The site at Breen's had only about one quarter of its diatom 

 flora in common with the North Fork site (Similarity Index = 

 23.36%, Table 7), indicating a significant change in diatom 

 composition and environmental conditions between these two sites. 

 Diatoms introduced from the South Fork and from McDonald Creek 

 may explain much of this observed change in floristics. 



Reduced flows and the concentration of nutrients may 

 explain the slightly depressed pollution index at Breen's (Table 

 7). Otherwise, water quality was excellent at this site and 

 aquatic life uses would be fully supported here if streamflows 

 were restored during the irrigation season. 



The siltation index (percent abundance of motile diatoms) 

 indicated minor impairment by sediment at Highway 89 (Table 7) . 

 Deep Creek and eroding stream banks upstream are possible sources 

 of sedimentation at this site. The Choteau lagoon apparently did 

 not have a significant effect at this site because the pollution 

 index was still within the "excellent" range and somewhat higher 

 than it was at Breen's. All other periphyton metrics indicated 

 full support of aquatic life uses at Highway 89. 



Highway 89 had less than a quarter of its diatom flora in 

 common with the next upstream site at Breen's (Similarity Index = 

 22.46%, Table 7), indicating a significant change in species 

 composition and environmental conditions between these two sites. 

 Deep Creek is a potential source of floristic change between 

 Breen's and Highway 89. 



The pollution- tolerant diatoms Gomphonema parvulum and 

 Cymbella silesiaca peaked in abundance at Miller's, resulting in 

 a depressed pollution index that indicated minor impairment 

 (Table 7) . The combined discharge from the Priest Butte Lakes 

 and waterfowl-rich Freezeout Lake may be a significant source of 

 organic nutrient loading to the Teton River at Miller's. 



14 



