Ladybower Reservoir is a large Y-shaped, artificial reservoir, the lowest of three in the Upper Derwent Valley in Derbyshire, England. The River Ashop flows into the reservoir from the west; the River Derwent flows south, initially through Howden Reservoir, then Derwent Reservoir, and finally through Ladybower Reservoir. The reservoir is owned by the water company Severn Trent.
The area is now a tourist attraction, with the Fairholmes visitors' centre located at the northern tip of Ladybower. The east arm of the reservoir, fed by the Ladybower Brook, is overlooked by Hordron Edge stone circle.
Design and construction
thumb|left|alt=Ladybower Reservoir, showing two viaducts carrying roads over the water|The Ashopton Viaduct (left) carries the [[Snake Pass A57 road and the Ladybower Viaduct carries the A6013 to Bamford over the reservoir.]]
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 around 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.
During the 1990s, the wall was raised and strengthened to reduce the risk of over-topping in a major flood. The original dam wall contains 100,000 tons of concrete, over one million tons of earth, and 100,000 tons of clay for the core. The upstream face is stone-faced. Materials were brought to the site on the Derwent Valley Water Board's own branch line and their sidings off the mainline in the Hope Valley.
thumb|left|The reservoir
The dam's design is unusual