other macroinvertebrates, and plants. Severe depletions of 

 dissolved oxygen can cause fish and insect kills. Chronic- 

 ally low levels can cause a decrease in diversity and 

 quality of aquatic life (DHES 1985) . Montana Water Quality 

 Standards (DHES 1988a) for most of the Clark Fork do not 

 permit induced reductions of DO below 7 mg/1. Between Warm 

 Springs Creek and Cottonwood Creek, DO concentrations cannot 

 fall below 6 mg/1 from June 2 to September 30 or below 7 mg/1 

 between October 1 and June 1. 



The variables that affect dissolved oxygen levels 

 include water temperature, biological activity such as 

 photosynthesis and respiration, oxidation of inorganic 

 compounds, decomposition of organic matter, and reoxygenation 

 from water turbulence. These variables, along with diurnal 

 and seasonal variations, interact in complex ways to 

 determine instream dissolved oxygen concentration (DHES 

 1985) . Algae and other aquatic plants produce oxygen in 

 sunlight and consume oxygen during nighttime respiration; 

 therefore, very productive streams may have severe nighttime 

 sags in DO (Braico 1973) . 



Although temperature and DO data for the Clark Fork are 

 limited, several studies have been completed by the WQB. The 

 first was done in August 1973, by Braico, who measured DO and 

 temperatures at frequent intervals during a 24-hour period 

 (called "diel" monitoring) at 12 stations along the Clark 

 Fork and at single sampling sites on Rock Creek (near 

 Clinton) , the Blackfoot River, and the Bitterroot River. 

 The author reported the following results: 



• The highest temperature was measured in the Clark 

 Fork just above the Rock Creek confluence where a 

 maximum temperature of 76' F was recorded. 

 Temperatures reached 72° F on the mainstem at 

 Garrison, Drummond, and Turah. 



• Maximum temperatures in Rock Creek, the Blackfoot 

 River, and the Bitterroot River were 68° F, 70° F, 

 and 74° F, respectively. 



• At stations below the Bitterroot confluence, where 

 the Clark Fork becomes quite large, stream 

 temperatures were least affected by diurnal 

 variations in air temperature. 



• The lowest DO concentrations of 5.9 and 5.2 mg/1 

 were observed in the Clark Fork at Deer Lodge and 

 the Rock Creek Bridge, respectively. Conditions 

 were critical at the latter station when high 

 temperatures (above 68° F) and low DO levels 

 coincided for over five hours. 



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