Entwistle Dam was designed by Thomas Ashworth, a local land surveyor, overseen by Jesse Hartley, the Liverpool Docks engineer. Other works were by Joseph Jackson, an engineer and surveyor from Bolton. It was built for the Commissioners of the Turton and Entwistle Reservoir, a group of local mill owners who obtained an enabling Act of Parliament in 1832 to regulate the supply of water in Bradshaw Brook for water power for the finishing textiles.[2]
Records suggest it was built entirely of puddle clay with no distinct core. Earth dams usually have a waterproof cutoff under their earthworks designed to stop seepage under the dam but it is doubtful whether there was any such a cutoff at Entwistle. The reservoir has a rock-cut outlet tunnel driven through the valley side rather than a culvert or pipeline and a siphon draw-off pipe. The dam is 108 ft high and 110 metres long at the crest. The present overflow channel and valve tower were added by the Bolton Corporation Water Works who took over the reservoir in 1864.