Metric values from Mol Heron Creek were compared to numeric 

 biocriteria or threshold values developed for streams in the 

 Rocky Mountain Ecoregions of Montana (Table 2) . These criteria 

 are based on metric values measured in least- impaired reference 

 streams (Bahls et al . 1992) and on metric values measured in 

 streams that are known to be impaired by various sources and 

 causes of pollution (Bahls 1993) . 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 . 



The criteria in Table 2 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. 



Quality Assurance. Several steps were taken to assure that 

 the study results are accurate and reproducible. 



Upon receipt of the sample, station and sample information 

 were recorded in a laboratory notebook and the sample was 

 assigned a unique number compatible with the Montana Diatom 

 Database: 1974-01. The first part of this number (1974) 

 designates the sampling site (Mol Heron Creek at canyon mouth) ; 

 the second part of the number (01) designates the number of 

 periphyton samples that have been collected at this site to date 

 for which data have been entered into the Montana Diatom 

 Database . 



Sample observations and analyses of soft (non-diatom) algae 

 were recorded in a lab notebook along with station and sample 

 information provided by MDEQ . A portion of the raw sample was 

 used to make duplicate diatom slides. After completing the 

 diatom proportional count, the slide used for the count will be 

 deposited in the University of Montana Herbarium in Missoula. 

 The other slide will be retained by Hannaea in Helena. 



