metal sources. Although these investigations are part of a 

 larger investigation of rivers in general, the data should be 

 useful for understanding Clark Fork problems. The investi- 

 gations involve the mainstem Clark Fork and several major 

 tributary streams (Luoma 1988) . 



Reservoir Sediments 



Milltown Reservoir acted as a primary catch basin for 

 mining-related sediment from the time of its construction 

 (1906) until the construction of the Warm Springs Ponds 

 (1911) . This reservoir is basically full, with an estimated 

 120 million cubic feet of metals-contaminated sediment 

 behind the dam (Woessner et al. 1984). Johns and Moore 



(1986) undertook a study to demarcate the lower boundary of 

 detectable metals-contaminated sediments derived from mining 

 and smelting activities in the headwaters. They collected 

 samples from the Thompson Falls, Noxon Rapids, and Cabinet 

 Gorge reservoirs in the lower portion of the Clark Fork 

 Basin. Samples were also collected from three drainages 

 tributary to Noxon Rapids and Cabinet Gorge reservoirs to 

 serve as background checks. Data from these lower reservoirs 

 and tributaries were compared with data from the Clark Fork 

 and Blackfoot arms of the Milltown Reservoir collected during 

 the Milltown Superfund Remedial Investigation. 



Results of this study are summarized in Table 3-13. 

 Total metals concentrations, measured in micrograms per gram 

 (ug/g) , in the sediments of all four reservoirs are clearly 

 elevated compared with Blackfoot and tributary sediments. In 

 almost all cases, total metals levels in the reservoirs 

 decreased progressively downstream. The same trends were 

 evident for acetic acid-extractable metals, as illustrated by 

 the copper and zinc plots in Figures 3-12 and 3-13. 



Although some of the metals concentrations in the three 

 lower reservoirs were not highly enriched over background 

 levels, it is clear that elevated levels of copper and zinc 

 occur as far downstream as Cabinet Gorge Reservoir, some 34 

 miles from the major source of those metals. Transport of 

 the metals-laden sediment down river may have occurred prior 

 to construction of the Milltown Dam, during exceptional 

 events such as dike breaches at the Warm Springs Ponds, 

 during operational and maintenance drawdowns of the Milltown 

 Reservoir, and as part of the current total suspended 

 sediment load in the Clark Fork. Metal-rich sediments were 

 and are likely diluted by additions of "clean" sediments 

 from major tributaries such as the Blackfoot, Bitterroot, 

 Flathead, and St. Regis rivers (Johns and Moore 1986) . This 

 conclusion appears to contradict the findings of Andrews 



(1987) . 



3-49 



