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Lake Ichinomegata, located in the Oga peninsula, Akita Prefecture, is a maar. Since the lake water has been used as water-supply for domestic and irrigation, river water has been conveyed into the lake from autumn to spring in order to replenish the lake water. In this study, processes of thermal stratification, circulation of the lake water, and mixing between the lake water and the conveyed river water were discussed based on seasonal variations of vertical temperature profile observed in the center at the lake. Lake water was thermally stratified from spring to autumn, while water temperature in whole depth was almost homogeneous in lower than 4°C during late winter, suggesting that overturn of lake water occurred. Higher wind velocity during winter season was considered as one of factors for overturn. Because the period of ice cover was estimated to not last for long time during winter season, it was suggested that vertical circulation of the lake water was easily caused by strong wind. Therefore, the lake was classified into monomictic lake. River water conveyed into the lake during the period that the lake water was thermally stratified was considered to be stored in the depth shallower than hypolimnion, because the temperature of the river water was higher than that of the hypolimnion. While river water was suggested to be mixed into whole lake water because of overturn in winter season.