When constructed in 1832 the Entwistle Dam was the highest in Britain; it rises 108 feet from the base.

[1] Entwistle Dam was designed by Thomas Ashworth, a local land surveyor, overseen by Jesse Hartley, the Liverpool Docks engineer.

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, the Turton and Entwistle Reservoir Act 1832 (2 & 3 Will.

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 present overflow channel and valve tower were added by the Bolton Corporation Water Works who took over the reservoir in 1864.