The Folsom Powerhouse sits on the south bank of the American River just above historic Folsom, in a stone building completed in 1895. It was the site of the first long-distance alternating-current power transmission in the United States: electricity generated here travelled 35 kilometres to Sacramento along wooden poles, lighting the streets and powering the city's new electric railway.
The building is now a California State Historic Park and is open for guided tours on weekends. Inside, two of the original generators have been restored and can be seen in the turbine hall — massive machines by the standards of the day, producing 800 kilowatts each. The surrounding grounds include a short trail down to the river and good views of the granite-walled gorge below the powerhouse intake. Check the State Parks website for current tour times; the park is sometimes closed mid-week.