Llwyn-on Reservoir

the folk song Llwyn Onn

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.

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 32 mile (51 km) 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 4 mile (6 km) upstream from Merthyr Tydfil to create a reservoir with a capacity of .

Louis Nott is known to have brought at least six locomotives to Llwyn on during the early phase of the project. All had previously worked at Birkenhead, where he had built a dry dock for Cammell Laird. Three may have been requisitioned by the Ministry of Munitions during the First World War. After Nott died in 1916, Robert Brodie traded as Executor of L P Nott, and then Nott, Brodie. Brodie's companies re purchased two of the locomotives. Chepstow: 0 6 0 saddle tank, Manning Wardle 738 of 1881. May have gone to the Ministry of Munitions. Norman: 0 6 0 saddle tank, Hunslet 454 of 1888. Subsequently worked at Eastleigh aerodrome, and then possibly for the Ministry of Munitions. Sharpness: 0 6 0 side tank, Sharp Stewart 3472 of 1888. May have gone to the Ministry of Munitions. Bought by Brodie in 1922 for use on the construction of Avonmouth Portway. Tuxford: 0 6 0 saddle tank, Hunslet 579 of 1893. Remained on site into Corporation era. Liverpool: 0 4 0 saddle tank, Manning Wardle 1518 of 1901. Bought by Robert McAlpine & Sons in 1917. Tranmere: 0 6 0 saddle tank, Hudswell Clarke 654 of 1903. Subsequently used on an ordnance factory project and then bought by Brodie in 1918 for use on the Blaen y Cwm reservoir and other projects. When Cardiff Corporation restarted the project in 1919, they acquired one locomotive from Nott's executors, two second hand ones, and bought three new ones. Tuxford: 0 6 0 saddle tank, Hunslet 579 of 1893. Acquired from Nott's executors. Repairs were made to it in 1919, but it was then mounted on blocks at Llwyn on, to supply steam to a pump. Nelson: 0 6 0 saddle tank, Kitson 1786 of 1871. It was driven from J F Wake's yard in Darlingon in September 1919. Some time after 1921, it was also mounted on blocks to power a stationary pump. Dan y Graig: 0 6 0 saddle tank, Manning Wardle 835 of 1882. Brought from Swansea in July 1920. Later worked at the shipbreaking yard of T W Ward at Briton Ferry until the 1930s. Llwyn on: 0 6 0 saddle tank, Hudswell Clarke 1429 of 1920. Bought new. Some teething problems had to be fixed by the manufacturer. Sold to the dealers T W Ward of Sheffield on 30 September 1927, and finally scrapped around 1957 at Felsted in Essex. Cwm Taff: 0 6 0 saddle tank, Hudswell Clarke 1466 of 1921. Sold to the dealers T W Ward of Sheffield on 30 September 1927, and subsequently worked at various locations, including the Ewden Valley Waterworks for Sheffield Corporation. Abernant: 0 6 0 saddle tank, Manning Wardle 2015 of 1921. Sold to the dealers T W Ward of Sheffield on 30 September 1927. Worked at the Austin Motor Company at Longbridge until 1963, and was then moved to a children's playground in Birmingham. Subsequently preserved, and was awaiting restoration at the Great Central Railway (Nottingham) in 2012.

To the east of the reservoir is the mountain of Garn Ddu, and to the west is the mountain of Cadair Fawr. There are a variety of guided walks and waymarked paths. Environmental sculptures can be found on the Wern and Willow walks. The Taff Trail path links Cefn coed y cymmer to Brecon. There is one bird hide. In 2020, Welsh Water blocked off lay bys around the Llwyn Onn reservoir because of persistent complaints about them being used for antisocial behaviour such as dogging.