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ABSTRACT 



The effects of varied flow patterns on trout populations and the 

 physical and hydraulic characteristics in a 6455 ft section of the 

 upper Beaverhead River below Clark Canyon Reservoir, Montana, were 

 measured between 1966 and 1976. Population estimates were made using 

 electrofishing techniques. The WSP (Water Surface Profile) computer 

 program of the Bureau of Reclamation was used to predict physical and 

 hydraulic values at various flows. 



The survival of age III and older rainbow trout was directly related 

 to the magnitude of flows during the nonirrigation season (approximately 

 October 15 to April 15). During this period, water for irrigation is 

 being stored in the reservoir and releases into the river are minimal. 

 Results of this study suggest that average daily flows greater than 

 approximately 250 cfs are needed to provide a high quality, trophy 

 rainbow trout fishery. Flows during this study were not sufficiently 

 reduced to adversely affect the survival of older brown trout. 



Poor reproductive success was the major factor limiting the total 

 numbers and biomass of trout throughout much of this study. Repro- 

 ductive success, as measured by the estimated numbers of age I trout, 

 appears to be related to flow patterns during the brown and rainbow 

 trout spawning periods. With flow releases favorable to both repro- 

 duction and the survival of older trout, the upper Beaverhead River 

 is capable of supporting greater numbers and biomass of trout of all 

 age groups than those which existed throughout much of this study. 



The physical and hydraulic characteristics most affected by flow 

 reductions were cross sectional area and current velocity. Top 

 width and wetted perimeter were least affected. The rate of loss for 



