Improvement of River Environment through Dam Reconstruction - April 2007

Improvement of River Environment through Dam Reconstruction - April 2007

2000s
In Hita City, Oita Prefecture, the water volume of the Oyama River was drastically reduced due to the transmission of dam water to a power plant that began in 1973, and the river environment deteriorated significantly. In response to this problem, a citizens' group, the Mikuma River Water Volume Increase Promotion Committee, began activities, and in 2001, the dam facilities were reconstructed through a public-private partnership, successfully tripling the water volume. As a result, the river environment was greatly improved, contributing to the preservation of the living environment of local residents. In 2007, the organization changed its name to "Suigo Hita Restoration Committee" and further strengthened its water quality improvement activities. This period saw the acceleration of academic research and policy initiatives to pass on the local environment to the future.

The 2010s
In the 2010s, the aim was to further improve the environment, and in addition to water quality protection, consideration for the local ecosystem was strengthened. 2014 saw the launch of a river cleanup and reforestation project with the cooperation of local businesses. Local schools and residents were encouraged to participate in this comprehensive activity, which included environmental education. Furthermore, in 2017, new water quality measurement technology was introduced in the Oyama River watershed, and a system for detailed analysis of water quality data for the entire watershed was established. This enabled scientific evaluation of the progress of improvements and more effective measures to be developed.

The 2020s
In the 2020s, flash release was introduced as a high-profile measure: the Mikuma and Oyama Rivers flash release experiment conducted in 2021 confirmed the effectiveness of improving the feeding environment for ayu by removing riverbed sediments before releasing juvenile ayu (see the March 31, 2021 issue of "Mikuma River and Report of the Mikuma River and Oyama River Flash Release Experimental Study" issued on March 31, 2021). In addition, it has been reported that the river improvement work that has been carried out so far contributed to the reduction of inundation damage during the torrential rains in July 2023. These activities are being carried out in collaboration with local residents, government agencies, and related companies, and are making a significant contribution to the preservation of the river environment and local disaster prevention.

Concluding Remarks
The improvement of the river environment of the Oyama River has been achieved step by step through public-private partnership efforts that have been ongoing since the 2000s. Through the introduction of science-based measures and the participation of local residents, the local natural environment has been protected and sustainable development for the future has been promoted. These activities are also attracting attention as a model case for improving river environments nationwide.