59 



Only the Hi ldreth Extended (2) and Barretts Downstream (18) Sections 

 showed significant increases in both the numbers and biomass of brown 

 trout between 1975 and 1976. Biomass actually decreased significantly 

 in three of the study sections. The results presented earlier (Hildreth 

 Section) show that the flow increases that occurred between the 1975 and 

 1976 estimates were not the primary factor causing the population in- 

 crease in the Hildreth Extended (2) Section. It appears that the volume 

 of flow is not the only factor limiting trout populations in much of 

 the Beaverhead River. 

 Comparison of trout populations between sections 



Differences in the estimated numbers and biomass of trout between 

 study sections (Figures 15 and 16) reflect a myriad of limiting factors 

 not common to all sections nor of the same magnitude. In addition to 

 flow differences, sedimentation and cover are factors of major concern. 

 Although the effects of sedimentation were not quantified in this study, 

 the decline in numbers and biomass of trout between the upper and lower 

 river is believed to partially reflect increasing sedimentation. 

 Sediment transport in the Beaverhead system is hindered by the diversion 

 of reservoir releases for irrigation. Removal of large amounts of water 

 reduce water velocities in the river to a point where the sediments can 

 no longer remain suspended and, therefore, are deposited in the lower 

 river. Mean turbidity measurements show that levels of suspended 

 sediments were lowest in the upper river (Table 23). Grasshopper Creek, 

 a major contributor of sediments to the Beaverhead River, is primarily 

 responsible for the increase in mean turbidity between miles 8 and 18. 



