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Deer Creek. The only indication of stress in Deer Creek was an elevated number of 

 teratological diatom valves in the species Synedra ulna. This may indicate chronic toxicity from 

 heavy metals or some other stressor. The percentage of motile diatoms approached but did not 

 exceed the threshold for minor impairment from sedimentation. Otherwise, diatom metrics 

 indicated excellent biological integrity and full support of aquatic life uses in Deer Creek. 



Overwhich Creek. Diatom metrics indicated minor impairment from organic loading 

 and sedimentation at the upper site on Overwhich Creek. The dominant species here was 

 Synedra ulna, which is somewhat tolerant of organic loading. At the downstream site, the 

 percentage of motile diatoms exceeded the threshold for moderate impairment from 

 sedimentation. The dominant diatom here was Nitzschia dissipata, a highly motile and 

 eutraphentic species. The two sites on Overwhich Creek shared slightly less than half of their 

 diatom assemblages, indicating that minor to moderate change occurred between them. 



Buck Creek. The only indication of stress at this site was an elevated number of diatoms 

 in the species Meridion circulare. This is a pollution sensitive species that prefers cool water 

 temperatures and constant flows. Being neither attached nor motile, M. circulare cannot 

 withstand high current velocities or large amounts of sediment (bedload). Nevertheless, the 

 sedimentation index in Buck Creek approached but did not exceed the criterion for minor 

 impairment. In other respects, Buck Creek demonstrated excellent biological integrity and no 

 impairment of aquatic life uses. 



Nez Perce Fork. The upper site on the Nez Perce Fork of the Bitterroot River had the 

 lowest pollution index and the highest sedimentation index of the three sites. The pollution 

 index approached but did not exceed the threshold for minor impairment, but the sedimentation 

 index did indicate minor impairment. The dominant diatom here was Planothidium lanceolatum, 

 a species adapted to living on sand grains. The abundance of Planothidium species here and at 

 other sites in the study area may reflect an adaptation to sandy substrates of decomposed granite 

 that is typical of these streams. Nitzschia dissipata — a highly motile and eutraphentic species — 

 ranked second in abundance at the upper site on the Nez Perce Fork. 



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