排砂バイパスを導入したダム下流における河床環境と底生動物群集

Abstract

In order to understand effects of sediment bypass on ecosystem downstream of dams, we surveyed riverbed environment and invertebrate community both at upstream and downstream of 4 dams with sediment bypass tunnel in Japan and Switzerland. Bed materials were coarse, in terms of a slope-size relationship and dimensionless shear stress at downstream of dams with new bypass tunnel, while they were as fine as upstream of dams at downstream of dams with older bypass tunnel. Sites of below dams and below bypass outlet of new tunnel dams were characterized by the occurrence of organic microhabitats (e.g., moss-mat, mud, filamentous algae), fewer glider mayflies against net-spinning caddisflies, and fewer riffle specialist species against pool specialist species. Our result suggests that sediment supply to downstream by bypass tunnel would promote filling or covering of coarse bed materials, increase the frequency of bed movement while decrease stagnant patches on bottom and periphery, and consequently encourage a formation of invertebrate community that is similar to upstream reaches after certain years of bypass operation.