postassium dichromate, and permanent diatom slides were prepared 

 using Naphrax, a high refractive index mounting medium, following 

 Standard Methods for the Examination of Water and Wastewater 

 (APHA 1998) . 



Between 428 and 454 diatom cells (856 to 908 valves) were 

 counted at random and identified to species. The four volume 

 series by Krammer and Lange-Bertalot (1986, 1988, 1991a, 1991b) 

 was used as the main taxonomic and autecological reference for 

 the diatoms. Lowe (1974), Bahls et al . (1984), van Dam et al . 

 (1994) , and Lange-Bertalot (1996) were also used as ecological 

 references for the diatoms. 



The diatom proportional counts were used to generate an 

 array of diatom association metrics (Table 1) . A metric is a 

 characteristic of the biota that changes in some predictable way 

 with increased human influence (Barbour et al . 1999). 



Metric values from Cottonwood Creek were compared to numeric 

 biocriteria or threshold values developed for streams in the 

 Rocky Mountain and Great Plains Ecoregions of Montana (Tables 2 

 and 3) . 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) . 



The criteria in Tables 2 and 3 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 



