Long-term and sequential restoration trials are often required for the conservation of stream ecosystems at downstreams of dams. In the Koshibu River, Japan, sediment additions through a sediment bypass tunnel of Koshibu Dam have been conducted seven times during 2016-2018 to restore the instream substrate environments and inhabiting organisms. To assess the effects of sediment addition on riverbed environments and aquatic insects, we measured size of substrate particles and collected insects at impacted (downstream of Koshibu Dam) and reference reaches (upstream of Koshibu Dam and Toyama River) during Jun 2016-Nov 2018.Before the sediment additions, the impacted reaches lacked gravels (2-16 mm) and pebbles (16-64 mm), and the biotic assemblages in the impacted reaches were significantly different to those in the reference reaches. After the first sediment addition (Oct 2016), there were no difference of gravels and pebbles between the impacted and reference reaches, whereas assemblages remained different between those reaches. However, from third-fourth sediment additions (Oct 2017), assemblages in the impacted reaches were gradually close to those in the reference reaches. These results indicate that shifts in community assembly follow abiotic habitat changes and require long-term and sequential sediment additions. |