From Wiki: Derwent Dam and Reservoir - Construction of the neo-Gothic solid masonry dam began in 1902, a year after the building of Howden commenced, and proved a mammoth task. The chief engineer was Edward Sandeman. He was also in charge of building nearby Howden dam. The huge stones that formed the walls of the dam were carried along a specially created railway from the quarries at Bole Hill near Grindleford. The filling of the reservoir began in November 1914, and overflowed for the first time in January 1916, with the water almost immediately passing into supply. The dam can support a total of 9.64 million cubic metres of water. Only two years after the dam's completion in 1916, it was decided that the flow from the reservoir was insufficient to support the surrounding population. As a result, between 1920 and 1931 the rivers Alport and Ashop were also diverted from the Ashop valley into the reservoir using tunnels . The diversion helped hold back water during the construction of the Ladybower Reservoir to the south, which was constructed between 1935 and 1945.