Management of River Environment Preservation at Hitokura Dam : Collaboration with Regional Residents

Hitokura Dam is the multi-purpose dam whose functions are flood control, domestic water supply and maintenance of river environment etc., is constructed in the Ina River in the Yodo River System. Hitokura Dam’s location is very close to the urban area and downstream of Hitokura Dam, there are Kawanishi City, Ikeda City, Amagasaki City and other cities. Due to the location, around 300,000 of people visit Hitokura Dam area every year to enjoy the rich environment around there. Under the circumstances, we have been implementing various measures to keep and improve the environment in the reservoir and downstream of the dam. We have been returning sedimentation in the reservoir to downstream of the dam by conducting flashing discharge (artificial small-scale floods) since April 2003 in order to improve the habitat in the downstream of the river. We also have been monitoring creatures in the river periodically to check the effectiveness of the flashing discharge. We have the meetings to exchange opinions with people in the neighborhood, environment NPOs, fishery cooperatives and residents’ associations about how to keep and improve the river environment on regular basis. We also release fry of Ayu fish into the Ina River together with local residents experimentally. During the spawning season (from early June to mid July), there were not enough river water for fish downstream of the dam. To solve this problem, we operate the water level higher than the flood season control level on trial basis to increase the discharge to downstream and improve the river environment during this season. 4. The results of the survey about fish showed that almost of the half of fish in the reservoir is alien fish, bluegill and they endanger indigenous fish. To get rid of bluegill, we catch fish during the water level declining operation to the flood season control level from the normal water level. We use the fixed nets to catch fish. This way was newly developed for this purpose and it gives less damage to fish so that we can release indigenous fish into the river alive. We produce fertilizer from those alien fish and use it to grow vegetables in the garden at our office, then we use them for our lunch. We are also experimenting and researching the effective use of the fertilizer with local farmers. By operating the water level in the reservoir higher than the flood season control level, we can expand and create spawning grounds in shallow areas of the reservoir. On top of this, we put some artificial floating islands in the reservoir to secure spawning grounds that can catch up with the water level fluctuation