Part of a nuclear power station was shut down and another plant - the oldest in the US - was put on alert after waters from superstorm Sandy rose six feet above sea level.
One of the units at Indian Point, about 45 miles north of New York City, was shut down around at 10.45pm yesterday because of external electrical grid issues, said Entergy, which operates the plant.
The company said there was no risk to employees or the public, and the plant was not at risk due to water levels from the Hudson River, which reached 9ft 8ins and was subsiding. Another unit at the plant was still operating at full power.
The oldest US nuclear power plant, New Jersey's Oyster Creek, was already out of service for scheduled refuelling. But high water levels at the facility, which sits along Barnegat Bay, prompted safety officials to declare an "unusual event" around 7pm.
About two hours later, the situation was upgraded to an "alert", the second-lowest in a four-tiered warning system.
Conditions were still safe at Oyster Creek, Indian Point and all other US nuclear plants, said the US Nuclear Regulatory Commission (NRC), which oversees plant safety.
A rising tide, the direction of the wind and the storm's surge combined to raise water levels in Oyster Creek's intake structure, the NRC said.
The agency said that water levels are expected to recede within hours and the plant, which went online in 1969 and is set to close in 2019, is watertight and capable of withstanding hurricane-force winds.
The plant's owner, Exelon, said power was also disrupted in the station's switchyard, but backup diesel generators were providing stable power, with more than two weeks of fuel on hand.
In other parts of the East Coast, nuclear plants were weathering the storm without incident.
Nuclear power plants are built to withstand hurricanes, plane crashes and other major disasters, but safety procedures call for plants to be shut down when hurricane-force winds are present, or if water levels nearby exceed certain flood limits.
At Pennsylvania's Susquehanna plant in Salem Township, officials were ready to activate their emergency plan, a precursor to taking the plant offline, if sustained winds hit 80 mph.