Tennant method is 

 partially the result of conflicting definitions. Tennant 's 

 minimum is defined as the flow that sustains short-term sur- 

 vival habitat for most aquatic life forms. Flows less than 

 the minimum result in the catastrophic degradation of the 

 fishery resource. The impact on the fishery of the absolute 

 minimum flow derived from the trout-flow data is less severe. 

 This minimum is defined as the lov;est flow that will sustain 

 intermediate or normal standing crops of adult trout or a par- 

 ticular group of adults, such as trophy-size trout. For Mon- 

 tana's nationally acclaimed wild trout fisheries such as the 

 Madison, Beaverhead, Gallatin and Big Hole Rivers, a minimum 

 flow that sustains less than normal population levels is 

 totally unacceptable. Considering these definitions, the 

 absolute minim\am derived from the trout-flow data is expected 

 to exceed Tennant 's minimvim. 



The flow regimen Tennant describes as fair or degrading 

 is- probably more compatible with the definition of the absolute 

 minimum recommendations. To provide fair or degrading aquatic 

 conditions, Tennant recommends a flow regimen of 10% of the 

 mean flow during the October-March period and 30% during the 

 April-September period. The 30% recommendation during the 

 April-September period compares favorably to the absolute min- 

 imum recommendations for the more typical rivers of the study 

 area while the 10% recommendation during the October-March 

 period is totally unacceptable. 



Presently, a fixed percentage method would only be used 

 by the Montana Department of Fish, Wildlife and Parks to make 

 preliminary flow recommendations in situations where time or 

 cost limitations prohibit field studies. More extensive 

 use of this method would depend on further testing of its 

 reliability. If proven valid, it is likely a fixed percentage 

 method would primarily be used to support the flow recommen- 

 dations derived from field methods, such as the single and 

 multiple transect methods previously discussed. 



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