organic loading. This site also had an unusually large percentage of teratological (physically 

 abnormal) cells. The cause of these abnormal cells is unknown. Diatom metrics also indicated 

 minor impairment from nutrient loading at the lower site on the South Fork of the Two Medicine 

 River (Table 8). Diatoma moniliformis was the dominant diatom species at this site. Dominance 

 by this species signals an increase in conductivity at the downstream site. Diatoma moniliformis 

 also tolerates some organic loading. An elevated percentage of motile diatom species indicates 

 moderate impairment from sedimentation and only partial support of aquatic life uses at the 

 lower site on the South Fork of the Two Medicine River. The reason for this sedimentation is 

 unknown, but it may be related to major disturbances to riparian areas caused by the 1964 flood. 



An elevated percentage of motile diatom species indicated minor impairment from 

 sedimentation in Railroad Creek (Table 8). Other diatom metrics indicated excellent biological 

 integrity and no impairment of aquatic life uses in Railroad Creek. The pollution index, 

 however, was borderline on minor impairment from organic loading. 



The only indicator of stress in the South Fork of Birch Creek and the North Fork of 

 Dupuyer Creek was an elevated percentage of Achnanthidium minutissimum (Table 8). This 

 minor stress was probably natural and caused by the relatively steep gradients, cold water 

 temperatures, and low nutrient concentrations of these streams. A large percentage of A. 

 minutissimum also indicated minor natural stress in the South Fork of Dupuyer Creek. However, 

 this site also supported an elevated number of motile diatoms, which indicates minor stress from 

 sedimentation. The North and South Forks of Dupuyer Creek shared slightly less than half of 

 their diatom floras, indicating that the two sites were quite similar in terms of diatom species 

 composition and ecological conditions. 



Two Medicine Drainage — Plains Sites. Diatom metrics indicated only minor 

 impairment in the Two Medicine River and at main stem sites on Birch Creek and Dupuyer 

 Creek (Table 9). Loading of organic nutrients was the cause of minor impairment at the mouth 

 of the Two Medicine River and at the lower 2 sites on Birch Creek. A significant change in the 

 diatom association of Birch Creek occurred between sites 2 and 3, which shared only about 20% 

 of their floras. This indicates a significant change in ecological conditions between these 2 sites. 



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