(Corrects height of dam to 550 feet (168 meters) from 5-1/2 stories in paragraph 8)
Sept 1 (Reuters) - A magnitude 4.2 earthquake struck near Grand Coulee Dam, the largest U.S. hydropower facility, in northeastern Washington state on Tuesday, prompting an immediate inspection of the facility but leaving no visible damage, a spokeswoman said.
Operation of the Columbia River dam, which supplies electricity to 11 Western states, was not disrupted by the tremor, according to the spokeswoman, Lynne Brougher of the U.S. Bureau of Reclamation, the agency that runs the dam.
The U.S. Geological Survey reported the quake was centered about 25 miles (40 km) north of the dam near the town of Nespelem in heart of the sprawling Coleville Indian Reservation, an area ravaged by a major wildfire in recent days.
Weak to light shaking was felt over a wide region, but there were no immediate reports of damage of injuries.
Earthquakes of that magnitude are not uncommon in seismically active Washington state, but Tuesday's 4.2 tremor was the first to strike this close to the dam "in quite a while," Brougher said.
"Oh, it got our attention. Everybody felt it," she said, adding that the trembling went on for roughly 30 seconds.
She said it would take several hours for dam officials to complete their inspection of the mammoth facility.
Grand Coulee stands 550 feet (168 meters) tall, is 500 feet (152 meters) thick at its base, and stretches nearly a mile (1.6 km) across the Columbia River to form Lake Roosevelt, which provides drinking water and irrigation supplies for communities and farms in the region.
The dam includes three major hydroelectric power-generating plants and a pump-generating plant that average a combined 21 billion kilowatt hours of electrical output a year. It also controls stream flow for flood management, fish migration and recreation downstream.
The original dam structure was completed in 1941, with additions made in the 1960s and '70s. (Reporting by Steve Gorman from Los Angeles; Editing by Sandra Maler)