Summary 



In August and September 2002, periphyton samples were collected from 19 sites on 10 

 streams in the Cut Bank — Two Medicine TMDL planning area in north central Montana for the 

 purpose of assessing whether these streams are water-quality limited and in need of TMDLs. 

 The samples were collected following MDEQ standard operating procedures, processed and 

 analyzed using standard methods for periphyton, and evaluated following modified USEPA rapid 

 bioasscssment protocols for wadeable streams. 



Diatom metrics indicate minor impairment from organic loading at both sites on Old 

 Maids Coulee. In addition, an elevated number of motile diatoms indicate minor impairment 

 from sedimentation at the downstream site. Diatom metrics indicate only minor impairment in 

 Cut Bank Creek due to low species richness and diversity, and decreasing concentrations of 

 available nutrients between the upstream and downstream sites. 



Diatom metrics indicate minor impairment from nutrient loading at the upper site on the 

 South Fork of the Two Medicine River. This site also had an unusually large percentage of 

 abnormal diatom cells. The cause of these abnormal cells is unknown. Diatom metrics also 

 indicated minor impairment from nutrient loading at the lower site on the South Fork of the Two 

 Medicine River. An elevated percentage of motile diatom species here indicates moderate 

 impairment from sedimentation. The reason for this sedimentation is unknown, but it may be 

 related to major disturbances to riparian areas caused by the 1964 flood. 



An elevated percentage of motile diatoms indicate minor impairment from sedimentation 

 in Railroad Creek. Other diatom metrics indicate excellent biological integrity and no 

 impairment of aquatic life uses in Railroad Creek. The pollution index, however, was near the 

 threshold for minor impairment from organic loading. 



The only indicator of stress in the South Fork of Birch Creek and the North Fork of 

 Dupuyer Creek was an elevated percentage of Achnanthidium minutissimum. This minor stress 

 was probably natural and caused by the relatively steep gradients, cold water temperatures, and 

 low nutrient concentrations of these streams. A large percentage of A. minutissimum also 

 indicated minor natural stress in the South Fork of Dupuyer Creek. However, this site also 

 supported a large number of motile diatoms, which indicates minor stress from sedimentation. 



Diatom metrics indicate only minor impairment in the Two Medicine River and at main 

 stem sites on Birch Creek and Dupuyer Creek. Loading of organic nutrients was the cause of 

 minor impairment at the mouth of the Two Medicine River and at the lower 2 sites on Birch 

 Creek. A significant change in the environmental conditions of Birch Creek occurred between 

 sites 2 and 3, which shared only about 20% of their diatom floras. In Dupuyer Creek, there was 

 no indication at any site of excessive organic loading, excessive sedimentation, or excessive 

 salinity for a prairie stream. 



