Dam Renovation and River Environment Improvement - April 2007

Dam Renovation and River Environment Improvement - April 2007

2000s
In Hita City, Oita Prefecture, the diversion of dam water to a power plant since 1973 caused a significant decrease in water flow in the Oyama River, severely deteriorating the river environment. In response, a citizens' group named "Mikuma River Water Volume Increase Promotion Committee" initiated activities. In 2001, the dam facilities were renovated through public-private collaboration, successfully tripling the water volume. This led to significant improvements in the river environment and contributed to preserving the living environment of local residents. Additionally, in 2007, the group was renamed "Suigo Hita Revitalization Committee," and efforts to improve water quality were strengthened. This period also saw accelerated academic research and policy measures to pass on the regional environment to future generations.

2010s
Entering the 2010s, further environmental improvements were targeted, including water quality preservation and consideration for local ecosystems. In 2014, local companies collaborated on a river cleaning and vegetation restoration project. This involved local schools and residents, incorporating environmental education into comprehensive activities. In 2017, a new water quality monitoring technology was introduced in the Oyama River basin, enabling detailed data analysis and the development of more effective measures.

2020s
In the 2020s, flash releases became a prominent measure. In 2021, the "Mikuma River and Oyama River Flash Release Experiment" demonstrated that removing sediment from the riverbed before releasing juvenile sweetfish improved their feeding environment (detailed in the "Mikuma River and Oyama River Flash Release Experiment Report," published March 31, 2021). Additionally, in July 2023, flood control measures from previous river improvement projects significantly reduced damage during heavy rains. These initiatives, driven by local residents, government agencies, and related companies, have greatly contributed to the conservation of river environments and regional disaster prevention.

Summary
The Oyama River environmental improvements have achieved phased results through ongoing public-private efforts since the 2000s. Introducing scientifically grounded measures and participatory approaches has protected the region's natural environment and promoted sustainable development. These activities are recognized as a national model case for river environmental improvement.