River reaches are often subject to multiple co-occurring anthropogenic stressors. Disentangling the relative impacts of different stressors is important in developing river management strategies. Bio-assessments using taxonomically based metrics (e.g. taxa richness and diversity) are frequently used as a means of determining the impact of stressors on river reach quality. Although this method has been successful in determining river reach quality, taxonomically based metrics are often limited in their ability to distinguish mechanisms of impact and thus the importance of different stressors. In contrast, biological traits can provide a mechanistic understanding of anthropogenic stressor impacts and therefore have the potential to identify the importance of different co-occurring stressors. Using both taxonomically based metrics and biological traits, we examined the impact of two common co-occurring stressors, summer water withdrawal and channelization, on the macroinvertebrate assemblage of a Columbia River tributary Both metrics and traits showed a strong shift at the point where intense channelization began. However, the metrics failed to distinguish whether channelization or co-occurring water withdrawal was the driver of change. In contrast, biological traits indicated that the assemblage within the heavily channelized area shifted to one dominated by resilience traits often found in river reaches subject to scouring flows. This result indicates that scouring flows associated with intense channelization are the primary driver of change in the macroinvertebrate assemblage and that water withdrawals had little observable impact. Our results demonstrate the utility of biological traits for disentangling the impact of multiple stressors in river systems. Copyright © 2011 John Wiley & Sons, Ltd.