Invertebrate community changes in the downstream of dam after the operation of sediment bypass tunnel

Abstract

To understand the effect of sediment bypass tunnel (SBT) on the recovery of invertebrate community in degraded channels below dam and the time required for the recovery, 17 years monitoring data since the start of SBT operation in Asahi Dam were analyzed. Invertebrate community recovered toward an upstream state within 2–3 years after the SBT operation, in terms of taxon richness and Bray-Curtis similarity index (BC). Taxon richness in the downstream was lower in the last few years, partly because it was also affected by flood occurrence of the years, while BC was consistently high in the later years. Data of national survey done every 5 years were used to analyze invertebrate community patterns in Miwa and Koshibu dams with SBT. Although the downstream channel of Koshibu was degraded before the SBT construction, taxon richness was higher in the downstream than upstream, while BC between upstream and downstream was consistently low. BC is suggested to be better as indicator of environmental recovery of downstream than taxon richness. Although SBT had been operated for more than 7 years in Miwa, no evidence of the recovery in invertebrate community was detected.