ダム撤去後に下流に形成した砂州の生物生息場機能

抄録

Changes in bed topography and invertebrate community after a sediment supply event were investigated in a downstream reach of Arase Dam, which was in the last step of dam removal, in the Kuma River, Kyushu. A gravel bar was newly formed in the reach after the removal of the major walls and the following flood in June 2015. Cross-section profile of the reach suggested that shallow submerged (<1m water depth) and emerged (<1m up from water surface) areas increased substantially. According to a steady flow calculation of the reach, average flow velocity at baseflow increased 1.8 times more than the previous for a given discharge. Invertebrate species that prefer faster currents increased in the community of the main channel. We also estimated that subsurface and upwelling (spring) flow through the bar was more than 3% of the total river discharge. The spring with a clear and stable flow fed a lentic habitat (wando), which supported higher species richness than before.