Skip to main content
Completion Date
Project Type
Power Generation
New Milford, CT
41 34 49.7 N
73 26 13.2 W


The Rocky River Pumped Storage Hydroelectric Plant was the first major pumped storage hydroelectric project in the United States.

"The statement that a hydro-electric plant can pump its own water supply sounds absurd on the face of it, yet this is virtually what happens in the case of the Rocky River Hydro Plant..."
- Chief Engineer Paul Heslop, Connecticut Society of Civil Engineering meeting, 1928

Rocky River Pumped Storage Hydroelectric Plant
Most residents of western Connecticut know 11-mile long Candlewood Lake as a recreational resource for boating, fishing, and swimming. Not many are aware that the lake is part of the first major project in the United States designed to generate electricity from water pumped into a reservoir. The project's design is made possible by the economics of the electricity market, where power provided during periods of greatest demand generates the highest rates.


To create Candlewood Lake, builders dammed a pair of tributaries to the Housatonic River, forming the basis for the reservoir. The eight-square-mile storage area is then filled by pumping water from the Housatonic at periods when electricity to run the pumps is least expensive. Power generated by releasing the same water can then be sold at higher rates during periods of peak demand. The technology pioneered at the Rocky River project-reversible pumps that also act as generators-was not widely used in other U.S. projects until the 1950s and 1960s.


  • The first pumped-storage facility in the world was built in 1909 near Schaffhausen, Switzerland. Unlike the Rocky River plant, it used a pump to store water and a separate turbine to generate electricity. By the time of the Rocky River project, more than 40 pumped-storage hydroelectric facilities had been built throughout Europe.
  • 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.