' ■ . '.-!- ,.f — ij :- SUMMARY,. ^^ ',-^*:< ■ ! .^ . •. .; 



Four instream flow methods were applied to five reaches 

 of the Madison, Beaverhead, Gallatin and Big Hole rivers of 

 southwest Montana. The methods were: 



(1) a single transect method in which the minimum flow 

 recommendation is selected at the inflection point 

 on the wetted perimeter-discharge curve for a 



- single riffle cross-section, j, t ;rj t 



(2) a multiple transect method in which the minimum 



. flow is selected at the inflection point on the j^r,-, . 

 wetted perimeter-discharge curve for a composite 

 of channel cross-sections, 



(3) the Tennant method, and r , ^ 



(4) the incremental method developed by the Cooperative : 

 Instream Flow Service Group (IFG) of the U.S. Fish 

 and Wildlife Service. 



Recommendations derived from the four methods were com- 

 pared to those derived from long-term trout standing crop 

 and flow data. The trout-flow data generally provided two ,' 

 minimum flow recommendations for each reach. Flows less than 

 the absolute minimum recommendation appear to lead to sx±)stan- 

 tial reductions in 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 most desirable minimum 

 recommendation sustained the highest standing crops. The 

 optimum flow should either equal or exceed the most desirable i 

 minimum. 



The recommendations generated by the single transect .~ ;• . 

 method for all five reaches compare favorably to the absolute 

 minimums derived from the trout-flow data. Single, well 

 defined inflection points were generally present and easily <* ■ 

 interpreted. In addition to providing reliable and consis- 

 tent recommendations, the single transect method was also 

 the most time and cost efficient of the three field methods. f- 



The multiple transect method provided acceptable absolute 

 minimum recommendations for the four reaches having discernible 

 inflection points. Inflection points, when present, were gen- 

 erally not as well defined as those on the wetted perimeter 

 curves derived for the single transect method. In the two 

 reaches having more than one inflection point, the lowermost 

 occurred at the flow approximately equal to the absolute mini- 

 mum recommendation. While the multiple transect method did 



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