SUMMARY 



Composite periphyton samples were collected from natural 

 substrates in lower Big Dry Creek and lower Little Dry Creek in 

 eastern Garfield County, Montana. Samples were collected 

 following standard operating procedures of the Montana Department 

 of Environmental Quality, processed and analyzed using standard 

 methods for periphyton, and evaluated following modified USEPA 

 rapid bioassessment protocols for wadeable streams. 



Most diatom association metrics indicated that Big Dry Creek 

 had good to excellent biological integrity when compared to 

 least-impaired reference streams elsewhere in eastern Montana. 

 However, a very low pollution index resulted in a rating of fair 

 biological integrity, moderate impairment, and only partial 

 support of aquatic life uses. The low pollution index was the 

 result of excessive organic loading, either from internal sources 

 or from wastewater discharges upstream, or both. The siltation 

 index also indicated problematic conditions in Big Dry Creek. 



Potentially toxic blue-green algae were present in Big Dry 

 Creek, but not in sufficient quantities to pose a problem for 

 livestock producers. 



All diatom metrics indicated full support of aquatic life 

 uses in Little Dry Creek when compared to other prairie streams . 

 Only a slightly depressed pollution index resulted in a rating of 

 good rather than excellent biological integrity. Little Dry 

 Creek proved to be a suitable local reference stream. 



When compared to metric values for Little Dry Creek, the 

 pollution and siltation indexes for Big Dry Creek indicated only 

 minor impairment, good biological integrity, and full support of 

 aquatic life uses. The diatom species diversity index was within 

 the range of no impairment and excellent biological integrity. 



i 



