2006 Volume 12 Pages 443-448
The maintenance flow discharge of the Ohyama River, which is located in the upper Chikugo River, had been restricted to l.5m3/s by a hydroelectric dam. To improve the river environment, the river administrator has changed the maintenance flow discharge to 4.5m3/s in summer and to l.8m3/s in winter. In this study, we aim at proposing an optimal control of the maintenance flow discharge for the river environment. Firstly, we carried out field surveys before and after reducing the river discharge in order to figure out effects on hydraulic conditions and attached algae. The results of the surveys show that hydraulic conditions become worse for ayu (plecoglossus altivelis) and attached algae. Also, there is a tendency to stagnate at the pool when the maintenance flow discharge decreases. Secondly, because it was reported that the length of ayu became larger than before changing the maintenance flow discharge, we attempted to analyze carbon and nitrogen stable isotope ratio of ayu and attached algae. We confirmed an obvious difference in food web structure between natural and cultured ayu.
