Cantref Reservoir dam, including valve house, draw-off tower and spillway, (partly in Hirwaun)

The Cantref Reservoir dam, which includes a valve house, draw-off tower, and spillway, is a structure built in the late 19th century. The dam features a clay core and earth bund construction, with a pitched stone inner face that ends in a battered rock-faced masonry wave-wall topped with simple cast-iron railings. The outer face is turfed.

On the east side of the dam, there is a single-storey draw-off tower made of coursed rock-faced stone. It has a flat roof that is hidden behind a parapet. The tower has a round-headed doorway with replacement doors and a metal plaque in the tympanum that commemorates the opening of the reservoir in 1892. Above the doorway is a moulded cornice, and there is a single window at the rear, which has been renewed in the original round-headed opening.

At the west end of the dam, there is a spillway that is marked by a short pier with a rounded end, constructed of rock-faced stone facing the reservoir, and dressed stone leading to the spillway. A segmentally-arched bridge spans the spillway at the dam's end. The spillway consists of a stone-paved channel with dressed stone revetment on the west bank, below a rock-faced dwarf wall with railings and a gateway. The channel continues downstream with a series of weirs that feature stepped and ramped dressed stone revetments topped with free-stone copings. An auxiliary syphon spillway was added in the later 20th century.