Llwyn-on Reservoir (or Llwyn Onn Reservoir) is the largest and southernmost of the three reservoirs in the Taf Fawr valley in South Wales. Cardiff Corporation Waterworks obtained an Act of Parliament in 1884 to authorise construction of the reservoirs, to increase the water supply for Cardiff, but construction of Llwyn-on Reservoir did not start until 1911 and was completed in the 1920s.
The reservoir, which has been owned and managed by Welsh Water and its predecessor organisation, the Welsh Water Authority, since 1973, is located within the Brecon Beacons National Park with its eastern half in the Merthyr Tydfil unitary authority area and the western half in Rhondda Cynon Taf. The village of Llwyn-On is nearby and the A470 trunk road runs along its eastern shoreline.
History
Cardiff Corporation had bought out the Cardiff Waterworks Company in 1879, and in March 1881 asked their Borough Engineer, John Avery Brandon Williams, to produce a report on all possible sources of water for Cardiff, so that they could plan for the future. Williams presented his reports in May and August 1881, suggesting that the best scheme was to impound the waters of the upper Taff Fawr. After consulting the water engineer John Frederick Bateman, who also recommended the Taff Fawr scheme in June 1882 as the one most likely to meet the needs for water at the lowest cost, the Corporation presented a bill to Parliament in November 1883, which was considered by a House of Commons Committee between 1 May and 20 May 1884. It faced serious opposition from riparian landowners and there were 16 petitions against it, but several influential civil engineers was called to give evidence, notably Bateman, Thomas Hawksley James Mansergh, and George H. Hill. J A B Williams, their own Engineer, and the meteorologist George James Symonds also gave evidence, and although it cost the Corporation £3,652 to fight the case, Royal Assent was obtained for the Cardiff Corporation Act 1884, a private Act of Parliament, on 7 August.
The catchment of the upper Taff Fawr was but it was split into an upper area of and a lower one of . Any works had to provide compensation water to maintain the flow in the Taff Fawr, and working on the upper area first meant that they only had to supply per day of compensation flow until work started on the lower area. Cantref Reservoir, at the time known as Cantreff, was the first to be built, together with a pipeline to Lisvane Reservoir and Llanishen Reservoir and intermediate balancing reservoirs at Blackbrook, Cefn, and Rhiwbina. Work on the pipeline was completed in September 1888, and on the balancing reservoirs in November 1888. Cantref Reservoir was completed on 14 September 1892, and Beacons Reservoir was completed on 30 September 1897.
For Llwyn-on Reservoir, the 1884 Act specified a dam on the River Taf Fawr upstream from Merthyr Tydfil to create a reservoir with a capacity of . During 1902, the Corporation looked at increasing its capacity, so that Cantref and Beacons could be used exclusively for water supply, and all of the compensation flow for the Taf Fawr, which would then be per day, could be provided by the new reservoir. They also looked at the possibility of building a fourth reservoir between Llwyn-on and Cantref. They had purchased of land which was adequate for the original reservoir, but considered buying more land. When the Corporation decided to proceed, they elected for a reservoir that was considerably bigger than their 1902 plans, and in 1909, a new Act was obtained that doubled the permitted size to .