FRANKFORT, Ky. (AP) – Fearing a dam break that could cause catastrophic flooding in Kentucky and Tennessee, the U.S. 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 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.

Bill Peoples, spokesman for the Corps of Engineers in Nashville, said dam failure isn’t imminent, but the risk exists. Peoples said families living in downstream towns like Burkesville in Kentucky and Celina, Carthage, Clarksville, Gallatin, Hendersonville and Nashville in Tennessee should have evacuation plans in place.

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 could 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.