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

The Llwyn-on Reservoir dam, which includes a valve house and spillway, is a structure built with a clay core and earth bund construction. The inner face of the embankment is finished with pitched stone, leading to a battered rock-faced masonry wave-wall that serves as a parapet for the roadway, while the outer face is turfed.

On the east side of the dam, there is a circular two-stage valve house. This house features cast-iron gates with flanking railings that create a break in the parapet. The valve house has an embattled parapet supported by a corbel table, and a higher corbelled stair-turret on the southeast side. It includes a pointed arched entrance with replacement doors and a metal plaque in the tympanum that commemorates the opening of the reservoir in 1926. The sides of the valve house have windows that have been replaced in their original openings, and above the lower stage, there are narrow loops that provide light to the stair and upper stage.

The spillway is located on the east side of the dam, where it ends with a short pier that has a rounded end. A road crosses over the spillway via a 2-arch bridge, which features elliptical arches and pointed cutwaters on the central pier. Beyond this, the parapet wall curves around with the road, forming a revetment to a steep bank further east. The spillway itself is stone-paved and has a revetment wall along the bank on the east side. Additional revetments enclose the stilling basin, overflow channel, and weirs located below the dam. There is also a tunnel portal at the base of the dam alongside the spillway, characterized by rusticated stone voussoirs and a parapet. A later 20th-century siphon spillway with a submerged intake is situated in the center of the dam.