Improving stream bioassessment methods for the Montana valleys and foothill prairies ecoregion

The Montana Department of Environmental Quality (MT DEQ) uses a modificatio n of US EPA's Rapid Bioassessment Protocols for stream assessment and monit oring. These protocols evaluate a battery of aquatic macroinvertebrate comm unity metrics and physical habitat assessments as indicators of stream cond ition. To evaluate the metric battery DEQ currently uses, 93 stream sites w ere sampled in the Montana Valleys and Foothill Prairies ecoregion and divi ded into low, medium and high impact categories based on visual assessment and watershed information. Grazing was the main source of impacts. Of the m etrics tested, only eight were able to distinguish low from high impact sit es. Seven of these impact sensitive metrics were highly correlated with cer tain habitat variables. Anthropogenic impacts explained over half the varia bility in 3 of these metrics (elevation was the natural factor that explain ed the most variability). Based on these results, a battery of seven metric s was developed that proved more sensitive to impact, better related to hab itat variables and more consistent in assigning impairment categories than the battery used by MT DEQ.