He presented his findings in May and August 1881, and concluded that reservoirs on the upper Taff Fawr, on the southern edge of the Brecon Beacons would be the best solution.

He also recommended that the Llanishen Reservoir, which had been authorised in 1878 by an Act of Parliament obtained by the Cardiff Waterworks Company prior to the Corporation buying them out, should be constructed urgently.

The corporation were unsure how to proceed and sought the advice of the water engineer, John Frederick Bateman who agreed with Williams that the Taff Fawr scheme was the most promising solution which would fulfil the town's requirements at the least cost.

They decided to build Llanishen Reservoir first, so that surplus water from the Lisvane catchment would not be lost, while the larger Taff Fawr project was under construction.

There is no connection to local streams, such as the Nant Fawr, which runs around the edge of the reservoir, so it was initially filled by rainwater.

It has a maximum surface area of water of 23.8 hectares (59 acres) and a capacity of 1,440,909 cubic metres (50,885,200 cu ft).

Welsh Assembly members were concerned that the takeover would mean jobs being transferred to Bristol and Cheshire, and at its effect "on consumers' interests, on the environment, and on the wider economy".

[12][13] WPD acquired the reservoir from Welsh Water in 2001 when Welsh Water was acquired by the not-for-profit organisation Glas Cymru:[14][15][16] the reservoir was sold to a WPD subsidiary, Hyder Industrial Group Ltd,[15] which was later renamed Western Power Distribution Investments Ltd.

[1] The reservoir is listed as a Site of Importance for Nature Conservation (SINC) as it has pondweed and stonewort, and common toads breed there.

[19] The banks of the reservoir are listed as a Site of Special Scientific Interest (SSSI) due to the diversity of waxcap fungi discovered growing on them.

[6][20] WPD appealed against this listing stating the Countryside Council for Wales's (CCW) decision was 'premature, arbitrary and unfair',[21] but in January 2007 a High Court Judge upheld the SSSI designation and said it was 'an important site'.

In March 2010, work started to drain the legal limit of 3 metres (9.8 ft) of water[24] from the reservoir into the nearby Nant Fawr stream.

[18] In May 2010, WPD applied to the Environment Agency for a discharge licence in order to completely drain the reservoir, which was granted in July 2010.

[44][45] In March 2011, WPD applied to Cardiff County Council for planning permission for a miniroundabout on Lisvane Road and listed building consent to alter the structure of the reservoir.

In tandem with refilling of the reservoir, redevelopment works at the site included the construction of a two-storey visitor centre and restaurant, nature trails, and bird hides.