Innovative approach in which stream sections were isolated with weirs and wild 

 rainhow trout added during the high flow period, saturating the habitat, was 

 used. Changes in trout carrying capacity, as determined by the movement of 

 trout out of the sections, were measured as the flow decreased. The derived 

 relationships between flow and trout carrying capacity were then compared to 

 the relationships between flow and various habitat parameters, including the 

 riffle wotted perimeter. The authors reported that in the pool-riffle 

 habitats of their study stream the wetted perimeter/inflection point method 

 worked well, while in run-riffle habitats the method underestimated the flow 

 that was needed to maintain rainbow trout at a reasonable level. In no case 

 did the method overestimate the summer instream flow needs. 



Met hod for the Low Flow Period - Rivers 



The Montana Department of Fish, Wildlife and Parks completed a study in 1980 

 that validated the wetted perimeter method as applied to the trout rivers of 

 southwest Montana (Nelson, 1980a, 1980b and 1980c). In this study, the actual 

 trout standing crop and flow relationship were derived from long-term data 

 collected for five reaches of the Madison, Gallatin, Big Hole and Beaverhead 

 Rivers, all nationally acclaimed wild trout fisheries. These relationships 

 provided a range of flow recommendations for each reach. Flows less that the 

 lower limit were judged undesirable since they led to substantial reductions 

 of the standing crops of adult trout or the standing crops of a particular 

 group of adults, such as trophy-size trout. Flows greater than the upper 

 limit supported the highest adult standing crops during the study period. 

 Flows hetween the lower and upper limits are broadly defined as those flows 

 supporting intermediate standing crops or those standing crops that normally 

 occur within each reach. The final recommendation was selected from this 

 range of flows. 



The range of flows derived from the trout-flow relationships for the five 

 river reaches were compared to those derived from the wotted perimeter method 

 as applied to riffle areas. The study results showed that the inflection 

 point flows had a somewhat different impact on the trout standing crops of 

 rivers than previously assumed for streams. For rivers, the flow at the upper 

 inflection point is a fairly reliable estimate of the lower limit of the range 

 of flows derived from the trout-flow relationships or, in other terms, flows 

 loss than the upper inflection point are undesirable as recommendations since 

 they appear to lead to substantial reductions of the standing crops of adult 

 trout . 



The flow at the upper inflection point is not necessarily the preferred 

 recommendation for all trout rivers. The "Blue Ribbon" rivers may require a 

 higher flow in order to maintain the sport fishery resource at the existing 

 level. In general, flows less than the upper inflection point are undesirable 

 as tlow recommendations regardless of the rating of the river resource. 



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