Sediment replenishment restores macroinvertebrate community structure downstream of a dam: lessons from a 7‐year study

Sediment replenishment, which involves the artificial addition of bedload‐sized sediment around the impoundment to the river channel downstream of a dam, is widely used to mitigate the adverse impacts of sediment transport disruption on river ecosystems resulting from dam construction. Although the response of biological communities to restoration activities often varies temporally, long‐term evaluations of the effect of sediment replenishment on biological communities are scarce; therefore, we aimed to address this. We surveyed macroinvertebrate community composition and streambed sediment deposition at four sites between 2009 and 2015: impact site, downstream of a sediment replenishment point; control site, below the dam without sediment replenishment; and two reference sites, above the dam and tributary. Sediment replenishment in this stream was initiated in 2009. We identified that sediment replenishment increased sediment deposition on the channel bed and restored the sediment regimes to natural conditions. Furthermore, we observed that the dissimilarity in community composition between the impact site and each reference site was lower over time than the dissimilarity between the control site and each reference site. These results indicate that sediment replenishment can effectively restore the macroinvertebrate community structure downstream of a dam over time by increasing sediment deposition. Consequently, sediment replenishment should be considered one of the effective measures to restore macroinvertebrate community structures from a condition of disrupted sediment transport by dam construction.