69 



Values of the six physical and hydraulic characteristics at various 

 flows were predicted for a total of 66 cross sections in 7 other study 

 sections in the Beaverhead River. Based on the values at 1000 cfs, 

 changes in the six characteristics with respect to discharge were 

 generally similar to those trends which occurred in pools, runs, and 

 riffles of the Hildreth Section. Appendix Table 33 lists those flows 

 yielding a 50% reduction in the measured characteristics based on the 

 mean values at 1000 cfs. Again, cross sectional area, an index of the 

 total amount of living space available for fish, was greatly affected 

 by changes in discharge. Based on 1000 cfs, flows of approximately 340 

 cfs throughout the Beaverhead River for all water types combined would 

 yield an approximate 50% reduction in the amount of living space avail- 

 able for fish. 



DISCUSSION 



The presence of large numbers of trophy size trout characterizes 

 the quality fishery of the upper Beaverhead River. Brown and rainbow 

 trout as large as 12.0 and 13.25 lbs, respectively, were captured by 

 electrofishing during this study. Numbers of trophy trout (> 5.0 lbs) 

 were not estimated due to small sample size. However, numbers of 

 trout > 5.0 lbs captured during each of the sampling periods between 

 1966 and 1976 provide an index of the abundance of trophy fish (Table 

 34). Trophy brown trout were not captured until the last five years of 

 the study. Changes in numbers captured did not appear to relate to flows 

 Trophy rainbow trout were captured throughout the study. Numbers 

 captured, which were greatest in 1973 and 1974, crashed in 1975. This 

 dramatic reduction followed the lowest mean nonirrigation flow (97 cfs) 

 of the study. 



