Flood retention dams in Japan and their impact on benthic invertebrate communities by affecting riverbed conditions

Flood Retention Dams (FRDs), designed solely for flood control with bottom outlets, generally have minimal influence on natural sediment regimes and benthic invertebrate communities. However, their actual ecological impacts remain unclear. Through a nationwide field survey, this study aims to quantify the effects of FRD on benthic invertebrate communities in downstream and reservoir areas. An inventory identified 24 existing FRDs in Japan; seven were selected for field investigation. Benthic invertebrates and riverbed conditions were assessed at upstream, reservoir, and downstream sites. The mean Bray-Curtis similarity between upstream and downstream communities was 0.73, comparable to that of unregulated rivers and unrelated to dam operation duration, indicating a minimal long-term downstream impact. Conversely, the similarity between upstream and reservoir sites was lower at 0.66. Reservoirs had finer, softer sediments with narrower interstitial spaces than upstream sites. As a result, taxa inhabiting or moving across stone surfaces were more common in reservoirs, while large-bodied, stable-bed-preferring taxa were less abundant. Moreover, variation in reservoir sedimentation volume—reflecting disturbance regimes—was associated with the Bray-Curtis similarity between upstream and downstream sites. Overall, FRDs exert limited influence on downstream communities but create distinct reservoir habitats shaped by finer sediment. Annual fluctuations in reservoir sedimentation can indicate ecological impacts related to FRD.