The effect of water conduction by a pump system on water quality of urban rivers was investigated. The object river system of this study is the Shinmachi river system which flows through the center of Tokushima city. By using field observations and a numerical analysis of Holly-Preissmann method, it was found that the concentration of BOD in the Suketoh river decreases from 14.7 ppm to 7.7 ppm when the pump system was operated only in the ebb tide.