algae according to the method described by Bahls (1993) . Soft 

 algae were identified using Dillard (1999) , Prescott (1978) , 

 Smith (1950), and Whitford and Schumacher (1984). These books 

 also served as references on the ecology of the soft algae, along 

 with Palmer (1969, 1977) . 



After the identification of soft algae, the raw periphyton 

 samples were cleaned of organic matter using sulfuric acid and 

 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 410 and 458 diatom cells (820 to 916 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 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 generated from the August samples were 

 compared to numeric biocriteria or threshold values developed for 

 streams in the Rocky Mountain Ecoregion 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 

 biocriteria in Table 3 do not apply to the May samples, which 

 were collected outside of the summer sampling "window" . 



