Llwyn-On Reservoir is the largest and southernmost of the three reservoirs in the Taf Fawr valley in South Wales, sitting within the stunning Brecon Beacons National Park at an elevation of 260 metres above sea level. Built in the 1920s following authorisation under the Cardiff Corporation Act of 1884, this 150-acre (54 hectare) impoundment is fed by the Taf Fawr, Nant Gwinau, Nant Car, and Garwnant rivers, with outflow returning to the Taf Fawr. The reservoir holds a water volume of approximately 5 million cubic metres, with a mean depth of 9.4 metres and a maximum depth of 22 metres, making it a moderately deep stillwater with significant variation across its basin. The water is characterised as humic with moderate alkalinity, giving it a slightly coloured appearance typical of upland Welsh reservoirs. The A470 trunk road runs along the eastern shoreline, while the western half of the reservoir sits within Rhondda Cynon Taf. Bordered by mixed woodland including conifers, the setting is scenic and the reservoir is a firm favourite among local anglers throughout the season.