Although conductivity nearly tripled at Miller's compared to 

 the Highway 89 site upstream (Figure 2), this increase did not 

 trigger a major change in the diatom assemblage (Similarity Index 

 = 51.55, Table 7) . Habitat conditions aside, water quality at 

 Miller's was good and aquatic life uses were fully supported. 



The brackish water diatom Diatoma tenue reached its maximum 

 abundance (15.12%) at Highway 221 (Table 7). This, together with 

 an abundance of other pollution- tolerant species {Cymbella 

 silesiaca and Fragilaria vaucheriae) , caused the pollution index 

 to remain at a level indicating minor impairment. Overall, the 

 diatom flora at Highway 221 was quite similar to the flora at 

 Miller's upstream (Similarity Index = 51.93%, Table 7). 



Highway 221 was the only site where teratological diatoms 

 were observed: four crooked valves of Diatoma tenue. Abnormal 

 diatoms are caused by a variety of factors, including heavy 

 metals (McFarland et al . 1997) . In Tenmile Creek near Helena, 

 acid mine drainage has produced abnormalities in up to 20% of the 

 cells at some sites (Bahls 1998) . 



Elevated salinity (Figure 2) was probably the cause of the 

 abnormal diatom valves observed in the Teton River at Highway 

 221. However, four deformed valves out of 800 (0.50%) is within 

 an acceptable range for full support of aquatic life uses (Table 

 3) . The pollution index and percent abnormal cells indicated 

 minor impairment but good water quality and biological integrity 

 at Highway 221. 



Lower Teton River 



When compared to criteria for plains streams (Table 8) , 

 diatom diversity at Highway 221 was judged to be somewhat 

 depresssed and indicating minor impairment. All other diatom 

 metrics at Highway 221 were within the fully supporting range for 



15 



