diversity and species richness values probably indicate natural 

 stress and represent baseline conditions for the East Boulder 

 River. As expected, the percent abundance of A. minutissiwa 

 declined in a downstream direction. The number of diatom species 

 nearly doubled from above to below Dry Fork Creek (Table 5) , 

 probably in response to an increase in nutrients. 



The pollution index indicated excellent water quality at all 

 sites except at Station 3 in 1999, where the pollution index 

 dipped into the range of minor impairment. The pollution index 

 approached, but did not dip below, the threshold for minor 

 impairment at Station 5. The number of diatom species increased 

 by nearly 50% between Station 4 and Station 5, perhaps indicating 

 an increase in nutrient concentrations below Elk Creek. Algae 

 samples collected by University of Montana researchers in 1998 

 indicated minor organic loading below Elk Creek (Bahls 1999) . 



The siltation index indicated little or no siltation at all 

 sites except at Station 5, where the siltation index exceeded the 

 threshold for minor impairment (Table 5) . Periphyton samples 

 collected by University of Montana researchers in 1998 indicated 

 moderate siltation below Elk Creek and near the mouth of the East 

 Boulder River (Bahls 1999) . 



Similarity indexes between adjacent sites indicated that the 

 upper three sites had very similar floras in 1998 (Table 5) . 

 There was less similarity between diatom assemblages at the three 

 downstream sites in 1999. Diatom assemblages collected on the 

 same day from adjacent riffles of the same stream usually share 

 between 60% and 80% of their diatom floras (Bahls 1993) . Lower 

 percentages may be due to intervening tributaries or to 

 discharges of pollution. Diatom assemblages collected at Station 

 3 in 1998 and 1999 had about half of their floras in common. 



10 



