Completed in 1958, the Stilling Basin allowed the river to flow through Oahe Dam until the Powerhouse became operation in 1962. The structure can release 748,000 gallons of water per second (110,000 CFS). The intakes are under 160 feet of water in Lake Oahe and cannot be seen. The 19 foot diameter, 3/4 mile long tunnels are flushed and inspected yearly. Ballards are concrete structures used to slow water, and they can also create dangers for swimmers and boaters.