This study dealt with the characteristics of hydrodynamics of Kojima Lake and the influence of a regional sewage treatment system in construction on the lake. Clockwise and anticlockwise circulations are caused by seasonal winds in summer and winter, respectively. The distribution of a conservative material continuously discharged off the shore of the sewage treatment plant is scarcely affected by seasonal winds and river discharges. The sewage treatment system improves the water quality of the lake except T-N.