2025 National Water Quality of Class A Rivers Released

Every July, during River Protection Month, the Ministry of Land, Infrastructure, Transport and Tourism releases water quality survey results for Class A rivers nationwide to deepen understanding of local water environments. The materials issued on July 24, 2026 cover the 2025 survey results.

The survey covered 160 rivers nationwide, of which 13 were designated as having the best water quality in 2025.

The published results provide a foundation for efforts to maintain and improve water quality across entire river basins and will also help further enhance the appeal of local communities by leveraging good water quality.

Publishing the survey results is intended to deepen understanding of water environments in each region.

A river is defined as having the best water quality when both the average of the annual BOD values calculated for each survey site across all sites in the river and the average of the BOD 75th-percentile values for each survey site across all sites are 0.5 mg/L. The 0.5 mg/L figure is the reporting lower limit for BOD established by the Ministry of the Environment.

Among the qualifying rivers, the Arakawa River in Fukushima Prefecture reached 16 consecutive years since 2010, the Kawabe River in Kumamoto Prefecture reached 20 consecutive years since 2006, and the Gokase River in Miyazaki Prefecture reached 13 consecutive years since 2013.

Water quality improvement over the past 10 years was evaluated by comparing annual average BOD values at each site in 2025 and 2016, based on the extent of improvement in sections under direct government management. The Tokura site on the Inagawa River in the Yodo River system ranked first.

The materials feature initiatives on the Gokase, Arakawa, Tenryu, and Karasu rivers that leverage clean water and water quality improvements. A local edition is also available in addition to the overview pamphlet and detailed version.