MONTANA STATE LIBRARY \v> 



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EVALUATION OF SELECTED INSTREAM FLOW METHODS IN MONTANA 



Frederick A. Nelson 

 Fisheries Biologist, Montana Department of Fish, Wildlife and Parks 



INTRODUCTION . .,.,,, ~ 



The Montana Department of Fish, Wildlife and Parks (MDFWP) began in 1966 

 to estimate standing crops (numbers and biomass) of trout in the rivers of the 

 Missouri drainage of southwest Montana. Presently, long-term standing crop 

 estimates are available for five reaches of the Madison, Beaverhead, Gallatin 

 and Big Hole Rivers, all nationally acclaimed wild trout fisheries. In these 

 five reaches, the flows, which are gaged by the USGS, are either regulated by 

 dams or altered by irrigation withdrawals. Annual variations of the standing 

 crops of adult trout within each reach were found to be related to annual flow 

 variations. From these relationships, instream flow recommendations were 

 derived. 



The use of long-term standing crop and flow data is not a practical means 

 of deriving future flow recommendations due to the excessive time, cost and 

 manpower required to collect data. Because of these limitations, flow recom- 

 mendations for other waterways in Montana will primarily be derived from in- 

 stream flow methods that incorporate limited biological data. However, the 

 reliability of the recommendations generated by the methods in current use has 

 not been adequately documented. Acceptance of these recommendations has gen- 

 erally been based on theoretical considerations and professional judgment 

 rather than biological proofs. The flow recommendations derived from the 

 standing crop and flow data for the five river reaches provide a biologically 

 derived standard for comparing and evaluating the recommendations of the 

 various instream flow methods. 



In this study four instream flow methods were applied to each of the five 

 river reaches and their recommendations compared to those derived from the 

 long-term standing crop and flow data. The four methods chosen were: (1) a 

 single transect method utilizing the wetted perimeter-discharge relationship 

 for a riffle cross-section, (2) a multiple transect method utilizing the 

 wetted perimeter-discharge relationship for a composite of cross-sections, (3) 

 the incremental method developed by the Cooperative Instream Flow Service Group 

 (IFG) of the U. S. Fish and Wildlife Service and (4) a non-field or fixed per- 

 centage method which utilizes historical discharge records. 



The policy of the MDFWP when deriving flow recommendations for Montana's 

 wild trout rivers is to address the needs of only the adult trout, the stage 

 that provides the recreational fishery. The reason for ignoring the other 

 trout life stages is based on the premise that the number of new recruits 

 entering a resident adult trout population is primarily dictated by the adults 

 and not by the number of eggs, fry or juveniles which produce the recruits. 



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