Long-term implications of dam removal for mesohabitat and macroinvertebrate communities in Michigan and Wisconsin rivers

The removal of the numerous aging dams in the United States has recently been a highly sought after stream restoration technique. The extent to which the damage done to streams by dams is reversed upon removal is unknown, especially on decadal time scales. The objectives of this study were to determine if mesohabitat heterogeneity and macroinvertebrate assemblages within rivers recover following the removal of a dam, and to estimate the time needed for recovery. A space-for-time substitution study was employed on 8 rivers in various stages of recovery following a dam removal, ranging from <1--40 years post-removal. Within each river, mesohabitat was mapped and macroinvertebrates sampled in a zone unaffected by the dam removal and two zones impacted by the dam removal (former impoundment and downstream zone), and compared to evaluate the extent of recovery. Mesohabitat recovery was variable, with some rivers recovering substantial heterogeneity within 7 years of removal while others exhibited much lower levels than the reference zone even after 40 years following dam removal. Generally, the macroinvertebrate community recovered 3--7 years following removal both in terms of taxonomic similarity and richness, although densities took decades to recover. Dam removal poses as a beneficial restoration technique yet the recovery of important stream components can be variable and may take longer than previous research has suggested.