The river continues southwards through a steep-sided wooded valley and is crossed by Pont Cwmyfedwen, an early- to mid-19th century single span bridge before it reaches Pentwyn Reservoir, the oldest of the four reservoirs. It was built by the Merthyr Board of Health between 1859 and February 1863, and the consulting engineer for the project was Thomas Hawksley, but despite this, the dam was built over a geological fault, which led to large volumes of leakage. This leakage was one of the main factors that led to the construction of the Lower Neuadd Reservoir. Remedial work was carried out at regular intervals, but never seemed to last for long. Repairs undertaken in 1912/13 reduced the level of leakage from which was sufficient until Pontsticill Reservoir could be built to impound the leaking water. The Corporation had obtained the Act of Parliament to authorise it in 1911, and awarded a contract to Sir Robert McAlpine & Sons on 16 June 1913. The contract price of £232,000 included various ancillary work including diverting the roads on both sides of the valley. The top of the earth dam was above the valley floor, and the reservoir was designed to hold , although this was subsequently increased a little by raising the water level by . Completion was delayed by the onset of the First World War, and when work resumed in 1922, the Corporation used direct labour. The Taf Fechan Water Supply Act had been passed in 1921, creating the Taf Fechan Water Supply Board, uniting the Rhymney Valley Water Board, the Pontypridd and Rhondda Joint Water Board, the Urban District Councils of Aberdare and Llantrisant & Llantwit Fardre, and Merthy Tydfil Corporation. The reservoir was opened by Lord Buckland of Bwlch on 21 July 1927.