Llwyn-on Reservoir dam, including valve house and spillway (partly in Hirwaun community)

Dam is clay core and earth bund construction, with pitched stone to inner face of embankment, terminating in a battered rock-faced masonry wave-wall forming parapet to roadway; turfed outer face. Circular two-stage valve house offset to east side of the dam, where cast-iron gates with flanking railings form a break in the parapet. The valve house has an embattled parapet on a corbel table and higher corbelled stair-turret to south-east. It has a pointed arched entrance with replacement doors, and metal plaque in tympanum recording the opening of the reservoir in 1926. The sides have windows replaced in original openings. Above the lower stage are narrow loops lighting the stair and the upper stage.

The spillway is on the east side of the dam, where the dam is terminated by a short pier with a rounded end. The road is carried by a 2-arch bridge over the spillway, with elliptical arches and pointed cutwaters to the central pier. Beyond this the parapet wall curves round with the road where it forms a revetment to a steep bank further east. The spillway is stone-paved and has a revetment wall to the bank on the east side. Similar revetments enclose the stilling basin, overflow channel and weirs below the dam. Tunnel portal alongside spillway at base of dam, with rusticated stone voussoirs and parapet. A later C20 syphon spillway with submerged intake is in the centre of the dam.