Middle Fork Dearborn River below FAS 434. As it did in the main stem Dearborn 

 River, the pollution index declined significantly between the upper site and the lower site on the 

 Middle Fork Dearborn River (Table 5). However, the pollution index here was still above the 

 threshold for minor impairment from organic loading and indicated excellent biological integrity 

 for a prairie stream, although marginal integrity for a mountain stream. Diatom diversity was 

 just below the threshold for minor impairment in a prairie stream, but diversity was excellent 

 when compared to criteria for mountain streams. Four abnormal cells of Cymbella excisa may 

 have resulted from the rapid population growth of this species, which was present in large 

 numbers at this site. The siltation index at this site was low and indicated an acceptable level of 

 siltation for a prairie stream or a mountain stream. This site shared less than a third of its diatom 

 association with the upstream site on the Middle Fork, indicating a moderate change in the 

 diatom flora and environmental conditions between the two sites. This is to be expected for sites 

 on the same sfream but on opposite sides of an ecoregional boundary. 



South Fork Dearborn River at Blacktail Ranch. Three of the four most abundant 

 diatom species here are somewhat tolerant of organic pollution (Table 5). This resulted in a 

 depressed pollution index that indicates minor impairment from organic loading when compared 

 to criteria for mountain streams. Other than two abnormal cells oi Achnanthidium pyrenaicum, 

 which also indicates minor impairment, diatom metrics here indicated excellent biological 

 integrity for a mountain stream. 



South Fork Dearborn River above FAS 434. Values for most diatom metrics, 

 including the pollution index, indicated improved water quality and biological integrity at this 

 site when compared to the upstream site at Blacktail Ranch (Table 5). The one exception was 

 the siltation index, which increased from 11% motile diatoms at Blacktail Ranch to 32% motile 

 diatoms at this site. Although this level of sedimentation is acceptable for a prairie stream, it 

 would have indicated minor impairment if compared to criteria for mountain streams in Table 2. 

 Two abnormal cells of Navicula reichardtiana indicate minor impairment at this site from an 

 unknown cause. The two sites on the South Fork Dearborn River shared about half of their 

 diatom associations in common, which indicates that changes in diatom species composition and 

 environmental conditions between the two sites were minor. 



11 



