Despite the serious dewatering that occurs in lower McDonald 

 Creek, water quality and biological integrity- -as indicated by 

 the periphyton community- -were excellent. If flows are restored 

 to lower McDonald Creek during the irrigation season, aquatic 

 life uses would be fully supported. 



Upper Teton River 



The attached, sensitive species Achnanthes minutissima ( = 

 Achnanthidium minutissima) dominated the diatom association in 

 the North Fork (Table 7) . Achnanthes minutissima is often the 

 dominant diatom in streams that have cold, fast, and nutrient- 

 poor waters, and sometimes this taxon achieves a relative 

 abundance of 60% or more where these natural stressors are 

 operating (Bahls et al . 1992). 



Dominance by Achnanthes minutissima resulted in a slightly 

 elevated disturbance index and percent dominant species index in 

 the North Fork, indicating minor impairment by natural stressors 

 (Table 7) . All diatom metrics indicated full support of aquatic 

 life uses in the North Fork and validated this as a suitable 

 reference stream. (The North Fork at the West Fork Guard Station 

 was sampled as a biological reference site by Bahls et al . 1992.) 



The predominance of tychoplanktonic diatoms (e.g., 

 Synedra ulna and Fragilaria construens) at Breen's may be due to 

 greatly reduced flows (Figure 1) and pooling at this site, or to 

 the floristic influence of McDonald Creek upstream. Two common 

 diatoms at Breen's were Diatoma tenue (7.25%), an unattached 

 species, and Cyclotella distinguenda (7.37%), a planktonic 

 species. Both diatoms prefer waters with elevated dissolved 

 solids but neither diatom was recorded at upstream sites in 

 McDonald Creek or in the North Fork, lending credence to the 

 pooling theory. However, there are other potential upstream 

 sources of these diatoms (e.g., South Fork and Eureka Reservoir). 



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