The Connecticut Light & Power Company pioneered the use of pumped storage in the United States at this hydroelectric station. First operated in 1929, the Rocky River Plant had two reversible pumps that somewhat resemble large hydroelectric turbines. This permitted significant improvements in the system efficiency of the company's network of hydroelectric and thermal-electric power generating plants.

With Candlewood Lake and the Housatonic River differing in elevation by 200 feet, the Rocky River plant uses a single penstock, 1,000 feet long, to carry water downhill and lift it uphill. The Rocky River plant's pair of 8,100-horsepower pumps together are capable of lifting a million gallons every four minutes. At the time of their installation, they were largest of their kind in the world.

The Rocky River plant pumps water when the Housatonic River runs high. When the river runs low, water released from the lake to create electricity also raises the river level, helping generate additional power at two downriver plants. For every kilowatt-hour used to pump water, the three plants together can generate 1.3 kilowatt-hours.

Open-loop Pumped Hydro Storage