耳川の通砂事業により西郷ダム下流に形成した砂州の湧水特性

抄録

Gravel bars were newly formed in the downstream of the Saigo Dam after sediment sluicing in 2017 in the Mimi River, Miyazaki. To evaluate effects of bar formation on river ecosystem, we measured discharge, water quality, water temperature, and fauna in the main-channel (surface flow) and secondary-channel (emerged subsurface/hyporheic flow), which flowed different sides of the bars. According to discharge and daily cycle of temperature, 33-50% of surface flow was submerged into the bars and re-emerged on surface with a hyporheic residence time of 2.5-6 hours. Spatial variation in the discharge of the emerged flow was roughly explained by hydraulic gradient estimated from surface water level of the bars. Turbidity decreased steadily in the downstream direction, which is likely due to a filtering process of gravel bars. Some benthic invertebrates and fish juveniles occurred in high density in the secondary-channel.