FRANKFORT, Ky. — Fearing a dam break that could cause catastrophic flooding in Kentucky and Tennessee, the Army Corps of Engineers began lowering the water level on Lake Cumberland on Monday.

The measure was aimed at reducing pressure on the weakened 240-foot-high dam, said Lt. Col. Steven J. Roemhildt, commander of the Corps of Engineers’ Nashville office.

“We must take this emergency action to reduce risk to the public and to the dam itself,” he said in a statement.

If the Wolf Creek Dam, which is nearly a mile long, were to break, flooding in communities downstream along the Cumberland River could kill people and cause an estimated $3.4 billion in damage, Roemhildt said. Cities along the Cumberland include Nashville, Tenn.

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.

The dam, which has a concrete core surrounded by earth, was built near Jamestown in the early 1950s. The lake it holds back was created as part of a federal plan to control floods along the Ohio and Mississippi rivers.

Roemhildt said water has been seeping under the dam and eroding the limestone on which the concrete rests. He said crews were pumping grout into the ground to counter the erosion.

Reducing the water level could have a major ecological and economic effect as well. Roemhildt said people can expect fish kills because of a rise in water temperature, and boats at marinas could be left high and dry.

Kentucky Commerce Secretary George Ward said as many as 90 percent of the launching ramps will be unusable because they won’t reach the water’s surface.

Lake Cumberland, about 100 miles southeast of Louisville and one of the nation’s largest freshwater reservoirs, is a popular destination for boaters. A thriving houseboat industry has sprung up around the lake, which has more than 1,000 miles of shoreline.