71 



The survival of the larger and older rainbow trout in the Hildreth 

 Section of the Beaverhead River is directly related to the magnitude of 

 flows during the nonirrigation season (approximately October 15 - April 

 15). During this period, Clark Canyon Reservoir is storing water for 

 irrigation and releases into the Beaverhead River are minimal. A measure 

 of the absolute minimal average daily flow that will maintain adequate 

 survival and numbers of older rainbow trout is provided by the distrib- 

 ution of average daily flows between successive Fall estimates (Appendix 

 Table 35). The three Fall to Fall periods having the lowest rates of 

 population decrease of age III and older rainbow trout and the greatest 

 numbers of age IV and older rainbow trout remaining at the end of each 

 period are 1970-71, 1971-72, and 1972-73. Compared to the other Fall to 

 Fall periods, 1970-71, 1971-72, and 1972-73 have fewer average daily 

 flows in the 101-150 cfs interval and none in the 57-100 cfs interval. 

 The data suggests that one average daily flow <_ 150 cfs is sufficient 

 to adversely affect the annual survival of older rainbow trout (Figure 

 18). 



Few rivers in Montana are capable of producing a trophy rainbow 

 trout fishery equal to the potential of the upper Beaverhead River. 

 The ability of the upper Beaverhead to produce trophy rainbow trout 

 is dependent on flow releases from Clark Canyon Reservoir. Appendix 

 Table 35 provides an index of the minimum average daily flow that will 

 maintain desirable numbers of trophy rainbow trout. The highest estimate 

 of numbers of age IV and older rainbow trout followed a period (1971-72) 

 in which only 6 of the average daily flows were <_ 250 cfs and 7 were 

 <_ 300 cfs. It appears that the management of the Hildreth Section of 



