EFFECTS OF SEDIMENT REPLENISHMENT ON THE RIVERINE ENVIRONMENT IN THE DOWNSTREAM REACHES OF THE HAMAHARA DAM, GONOKAWA RIVER, WESTERN JAPAN

We aimed to evaluate the effects of sediment replenishment on the riverine environment in the downstream reaches of the Hamahara Dam, Gonokawa River. We estimated the amount of the sediment transported from the replenished sediment by photogrammetry and morphodynamic simulation. We also examined differences in the riverbed environment and fish and benthic organism abundances between fall and winter after the summer floods in 2020. The 30% (2,625 m3) of the sediment replenished in the previous year was estimated to be transported to the site 3 km downstream from the replenished sediment. In the fall, at the site 1 km downstream of the replenished sediment, the values of riverbed penetration and environmental DNA (eDNA) concentrations for sand-dwelling fishes were higher than those of the upstream site. These results suggest that the riverbed environment at the site below the replenished sediment could have been improved by the sediment replenishment. In contrast, the values of riverbed penetration and the eDNA concentrations for sand-dwelling fishes had decreased and the biomass of net spinning caddisfly larvae had increased at the site below the replenished sediment by winter. These results indicated that the riverbed improvement was sustained only for ca. three months after floods.