Ladybower was built between 1935 and 1943 by the Derwent Valley Water Board to supplement the other two reservoirs in supplying the water needs of the East Midlands. It took a further two years to fill which was done by 1945. The dam differs from the Howden Reservoir and Derwent Reservoir in that it is a clay-cored earth embankment, and not a solid masonry dam. Below the dam is a cut-off trench deep and wide filled with concrete, stretching into the hills each side, to stop water leaking round the dam. The dam wall was built by Richard Baillie and Sons, a Scottish company. The two viaducts, Ashopton and Ladybower, needed to carry the trunk roads over the reservoir, were built by the London firm of Holloways, using a steel frame clad in concrete. The project was delayed when the Second World War broke out in 1939, making labour and raw materials scarce, but construction was continued due to the strategic importance of maintaining supplies. King George VI, accompanied by his wife, Queen Elizabeth, formally opened the reservoir on 25 September 1945.