RESULTS AND DISCUSSION 



Macroinvertebrate taxa lists, metric results and other information for each sample are 

 given in the Appendix. 



Flgnre 1. Bioatiewmeiit icorei. PUini ecoregloni itreamt, July 19 and 20, 1995 



100 



Plains Ecoregions reference 



Impairment classifications ranged fi^om slight impairment at both Corral Creek sites, the 

 Marias River site and the Sage Creek site, to moderate impairment at a Pondera Coulee site when 

 sites were compared to the Plains Ecoregions reference. The other Pondera Coulee site was rated 

 slight-to-moderately impaired. Figure 1 displays total bioassessment scores, based on the Plains 

 ecoregional reference, for each site. 



In this 

 assessment, the 

 highest 

 biointegrity 

 score was 

 assigned to the 

 Marias River 

 site. The biotic 

 index score 

 (3.28) indicated 

 a moderately 

 sensitive 

 community, 

 organic or 

 nutrient 



enrichment does 

 not seem to be 

 much of a 



problem at this site. A high proportion of the community, however, is comprised of taxa which 

 tolerate sediment, such as Tricorythodes sp., baetid mayflies and the caddisfly Brachycentrus sp. 

 Together, these three make up 69% of the sampled assemblage. 



Slight impairment was also indicated by the data from both Corral Creek sites in this 

 analysis, though biotic conditions differ markedly between the mouth and headwaters sites. At the 

 mouth, a high biotic index score (6.59), and high relative abundances of lymnaeid snails and 

 midges (10% and 73%, respectively, of the sampled community) strongly suggest organic and/or 

 nutrient enrichment here. The midge community is comprised of tolerant forms such as 

 Corynoneura sp. and Cricotopus spp. Only three EPT taxa occurred here, two mayflies and one 

 caddisfly. Indeed, the total bioassessment score is deceptively high; high diversity scores and lack 

 of dominance of a single taxon or a few taxa may make the biotic health seem better at this site 

 than it really is. Perhaps these indicate diverse habitat, but moderate water quality and/or thermal 

 impairment. 



Near the headwaters, however, the Corral Creek benthic community, though less diverse. 



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