This large wood and concrete ball plug was one of two used to halt the flow of water through the 42-inch-diameter power outlet works conduits at the base of the dam, allowing repair and maintenance of downstream machinery and equipment. To reach the submerged conduit openings on the upstream face of the dam, divers first removed trash rack structures and a cableway winch raised them to the surface. Assisted by divers, the ball plugs were then lowered by the cableway winch into position, where water pressure forced the balls against the conduit openings and sealed off the flow of water. Their wood-and-concrete construction provided strength while also allowing easy maneuverability underwater. During the Buffalo Bill Dam modification project of 1985-1993, two new ball plugs were obtained, and since the removal of the cableway winch in 1985, installing the new ball plugs has had to be done with a barge-mounted crane on the reservoir.