Dam, the assemblage remained constant but was suppressed 

 compared with Warm Springs Creek and stations downstream from 

 Milltown Dam. 



More recent investigations have documented improved 

 macroinvertebrate communities in Silver Bow Creek (Chadwick 

 et al. 1986) and in the upper Clark Fork (Canton and Chadwick 

 1985; McGuire 1987) . Macroinvertebrates began colonizing 

 Silver Bow Creek in 1975 when the Anaconda Minerals Company 

 began secondary treatment of the Weed Concentrator effluent 

 and the Butte sewage treatment plant ceased discharging 

 sludge into the stream (MultiTech and OEA Research 1986) . 

 By 1981, metal-tolerant midge species were present throughout 

 Silver Bow Creek, and a few other tolerant species were 

 established in the stream's lower reach (Gregson Hot Springs 

 to the Warm Springs Ponds) . Since 1981, the composition and 

 abundance of macroinvertebrate assemblages have been more 

 variable, indicating a gradual stabilization of environmental 

 conditions. Although much improved relative to historic 

 conditions. Silver Bow Creek remains severely polluted by 

 heavy metals, which results in an impoverished macroinverte- 

 brate fauna. 



Similarly, severe impacts from metals contamination have 

 been less frequent during the past ten years in the upper 

 Clark Fork (MultiTech and OEA Research 1986) . However, 

 metal-sensitive species are still precluded from much of the 

 river above Milltown Dam. As heavy metals pollution has 

 become less severe, other environmental conditions have 

 become more apparent. Densities of a few tolerant insect 

 species have increased dramatically in response to nutrient 

 and organic enrichment from municipal sewage treatment plants 

 and nonpoint sources (natural, agricultural, and forest 

 practices) . This response, previously suppressed by toxic 

 conditions resulting from metals contamination, is now 

 evident throughout the drainage. 



Milltown Dam to the Confluence of the Flathead River 



Pollution in the Clark Fork has had a less dramatic 

 effect on the biota downstream from Missoula than in the 

 headwaters. Impacts attributable to heavy metals have been 

 substantially less downstream from the Milltown Dam than in 

 the upstream reaches where metals pollution has historically 

 been more severe. The magnitude, frequency, and the 

 duration of exposure to elevated metals concentrations 

 downstream from Milltown Dam have been lessened as a result 

 of metal-bearing sediments being trapped in the reservoir 

 (Johns and Moore 1985) , and by dilution from the Blackfoot 

 River and Rock Creek. 



2-25 



