The reservoirs have their origin in the rapid growth of Cardiff as a major coal exporting port in the nineteenth century.  Unfortunately the town’s utility services did not grow as rapidly as the population and the insanitary conditions led to major outbreaks of diseases such as typhoid and cholera.  When it was realised that many of these diseases were caused by poor drainage and contaminated drinking water, Cardiff Corporation took steps to provide a clean water supply for the town.

Work started in 1864 to build a new storage reservoir at Lisvane and this was completed in 1865.  The new reservoir had a surface area of 20 acres and could hold 60 million gallons.  It was fed by the surrounding streams including the Nant Fawr, Nant-y-Felin and Nant Glandulas.  Filter beds were also constructed near the reservoir, on the bed of what would later become the adjacent Llanishen reservoir.

However, the population of Cardiff continued to grow and by the end of the 1870s water was again getting in short supply.  This time, however, Cardiff Corporation came up with a much more radical solution, designed by their Borough Engineer, Mr. J.A.B. Williams.  It involved building a number of reservoirs in the Taf Fawr valley of the Brecon Beacons and then constructing a 32 mile pipeline back to Cardiff where the water could be stored in a new holding reservoir at Llanishen, adjacent to Lisvane reservoir.

Work started on Llanishen reservoir early in 1884 and was finished in 1886.  At first the rest of the reservoirs in the Taf Fawr scheme weren’t completed, and Llanishen reservoir was used to store excess water from Lisvane reservoir, but following the completion of Cantref reservoir in 1892 water started to be piped down to Llanishen, and then on into Cardiff.  Llanishen reservoir has an area of 60 acres and can hold 317 million gallons.  When the Taf Fawr scheme was completed, the pipeline could deliver up to 12 million gallons a day to Llanishen reservoir.

The Taf Fawr Scheme continued to provide drinking water to Cardiff until the mid 1960’s, when Cardiff Corporation built a new reservoir at Llandegfedd, just outside Pontypool.  The Taf Fawr scheme became redundant: water was no longer supplied to Llanishen Reservoir, and none was taken from it.  It was just kept topped up by rainfall.

Although Llanishen was no longer used as a water supply, in the late 1960s the reservoirs started to be used as a venue for sailing.  In 1974 South Glamorgan County Council decided to set up a sailing school, run by its Education Department, which provided tuition mainly for young people, including school groups.  The sailing school flourished and became recognised as a centre of excellence for sailing in Wales, with many of its members going on to join national squads.  One of the most well-known sailors who started her career at Llanishen is Hannah Mills, who won a gold medal in the women’s 470 class at the 2016 Rio Olympics.

Another recreational use of both reservoirs was for fishing, which started not long after they were first opened.  The Cardiff Reservoirs Fly Fishing Club was founded in 1948 and the reservoirs became one of the most popular fishing venues in South Wales.  It was not uncommon to see up to 30 or 40 fishermen there, particularly on warm summer evenings.

The reservoirs have also been extensively used by bird-watchers and others who just appreciated the quiet enjoyment of spending time in pleasant natural surroundings.  An annual permit was needed to visit the site and cost a modest £5.

The reservoirs remained in Cardiff Corporation’s ownership until 1973 when they were taken over by the newly formed Welsh National Water Development Authority.  The Welsh Water Authority was privatised in 1989 by the Thatcher government and the resulting company, which rebranded itself as Hyder in 1996, got involved in other activities such as electricity supply, construction and hotels.  It over-stretched itself and got into financial difficulties.  On 15 September 2000 Hyder was bought by Western Power Distribution (WPD), a subsidiary of an American multi-national utility company: the Pennsylvania Power and Light Corporation.  WPD was mainly interested in Hyder’s electricity distribution business (SWALEC), so it rapidly disposed of the other assets, including the water supply and sewerage infra-structure, which was sold for £1 (along with £1.85 billion of debt) to a company called Glas Cymru.  At the time of this sale, WPD identified the Cardiff reservoirs as a site with development potential and inserted a clause in the sale agreement that gave them the option to buy back the reservoirs from Glas Cymru.

Western Power’s plans for the reservoirs became clear in the summer of 2001 when it was revealed that it intended to build a large scale housing development on both the Nant Fawr meadows and the site of Llanishen reservoir.  The surrounding community was outraged by the prospect of the potential loss of such a much-loved local beauty spot and in December 2001 the Reservoir Action Group (RAG) was formed to defeat this development proposal.

The campaign to save Llanishen and Lisvane reservoirs was a long and arduous one.  It lasted twelve years and involved four separate planning applications, three public planning inquiries, and numerous High Court Actions involving Western Power bringing cases against Cardiff Council, the Countryside Council of Wales, CADW (the Welsh Government’s historic environment service), and the Welsh Government itself.  Sadly, Western Power had Llanishen reservoir drained in 2010 and thereafter left it empty.  During the process of the campaign RAG managed to get the reservoir embankments notified as a Site of Special Scientific Interest for their grassland fungi and the reservoir itself listed as a structure of historical importance by CADW.  Both these designations played an important part in the eventual refusal of the planning application by the Welsh Government and the abandonment of the housing scheme by Western Power.

In September 2013 Western Power announced that they had sold the reservoirs to Celsa Steel UK, a Catalan company that runs a steel mill in Cardiff Bay.  Celsa were interested in the reservoirs because for many years water from Lisvane reservoir had been used in their Cardiff mill for cooling the steel.  Unfortunately, this time was a difficult one for the steel industry due to the high cost of energy in the U.K. and the international steel market being flooded with cheap imports from China.  Celsa didn’t have the available resources to do much with the reservoir site, although they were sympathetic to requests from the local community to get Llanishen reservoir refilled and the site re-opened to the public.

As a result, in January 2016, Celsa announced that it had leased the reservoirs to Dŵr Cymru / Welsh Water (DCWW) on a 999 year lease.  DCWW announced that it planned to repair and refill Llanishen reservoir and to reopen the site to the public with an emphasis on recreational activities and the conservation of biodiversity.  This work started in July 2016 and is due to be completed during 2023, when the site will be re-opened to the public.