You are already at the latest version In order to clarify effects of the volcanic activity on hydrological, thermal and chemical features of a crater lake, Lake Katanuma, Japan, temperature and electric conductivity at 25 °C (EC25) of lake water were monitored at two maximum-depth points in 2023-2025. Under closed condition of the lake without river outflow, the lake is characterized by the high acidity of pH = 2 on average and the existence of the surrounding geothermal area.