Bagnell Dam was designed by Stone & Webster Engineering Company of Boston, Massachusetts. The owner of the dam is AmerenUE of St. Louis, Missouri. (At the time of construction, AmerenUE was known as Union Electric Light & Power Company.) Construction began on August 6, 1929, and was completed in April 1931. Lake of the Ozarks began to fill on February 2, 1931, and would fill at an average rate of one-and-a-half feet per day. The water reached spillway elevation on May 20, 1931–seventy-eight days after the dam sluiceways were closed. However, the lake water would not reach full reservoir for the first time until the spring of 1932. The total cost of the project was $30 million.
The concept of an Osage River dam dates to 1912. After acquiring the necessary capital, Kansas City entrepreneur Ralph Street built support structures, roadways, and railroad tracks in the vicinity of the dam site, but financial difficulties brought his venture to a close in 1926. Street’s original plan called for a sixty-foot high hydroelectric dam. This would have created a lake of approximately forty miles in length with no major tributaries. (The Gravois, Grand Glaize, and Niangua Arms of the present lake merely would have been short, shallow coves!) One year later Union Electric, together with Stone & Webster, revitalized the project and expanded it to its present-day parameters.
The specific site for the dam was chosen for a variety of reasons. Amongst the major considerations were the presence of a projecting point on the south side of the floodplain directly opposite a steeply sloping ridge on the north side. The combination restricted the floodplain valley to one-half mile in width, which generally was narrower than the average width of the floodplain. Furthermore, bedrock at his point was relatively close to the surface, being approximately twenty feet below the level of the floodplain.
The dam is 2,543 feet long. It consists of three sections. The abutment section (1,512 feet) is a solid wall rising from bedrock to the roadway level. Most of the abutment section is on the south side, though a short section of it supports the curving bridge at the north end of the dam. The base of the abutment section is 93 feet wide.
The spillway section (520 feet) forms the middle portion of the dam. The spillway section consists of five sealed sluiceways at the base of the structure and twelve flood gates at the top. Each of the steel floodgates is 22 feet high by 34 feet wide. They are classified as tainter gates by their design and usage. They weigh 54,000 pounds each. The gates are opened by raising rather than lowering them. The water flows under the gates from a depth of twenty-two feet below the surface, thus avoiding a strong surface current when in use. The gates are raised and lowered by 70-ton traveling gantry cranes. These are the two house-like structures seen above the dam wall. In the event of mechanical failure, high water will overtop the floodgates before reaching the top of the dam. The base of the spillway section (131 feet) is wider than the rest of dam. The additional width, of stairstep construction, is located below the level of the Osage River and serves as a dissipater for water coming down the spillway.
The headworks, or powerhouse, section (511 feet) sits over the original bed of the Osage River. It consists of eight water-wheel turbines directly connected to eight electrical generators, plus one smaller station service unit. (Originally only six of the generators were installed. The other two were added in 1953.) The face of the headworks presents nine large openings into the lake. When the turbines are not in use, each of the openings is sealed by a steel headgate, measuring twenty-seven feet square and weighing 70 tons. Under a maximum head of ninety feet, the turbines revolve at 112-1/2 r.p.m. with a force equal to 33,500 horsepower each. The generators were designed to produce a maximum of 21,500 kilowatts each.
The overall dam structure was poured in 40-foot wide blocks that are designed to slide in the event of a breach. Therefore, instead of a total collapse, only one or two sections would move back while the remainder of the dam would remain solid and undamaged. In the 1980s holes were drilled down through the dam and tensioning rods were driven through those holes and into the bedrock below. The effect was to anchor the dam more securely to the bedrock and render the whole facility as strong as a rock. The dam actually is more solid now than when it was built. Regular visual inspections of the entire dam take place to ensure nothing has changed. Even if Truman Dam, located some 93 miles upstream, were to fail, and the ensuing flood tide were to overtop Bagnell Dam, it would not be destroyed.
Bagnell Dam carries U.S. Business Route 54, known locally as Bagnell Dam Boulevard, across its full length. The highway deck is approximately 10 feet above the full reservoir. The highway underdeck was rebuilt in 2004.
In case you’ve never been there, the picture above, taken from Kaysinger Bluff, shows the extreme upper end of Lake of the Ozarks, 92-1/2 channel miles above Bagnell Dam, and some two miles west of Warsaw, Missouri. The rock covered slope in the foreground is part of Truman Dam, which would form Missouri’s second largest reservoir–Truman Lake. The entire length of Lake of the Ozarks, including all major tributaries and long coves, is approximately 180 miles. The shoreline measures 1,100 miles and is longer than the Pacific coastline of California.