The community structures at Sts. 2–4 are simple and poor, and strongly reflect the influence of the effluent from the treatment plant. The community structure of benthic macroinbertebrate at St. 7 is less influenced by the effluent compared with other stations. In addition, Chironomidae and Hydropsychidae are considered as important indicators of the recovery of benthic macroinvertbrate abundance in the River Akagawa. The dissolved metals in the river water inhibit the inhabitation of Chironomidae and Hydropsychidae, whereas the restoration of pH and attached algae and the increase in
the concentrations of nutrients and organic matter promote the inhabitation of these families. Moreover, the sedimentation of metals would cause a severe damage to the inhabitation of Hydropsychidae compared with that of Chironomidae.