the Examination of Water and Wastewater (APHA 1998) . Between 400 

 and 406 diatom cells (800 to 812 valves) were counted at random 

 and identified to species. The following were used as the main 

 taxonomic and autecological references for the diatoms.- Krammer 

 and Lange-Bertalot 1986, 1988, 1991a, 1991b; Patrick and Reimer 

 1966, 1975. Lowe (1974) was also used as an ecological reference 

 for the diatoms . 



The diatom proportional counts were used to generate an 

 array of diatom association metrics (Table 2) . 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 and Rock Creek were 

 compared to numeric biocriteria or threshold values developed for 

 streams in the Rocky Mountain Ecoregions of Montana (Table 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 Table 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. 



Besides the ecoregional biocriteria listed in Table 3, lower 

 Rock Creek metrics were compared to metrics generated from an 

 upstream control site (Rock Creek Station 1) using Protocol II in 

 Bahls (1993) . Protocol II may be used on relatively short 

 segments of stream where an upstream control site fully supports 

 its aquatic life uses, that is, if it has a rating of "good" or 

 "excellent" biological integrity using Protocol I. Protocol II 



