The U.S. Army Corps of Engineers on Monday began lowering the water level on Lake Cumberland in what it called "emergency measures" to alleviate pressure on Wolf Creek Dam.
The leaky 240-foot-high dam is enough of a threat to break that officials were warning residents in the downriver areas to develop evacuation plans as they announced the project.
For many residents and tourists as well as many fish the results of the lake lowering will be severe. But a breach of the mile-long dam would no doubt be worse, causing an estimated $3 billion in damage to cities along the Cumberland River, which cuts through downtown Nashville, Tenn.
"We understand that this decision will adversely impact many people, communities and businesses that rely upon Lake Cumberland for project purposes and other uses," said Lt. Col. Steven J. Roemhildt, commander of the Corps of Engineers' Nashville office, in a statement. "But we must take this emergency action to reduce risk to the public and to the dam itself."
To counter the seepage that is threatening the dam, the Corps will pump grout into the dam's limestone foundation.
The draw-down will leave the lake level at 680 feet, 43 feet below the normal summer level. In the fall, the Corps will reassess that level.
A Corps web site states that the probability of the dam breaking "is high given the consequences of failure." Draining the lake as planned will reduce the pressure on the foundation by 10 to 25 percent, the Corps says, adding that:
• The fisheries of the reservoir and tailwater will suffer immensely, cost $200 million and require 15 years from the time when normal operations are resumed to restore them completely. The U.S. Fish and Wildlife Service hatchery likewise might not survive.
• The reduction in lake elevation to 680 feet will render unusable hundreds of private boat slips, dozens of launching ramps and three of the lake's 11 marinas.
• The initial water release what the Corps calls "pool reduction" will last at least until the spring.
• The cost of the full repair project, which will include fortifying the dam with grout to a depth of 275 feet and building a concrete cutoff wall, will be $309 million and, optimistically, will be complete in 2012
• Hydroelectric generation and the overall water supply will fall.
With more than 1,000 miles of shoreline and 50,000 acres of water, Cumberland Lake is the ninth-largest reservoir in the United States.
The Wolf Creek Dam, which has a concrete core surrounded by earth, cost $81 million to build in 1950 and nearly twice that to repair when leaks were discovered in 1967, according to the Corps of Engineers.
That project saw 290,000 cubic feet of grout pumped into the foundation of the earthen embankment, and a half-mile-long concrete diaphragm wall installed in the embankment.
Corps spokesman Bill Peoples said failure of the dam was not imminent. But he said people should have evacuation plans ready in Nashville and other downstream communities, including Burkesville in Kentucky and Celina, Carthage, Clarksville, Gallatin and Hendersonville in Tennessee.
Nashville officials said that they have a plan in place for any flooding but that any threat would be minimized once the lake's level is lowered.
"We have re-reviewed some of the plan and addressed specific things that may need to be included if there's a breach in the dam," said Amanda Sluss, a spokeswoman for the city Office of Emergency Management.
At a marina near Russell Springs, workers spent Monday moving million-dollar houseboats to moorings where they can stay afloat after the water recedes.
"We're kind of at a loss," said Estelee Slusser, who operates the Alligator Dock No. 1 marina. "It has just happened so quickly. We really don't know what to do."
The Army Corps notified local officials and business owners before making the plan public Monday. Slusser said she learned of it Friday.
"We spent the whole day yesterday on the phone with customers, trying to calm them down," she said.
The Associated Press contributed to this report.