Obonai hydroelectric power station was built in 1939.
Power generation began in January 1940. It used water from Lake Tazawa for power generation. In order to use lake Tazawa as a dam, highly acidic water from the Tamagawa River was drawn into Lake Tazawa, then it made the lake uninhabitable for fish.
In recent years, the water has been neutralized and fish can now be seen in the lake.
