Lake Vyrnwy Dam

Large masonry dam of the gravity type, 358m long and 44m high and topped by a spectacular carriageway of 31 masonry arches, each of 7.3m span. It was designed with an exceptional width of 39m at its base to ensure stability and was constructed of massive, irregularly shaped, blocks of stone which were locally quarried; mostly a variety of rubble but with much freestone facing and some vermiculated dressings, especially to the towers. Built in a neo-Baroque manner, in the deliberately massive style characteristic of the work of Sir John Vanburgh. Symmetrical design with clustered groups of four towers at either end of the nineteen-arch central section which comprised the principal overflow; full-length pierced parapet and piers flanking each arch. Each tower is capped by a corbelled pyramidal stone roof and has pierced upper stage over a pronounced bracket cornice; the main piers of the towers are rusticated to give a ribbed appearance and the doorways to the towers are tapered in Egyptian manner; on the south-east sides the towers have heavily key-blocked round-arched openings with pierced and bracketed balustrades. The dam has two inscriptions to the frieze on this side, reading: OPUS INCHOATUM MDCCCLXXX and OPUS ABSOLUTUM MDCCCXC.

In the rock face to the north of the dam (separately listed) are three commemorative plaques, the middle one, on the axis of the dam road, recording the laying of the foundation stone in 1881 by Lord Powis.

At the base of the dam, below the towers, there are two transverse discharge tunnels with valves operated hydraulically from a chamber near water level. Cast-iron valve controls were made by Guest and Chrimes of Rotherham. Tunnels and cast-iron staircases within tapering circular shafts provide access throughout the dam from the towers. A network of drainage tunnels was incorporated for safety purposes to prevent build up of water pressure underneath the base, and this was apparently the first dam to be constructed featuring such equipment.