percent of the samples in FY 86-87, with mean concentrations 

 averaging nearly three times the threshold value. 



Upper Clark Fork 



Measured total phosphorus concentrations and estimated 

 annual phosphorus loads for the Clark Fork from its head- 

 waters below Warm Springs Creek (station 7) to below Milltown 

 Dam (station 15) are presented in Figures 3-28 and 3-29, 

 respectively. Estimated annual loads for total inorganic 

 nitrogen are given in Figure 3-30. 



Nutrient concentrations in Warm Springs Creek and in the 

 Mill-Willow Bypass were significantly lower than those in 

 Silver Bow Creek. Each of these tributaries helped to reduce 

 the nutrient concentrations in the mainstem Clark Fork at 

 its headwaters. 



Nutrient concentrations in the upper Clark Fork 

 mainstem, in general, decrease below incoming clean tributar- 

 ies and increase below municipal wastewater discharges. In 

 Figure 3-28, notable increases in median total phosphorus 

 concentrations were observed between monitoring stations 9 

 and 10 and between stations 11 and 12. The primary point 

 sources of phosphorus in those reaches are the Deer Lodge, 

 Philipsburg (via Flint Creek) , and Drummond wastewater 

 discharges. The ground water system is also a possible 

 source of phosphorus. In 30 water samples collected from 

 1985 to 87 from 28 wells in the area between Deer Lodge and 

 Drummond, most concentrations of dissolved phosphorus were 

 less than 100 ug/1, and the maximum concentration was 300 

 ug/1 (USGS unpublished data) . The phosphorus load plot 

 (Figure 3-29) confirms that these reaches contain significant 

 phosphorus sources. However, the amount of ground water 

 inflow in this area has not been quantified. 



Comparing the measured phosphorus concentrations with 

 the EPA criterion indicates that concentrations in the upper 

 Clark Fork frequently exceeded the threshold value, but not 

 by a large margin. The phosphorus criterion was exceeded in 

 60 to nearly 80 percent of the samples below the Blackfoot 

 River during the FY 85-87 monitoring period. Average 

 concentrations ranged from 1.5 to 0.7 times the criterion. 

 The highest frequency of exceedence of the phosphorus 

 criterion anywhere in the mainstem Clark Fork during the FY 

 85-86 period consistently occurred at monitoring station 10, 

 below the Deer Lodge sewage outfall. This area corresponds 

 roughly to the uppermost extent of the Cladophora algal 

 blooms. Rock Creek marks the downstream extent of the most 

 serious Cladophora blooms. Cladophora is further reduced 

 below the Blackfoot River confluence. 



3-88 



