various sources and causes of pollution (Bahls 1993) . 



Because of inherent differences in periphyton composition 

 and community structure. between mountain streams and prairie 

 streams, two different sets of criteria are provided (Tables 3 

 and 4) . For the purpose of periphyton assessment, mountain 

 streams are those located in the Rocky Mountain and Montana 

 Valley and 'Foothill Prairies Ecoregions (Omernik and Gallant 

 1987) . These streams are generally 'classified B-1 and B-2 in the 

 Montana Surface Water Quality Standards. Prairie streams are 

 those located in the Great Plains Ecoregions and are generally 

 classified B-3 and C-3. 



Of the 6 sites addressed in this report, periphyton metrics 

 from the forks of the Teton River and from the site just below 

 their confluence will be compared to criteria for mountain 

 streams in Table 3 . Metrics from the site near the mouth at 

 Kelly Ranch will be compared to criteria for prairie streams in 

 Table 4. Metrics from the remaining site near Collins will be 

 compared to criteria for both mountain and prairie streams. 



The criteria in Tables 3 and 4 distinguish among four levels 

 of impairment and three levels of aquatic life use support: no 

 impairment or only minor impairment (full support) ; moderate 

 impairment (partial support) ; and severe impairment (nonsupport) . 

 These impairment levels correspond to excellent, good, fair, and 

 poor biological integrity, respectively. 



Only periphyton samples collected in summer (June 21- 

 September 21) can be compared with confidence to reference stream 

 samples because metric values change seasonally and summer is the 

 season in which reference streams and impaired streams were 

 sampled for the purpose of biocriteria development. 



Quality Assurance. Several steps were taken to assure that 



7 



