Ladybower Reservoir is a large Y-shaped reservoir, the lowest of three in the Upper Derwent Valley in Derbyshire. The dam's design is unusual in having two totally enclosed bell-mouth overflows (locally named the "plugholes") at the side of the dam wall. These are stone and of 80 feet (24 m) diameter with outlets of 15 feet (4.6 m) diameter. The overflows originally had walkways around them, but they were dismantled many years ago. The bell mouths are often completely out of the water and are only rarely submerged, usually after heavy rainfall or flooding.