SUMMARY 



In late July and early August 2000, periphyton samples were 

 collected at four stations on Cottonwood and Rock Creeks near 

 Clyde Park, Montana for the purpose of assessing whether the 

 creeks are 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 non-diatom algae indicated cold, fast-flowing waters and 

 low concentrations of dissolved nutrients. Algal standing crops 

 were evidently very small, judging by the sparse contents of the 

 samples . No algae were found in the sample collected at 

 Cottonwood Creek Station 1. 



Diatom association metrics indicated no stress or only minor 

 stress and full support of aquatic life uses at the remaining 

 three sites. Diatom indicator species confirmed these waters to 

 be cold and nutrient poor; at least some of the observed stress 

 was natural in origin. Phosphorus was likely the nutrient most 

 limiting to algal growth. 



The two Rock Creek stations had about half of their diatom 

 assemblages in common, which is to be expected given the 

 intervening tributaries between these sites. The diatom 

 assemblage at Rock Creek Station 2 was slightly more similar to 

 the assemblage at Cottonwood Creek Station 2 than it was to the 

 assemblage at Rock Creek Station 1. 



Using Rock Creek Station 1 as a local reference site under 

 Protocol II, Rock Creek Station 2 proved to have good to 

 excellent biological integrity, little or no stress, and no 

 impairment of aquatic life uses. 



