left behind. The Blackfoot River joins the Clark Fork just 

 above the Milltown Dam, and its clean water further dilutes 

 metals concentrations in the middle Clark Fork segment. 

 However, in the Clark Fork just below Milltown Dam, these 

 benefits are sometimes masked. 



Elevated metals levels periodically occur in association 

 with operational drawdowns of Milltown Reservoir that result 

 in the loss of metal-bearing sediments from the reservoir. 

 More recently, short-term increases in metals levels below 

 the dam have been associated with reconstruction of the dam's 

 aged spillway, which was severely damaged during the major 

 runoff of February 1986. The occurrence of sediment -metal 

 events resulting from drawdowns will be reduced by the 

 completion of the Milltown Rehabilitation Project. The 

 installation of a radial gate and fixed wheel panels will 

 allow the control of runoff up to 28,000 cfs without drawing 

 down the reservoir. A drawdown will be required only if 

 streamflow exceeds 28,000 cfs, which is an event that is 

 expected to occur on the average of about every 14 years. 

 These high flows will cause flow control gates to open to 

 accommodate the increased water quantity. A drawdown of the 

 reservoir is necessary to reset the gates once the high flows 

 recede. The Montana Power Company believes, based on past 

 experience, that such a drawdown will cause less sediment 

 loading than previously occurred because such high flows 

 (greater than 28,000 cfs) will have removed much of the 

 susceptible sediment from the reservoir. 



River monitoring by the Montana Power Company revealed a 

 brief increase in zinc concentrations in March 1987 before 

 the onset of the runoff period. Concentrations of 1,72 ug/1 

 and 1,120 ug/1 acid-soluble zinc were measured at Turah on 

 March 5 and 6, 1987 (MPC 1987a). River flow at Turah 

 increased 50 percent from 787 cfs on March 3 to 1,180 cfs on 

 March 5 after being stable (609-836 cfs) since January 1. 

 Total suspended sediment increased from 39.7 mg/1 on March 4 

 to 88.8 mg/1 and 88.9 mg/1 on March 5 and 6 at Turah. Acid- 

 soluble copper was less markedly elevated to 50 ug/1 on both 

 days — up from less than 10 ug/1 on March 2, 1987. 



Middle Clark Fork . Water Quality Bureau data indicate 

 that metals concentrations in the middle Clark Fork are 

 generally much lower than those in the upper Clark Fork 

 (Figure 3-20) . This is likely due to fairly large volumes of 

 clean dilution water provided by the Bitterroot and St. Regis 

 rivers and increasing distance from metals sources. 

 Exceedences of copper criteria were generally infrequent, 

 slight, and short-lived in this reach. Zinc criteria were 

 exceeded only once in the three-year monitoring period (in 

 February 1986) . 



3-73 



