SUMMARY 



From 1991 to 1993, seven composite periphyton samples were 

 collected from four sites on the upper Boulder River south of Big 

 Timber, Montana, following MDEQ standard operating procedures. 

 The samples were processed and analyzed using standard methods. 

 Diatom metrics generated from the samples were compared to 

 biocriteria developed for streams in the mountain ecoregions of 

 Montana and these comparisons were used to assess the support of 

 beneficial uses . 



All sites, including the control site, fully supported their 

 beneficial uses, but with minor impairment. Except for minor 

 sedimentation at the Flemming Bridge site, the underlying causes 

 of stress appear to b-^ natural in origin and related to the cold 

 waters and steep graaient of the upper Boulder River. 



Dominance by the diatom Gowphonewa olivaceoid.es at Two Mile 

 Bridge and Natural Bridge appears to result from the upwelling of 

 cold, nutrient-rich groundwaters in this reach. Outcrops of 

 limestone in this area may also contribute to the abundance of G. 

 olivaceoides and to the upwelling of groundwater. 



Conditions that favor the abundant growth of G. olivaceoides 

 may also favor the excessive growths of filamentous algae that 

 have been observed in this reach. Although diatom species 

 composition and community structure show that the Boulder River 

 above Natural Bridge fully supports its aquatic life uses, large 

 standing crops of filamentous algae in this reach may impair 

 certain beneficial uses. The status of the periphyton community 

 in the lower reach of the river, from Natural Bridge to Big 

 Timber, is unknown at this time. 



