In the Higashiyokobori and Dotonbori rivers, water quality is controlled by inflowing outside non-polluted river water from upstream gate and outflowing inside water from downstream one. However, there is hypoxic water mass during high-temperature period. In the present study, field surveys, laboratory experiments, and numerical simulations were conducted to evaluate the water-quality improvement effect of exchanging water mass with outer rivers through the gates and to propose more effective methods. The results showed that main factor of hypoxia was DO consumption process in the waters and that more than 2.0 m3/s discharge through the gates is needed to maintain a good DO environment in summer.
