to brackish, and prefers cooler water temperatures (Krammer and 

 Lange-Bertalot 1986) . 



Navicula gregaria is one of the most common diatoms in the 

 southern Fort Union Region. In this area, it prefers cool, mid- 

 size streams (1-100 cfs) , very hard and slightly brackish waters, 

 and pH values between 8.10 and 8.35. It tolerates some suspended 

 sediment and turbidity and reaches peak numbers in May and June 

 (Bahls et al. 1984) . 



The increase in Achnanthes minutissima at site #1 also 

 indicates cooler, better oxygenated water compared to site #3. 

 The decline in Navicula veneta between sites #3 and #1 indicates 

 lower concentrations of nutrients at site #1. Sites #3 and #1 

 shared a bit more than half of their diatom floras. 



The two replicate samples from site #1 contained very 

 similar diatom assemblages that generated very similar metrics 

 (Table 5) . The similarity index for the two replicate samples 

 was 85.72. This compares favorably with a mean similarity index 

 of 81 for counts conducted on replicate slides made from the same 

 sample (Bahls 1993) . One sample indicated moderate impairment 

 from siltation and the replicate indicated only minor impairment, 

 but both values were very close to the threshold. 



The station at the mouth of Squirrel Creek was one of nine 

 stream sites that were intensively monitored over a span of three 

 years as part of the Southern Fort Union biological baseline 

 study (Bahls 1980, Klarich and Regele 1980) . The site was also 

 included in an aquatic inventory of the CX Ranch conducted by 

 Olson-Elliott and Associates for Consolidation Coal Company. 

 Under these studies, a total of 31 periphyton samples were 

 collected at site #1 from June 1978 through October 1980. Diatom 

 proportional counts were completed on 30 of these samples and 

 Shannon species diversity and other diatom association metrics 



