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

Dam is clay core and earth bund construction, with pitched stone to inner face of embankment terminating in battered rock-faced masonry wave-wall with simple cast-iron railings; turfed outer face. Offset to the east side of the dam is the single storey draw-off tower of coursed rock-faced stone. Flat roof concealed behind parapet. Round headed doorway with replacement doors, and metal plaque in tympanum recording the opening of the reservoir in 1892. Moulded cornice above doorway, and single window, renewed in original round-headed opening, to rear.

At the west end of the dam is a spillway, where the dam is terminated by a short pier with rounded end, of rock-faced stone facing the reservoir, and dressed stone to spillway. A segmentally-arched bridge crosses the spillway at the end of the dam. Spillway comprises stone-paved channel with dressed stone revetment to west bank below a rock-faced dwarf wall with railings and gateway. The channel continues down-stream in a series of weirs with stepped and then ramped dressed stone revetments with free-stone copings. Auxiliary syphon spillway added in later C20.