The Blackfoot River also has low trace element concentra- 

 tions, despite some abandoned mine areas in its upper basin. 

 Because of their large flow contributions and relatively low 

 trace element concentrations, Rock Creek and the Blackfoot 

 River improve the water quality of the mainstem. 



Water Quality Bureau data indicate that water quality 

 varies considerably within different sections of the upper 

 river reach. Water quality is much improved below Warm 

 Springs Creek through a direct dilution of metals concentra- 

 tions and as a result of increased water hardness and 

 alkalinity that buffer the effects of metals. Warm Springs 

 Creek drains a limestone formation that contributes to its 

 high hardness and moderate alkalinity. Unfortunately, Warm 

 Springs Creek is severely dewatered for irrigation and it is 

 frequently nearly dry in the months of July and August. 



Metals concentrations in the Clark Fork tend to decrease 

 from its point of origin at Warm Springs to Dempsey, 

 presumably as a result of dilution from cleaner tributaries. 

 The copper criteria (Figure 3-20) were exceeded less than 

 half the time, and exceedences that did occur were usually 

 slight. Zinc criteria were rarely exceeded. From Dempsey to 

 the Little Blackfoot River, water quality progressively 

 deteriorates, especially during winter and spring months. 

 Metals concentrations and frequency of exceedences of the 

 aquatic life criteria tend to increase, despite the entry of 

 additional clean-water tributaries. The copper criteria were 

 exceeded up to half the time during the monitoring period in 

 the Clark Fork above the Little Blackfoot River, with some 

 measurements exceeding the criteria several-fold. Despite an 

 increase in zinc concentrations, criteria were infrequently 

 exceeded. 



Average annual copper loads (Figure 3-21) increased by 

 as much as 6.5 times, and zinc loads (Figure 3-22) increased 

 by more than three times in the Clark Fork from Warm Springs 

 to some 15 or more miles below Deer Lodge. Metals sources 

 are streamside tailings deposits and possibly inputs from 

 contaminated ground water. The rate of increase in metals 

 loading seems to be consistent progressing downstream in the 

 reach from Warm Springs Creek to Deer Lodge. However, from 

 Deer Lodge to the Little Blackfoot River, a major increase in 

 loading occurs. This may correspond to the presence of a 

 major ground water recharge zone and the presence of 

 localized tailings deposits in the river floodplain. 



Conditions generally improve in the Clark Fork from 

 Garrison downstream to the Blackfoot River as the contribu- 

 tions of clean water from major tributaries such as the 

 Little Blackfoot River and Rock Creek dilute metals con- 

 centrations and metals sources become less significant or are 



3-72 



