Summary 



In August and September 2002, pcriphyton samples were collected from 6 sites on the 

 Dearborn River and its Middle and South Forks 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 bioassessment protocols for 

 wadeable streams. 



Diatom metrics indicate good to excellent biological integrity and full support of aquatic 

 Ufe uses at all sites in the Dearborn River TMDL planning area. The sites with the lowest 

 biological integrity were on the South Fork of the Dearborn River, where nutrient loading and 

 sedimentation caused minor impairment. Dissolved oxygen was also lower at these sites. The 

 upstream site on the South Fork (Blacktail Ranch) was dominated by Oscillatoria (a cyanophyte) 

 and had more organic loading and lower dissolved oxygen than the downstream site (above FAS 

 434), while the downstream site had a higher sedimentation index than the upstream site. 



Moderate stress of natural origin was noted at the upstream site on the Middle Fork 

 Dearborn River near Rogers Pass. This natural stress was probably caused by steep gradients, 

 high current velocities, cold water temperatures, and low nutrient concentrations. The pollution 

 index indicated borderline impairment from organic loading at the lower site on the Middle Fork. 

 A few abnormal diatom cells were also recorded at this site. 



In the main stem of the Dearborn River, the only stress noted was probably natural in 

 origin. Although the percentage of pollution sensitive diatoms and the pollution index declined 

 significantly from the upstream site to the downstream site, these metrics were still at acceptable 

 levels for a mountain stream. The sedimentation index was very low at both sites. Oscillatoria, 

 an indicator of nutrient loading, dominated the periphyton community at the lower site. Only 

 one abnormal diatom cell was observed in the main stem (at the downstream site). 



