A large reservoir built to supply Cardiff with water. Originally authorised by Cardiff Corporation Act of 1884 with a capacity of 670 million gallons, and designated Reservoir No.3, Llwyn-on Reservoir was re-authorised by Act of 1909 with an increased capacity of 1260 million gallons. The top water level was 854ft (260m) OD. The dam was at the southern end (SN00911125 - SN01231146). Work commenced in January 1911 with Louis P Nott of Llanelly and Bristol as contractor. Work was suspended in 1915 due to the First World War and resumed using direct labour in 1919. The reservoir was inaugurated in June 1926, being fully commissioned in May 1927. Workshops were built close to the dam site, W of the main road, and accommodation E of the road, but on recommencement of the work, the original huts were demolished and replaced by new ones W of the road. The railway from Cefn Coed was relaid in 1911. The railway and the works site were removed by 1928.
D.J.Percival, RCAHMW, 09 August 2006.