Northfield Mountain (hydroelectricity facility)
Engineering studies for Northfield Mountain began in October 1964, with early site preparation starting three years later. In 1972 its 1,080 megawatts (1,450,000 hp) hydroelectric plant became operational as the largest such facility in the world.
The plant was built entirely underground, and located about 5.5 miles (8.9 km) up the Connecticut River from Turners Falls Dam. A stretch of the Connecticut River, extending some 20 miles (32 km) north from this dam to the Vernon Dam, Vermont, serves as the station's lower reservoir. During periods of lower electrical power demand, the plant pumps water from this lower reservoir through the Northfield Mountain Tailrace Tunnel to a man-made upper reservoir. At times of high demand, water is released to flow downhill from this upper reservoir through a turbine generator, where it then collects in the lower reservoir to be stored until again pumped to the upper reservoir.
Northfield Mountain's upper reservoir covers 300 acres (120 ha) at 800 feet (240 m) above the river, with total storage of 5.6 billion US gallons (21 Gl) of water. Its underground powerhouse lies at 700 feet (210 m) below the surface and is accessible through a 2,500-foot (760 m)-long tunnel; it includes four large reversible turbines, each of which can pump 15,200 cubic feet (430 m3) of water per second and release 20,000 cubic feet (570 m3) of water per second to generate 1,143 megawatts (1,533,000 hp) of electricity.[1]
The "lower reservoir" noted above is a pool along the Connecticut River above Turner's Falls Dam. Since the dam at Turner's Falls was raised in the early 1970s for Northfield Mountain, the banks of the Connecticut River have been re-adjusting to the new water level.