BIOASSESSMENT 



PROTOCOL I 



Most diatom association metrics for Big Dry Creek indicated 

 good to excellent water quality and biological integrity when 

 compared to least- impaired reference streams elsewhere in eastern 

 Montana {Table 5) . 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 source of the organic loading that resulted in this low 

 pollution index is unknown. It may be natural in origin, that 

 is, internal organic loading resulting from the decay of aquatic 

 plants. Or, it may originate from wastewater discharges upstream 

 in the Jordan area. Or, most likely, it is a combination of 

 these two sources. 



A low species diversity index and a large siltation index 

 indicated minor impairment yet good biological integrity and full 

 support of aquatic life uses in Big Dry Creek. However, the 

 siltation index was within one percentage point of the threshold 

 for moderate impairment for a prairie stream. Such a rating 

 would correspond to fair biological integrity and only partial 

 support of aquatic life uses. 



All diatom metrics indicated full support of aquatic life 

 uses in Little Dry Creek when compared to other prairie streams 

 (Table 5) . 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 

 for use in Protocol II. 



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