63 



as low as 22 cfs have occurred in the Grant (0.1) Section in recent years 

 (October 16, 1974). Fish kills, caused by toxic levels of hydrogen 

 sulfide in reservoir releases, were documented in the Grant (0.1) 

 Section at the beginning of this study. Ammonia concentrations were also 

 elevated (see page 19). The severity and frequency of water quality 

 problems in the Grant (0.1) Section throughout the study period are 

 unknown. 



Water quality problems also exist in other sections of the Beaver- 

 head River. Sampling by the Montana Department of Fish and Game and the 

 U.S. Food and Drug Administration in 1970-71 indicated that mercury 

 pollution occurred in the vicinity of Dillon. Some fish in this area 

 contained mercury greatly in excess of FDA standards. Natural spring 

 discharges, seed-potato treatment, the leaching of old mill tailings, 

 and possible industrial activity were considered the sources of mercury. 

 Above Grasshopper Creek, the Beaverhead River did not appear to contain 

 elevated levels of mercury. 



Growth, in addition to numbers of trout, influenced the differences 

 in biomass between sections. A measure of the differences in growth 

 rates between study sections is provided by the mean weights at capture 

 of age II brown trout in Spring, 1975 and 1976 (Figure 14). Mean weights 

 decreased dramatically between the Grant (0.1) and Barretts Downstream 

 (18) Sections. The growth differences in the upper river are partially 

 attributable to Clark Canyon Reservoir, which has reduced diel fluctuations 

 in water temperature and has probably provided temperatures more favorable 

 for trout growth. However, the influence of the dam on water temperatures 

 decreased rapidly in approximately the upper 15 miles of river. 



