1. Dam removal has great potential for restoring rivers and streams, yet limited data exist documenting recovery of associated biota within these systems following removals, especially on larger systems. This study examined the effects of a dam breach on benthic macroinvertebrate and fish assemblages in the Fox River, Illinois, U.S.A.
2. Benthic macroinvertebrates and fish were collected above and below the breached dam and three nearby intact dams for 1 year pre- and 3 years post-breach (2 years of additional pre-breach fish data were obtained from previous surveys). We also examined the effects of the breach on associated habitat by measuring average width, depth, flow rate and bed particle size at each site.
3. Physical habitat at the former impoundment (IMP) became comparable to free-flowing sites (FF) within 1 year of the breach (width and depth decreased, flow rate and bed particle size increased). We also found a strong temporal effect on depth and flow rate at all surveyed sites.
4. Following the breach, relative abundance of Ephemeroptera, Plecoptera and Trichoptera (largely due to hydropsychid caddisflies) increased, whereas relative abundance of Ostracoda decreased, in the former IMP to levels comparable to FF sites. High variation in other metrics (e.g. total taxa, diversity) precluded determination of an effect of the breach on these aspects of the assemblage. However, non-metric multidimensional scaling (NMDS) ordinations indicated that overall macroinvertebrate assemblage structure at the former IMP shifted to a characteristically FF assemblage 2 years following the breach.
5. Total fish taxa and a regional fish index of biotic integrity became more similar in the former IMP to FF sites following the breach. However, other fish metrics (e.g. biomass, diversity, density) did not show a strong response to the breach of the dam. Ordinations of abundance data suggested the fish assemblage only slightly shifted to FF characteristics 3 years after the breach.
6. Effects of the breach to the site immediately below the former dam included minor alterations in habitat (decreased flow rate and increased particle size) and short-term changes in several macroinvertebrate metrics (e.g. decreased assemblage diversity and EPT richness for first post-year), but longer-term alterations in several fish metrics (e.g. decreased assemblage richness for all three post-years; decreased density for first two post-years). However, NMDS ordinations suggested no change to overall assemblage structure for both macroinvertebrates and fish following the breach at this downstream site.
7. Collectively, our results support the effectiveness of dam removal as a restoration practice for impaired streams and rivers. However, differences in response times of macroinvertebrates and fish coupled with the temporal effect on several habitat variables highlight the need for longer-term studies.