SUMMARY 



On August 9, 2000, periphyton samples were collected from 

 two stations on the middle Judith River near Lewistown, Montana 

 for the purpose of assessing whether this segment of the river is 

 water-quality limited and in need of TMDLs . The samples were 

 collected following DEQ standard operating procedures, processed 

 and analyzed using standard methods for periphyton, and evaluated 

 following modified USEPA rapid bioassessment protocols for 

 wadeable streams . 



The Judith River heads in the Rocky Mountain Ecoregion, 

 flows through the Valley and Foothill Prairie Ecoregion, and ends 

 in the Northern Great Plains Ecoregion, where the study reach is 

 located. For this reason, Judith River metrics were compared to 

 criteria for both mountain streams and prairie streams . 



When compared to biocriteria for both mountain streams and 

 prairie streams, the large percentage of Achnanthes minutissima 

 and the large number of abnormal diatom frustules indicated 

 moderate impairment and partial support of aquatic life uses at 

 both sampling sites. The stress that caused this impairment may 

 be either natural or cultural in origin. 



The periphyton communities at both sites were represented by 

 the usual groups of algae with healthy genus richness. Although 

 a shift in abundance from cyanobacteria at J 1 to green algae at 

 J 3 may indicate some nutrient enrichment, values for the diatom 

 pollution index, which were nearly identical, indicated no 

 significant increase in organic loading between the two sites. 



The relatively large similarity index indicated that sites 

 J 1 and J 3 had very similar diatom floras and that little or no 

 environmental change had occured between the two sites. 



