Lake Ogawara is becoming eutrophicated very rapidly due to the contaminant upwelling from the anaerobic saline water layer. The authors have already developed a practical simulation model to predict the saline water intruding in winter. In this paper, the previous model is improved by adding the effect of thermal stratification and the current in saline layer. Then, year-round calculation was conducted to investigate the expansion of the anaerobic saline water layer in the recent ten years. The model reasonably reproduced the field measurement results on seasonal change and long-term expansion of saline water layer. The simulation result confirms that the expansion of saline water layer and the saline water entrainment are the primary reasons for the eutrophication in surface layer. It also suggests these events occur mainly in winter, when seawater intrudes frequently and thermal stratification is absent.