The metal concentrations in the mainstem floodplain are 

 generally several orders of magnitude above the levels 

 expected for noncontaminated sediments. It is interesting 

 to note that the farthest downstream site (Drummond) had the 

 highest average cadmium and copper levels and the second- 

 highest arsenic concentration, indicating that in this study, 

 metal levels did not decrease with distance downstream from 

 the source areas at Butte and Anaconda. Knudson (1984) noted 

 that the Drummond and Deer Lodge valleys are deposition zones 

 because of low stream gradients and suggested that contami- 

 nated sediments deposited in these areas may be sources of 

 metals to the lower reaches of the upper river. 



In 1983, Rice and Ray (1984) conducted a study of the 

 Grant-Kohrs Ranch at the north end of Deer Lodge (Figure 3- 

 8) . This ranch is a National Historic Site that commemorates 

 the development of the cattle industry in the West. 

 Approximately 75 percent of the ranch acreage is on the 

 floodplain of the Clark Fork, which bisects the site. The 

 study was conducted to describe the flora and fauna of the 

 site and to assess the extent and severity of metal con- 

 tamination in the ranch soils and biota. 



The researchers sampled soil and biota in four distinct 

 zones on the ranch: riparian zone (grass/shrub floodplain) , 

 meadov zone (grass/hay) , bench zone (grass) , and creek zone 

 (Cottonwood Creek, a minor tributary to the Clark Fork) . 



The same check and control plots established by Ray 

 (1983) (on Tin Cup Joe Creek, about five miles southwest of 

 the ranch, and along the Blackfoot River, 60 miles northwest 

 of the ranch) were used for this study. 



Soil profiles (0-10 inches) and a forage grass species 

 were sampled at 94 plots. Concentrations of soil arsenic, 

 cadmium, and copper in all four zones were greatly elevated 

 compared with the control plot in the Blackfoot drainage, 

 with the highest levels occurring in the riparian zone. 

 Metal concentrations in the grasses sampled were higher than 

 concentrations thought to be typical of grasses from 

 uncontaminated areas, but only copper in grass from the 

 riparian zone was significantly elevated relative to the 

 check plot (Rice and Ray 1984) . 



In a study by Moore (1985) for the EPA, samples of bank 

 sediment were collected at 26 sites along the mainstem Clark 

 Fork to determine if these floodplain deposits could be the 

 source of metals in the Milltown Reservoir. Bank sediments 

 in the Little Blackfoot River, Flint Creek, Rock Creek, and 

 the Blackfoot River were also sampled to assess the pos- 

 sibility of metal-rich sediments coming from the major 

 tributary drainages. To establish natural background levels 



3-45 



