The site at the Gaging Station below the forks was dominated 

 by the pollution sensitive diatoms Achnanthes winutissiwa and 

 Gomphonema bohewicum (Table 6) . The number of species counted, 

 diatom species diversity, pollution and siltation index values 

 all indicated excellent water quality. Only minor impairment was 

 indicated by the disturbance index and percent dominant species. 

 This stress causing this impairment was probably natural in 

 origin . 



The Gaging Station site had about half of its diatom flora 

 in common with the North Fork site upstream (Table 6) . This is 

 to be expected where two co-equal tributaries (i.e., the North 

 and South Forks) join and their floras are combined. A larger 

 change occurred between the Gaging Station and the next site 

 downstream at Breen's; these two sites shared less than a third 

 of their diatom floras. These 5 upper sites show progressively 

 increasing species richness and diversity (Table 6) , which is 

 probably caused by inorganic nutrient enrichment. The site at 

 Breen's shows minor impairment by nutrient enrichment. 



When assessed using criteria developed for mountain streams, 

 the site near Collins shows minor impairment because of depressed 

 diversity and species richness, and a large percentage of the 

 dominant diatom Amphora pediculus (Table 6) . Most species of 

 Amphora are motile (Round et al . 1990) . Amphora pediculus is an 

 alkaliphilous species, prefers large concentrations of inorganic 

 nutrients, and is indifferent to small amounts of salt (Lowe 

 1974) . It is rarely abundant in stream samples (Patrick and 

 Reimer 1975) . Large numbers of this diatom near Collins are 

 probably a response to siltation and nutrient enrichment. 



When compared to criteria developed for prairie streams, the 

 site near Collins exhibited moderate impairment and only partial 

 support of aquatic life uses (Table 7) . This is because prairie 

 streams are typically richer in species and have higher species 



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