Built primarily for flood control, the lake surface is kept at an elevation of 1,396 feet above sea level for recreational use during the summer. During the fall, the lake is lowered 16 feet to hold additional water from winter and spring runoff.
When flooding does occurs, the gates in the intake structure are closed to the lowest level. The water runs off from the 221 square mile drainage basin behind the Dam. The water is then stored in the reservoir to protect life and property downstream. When the danger of downstream flooding has passed, the intake structure gates are opened to lower the reservoir. Opening the gates is coordinated with other dams on the Big Sandy and Ohio Rivers as a part of a larger flood control system.
The Flannagan Dam has prevented millions of dollars of damage due to flooding in excess of the $20 million it cost to build.