The Taf Fawr rises below the peak of Corn Du, south-west of Pen y Fan and soon flows in a generally southerly direction through a steep-sided valley, which was identified by Cardiff's Borough Engineer John Avery Brandon Williams as the best source of water for the town when he assessed all possible sources of water for the newly-formed Cardiff Corporation Waterworks in 1881. A bill was presented to Parliament in November 1883, and despite serious opposition from riparian landowners, Royal Assent was obtained for the Cardiff Corporation Act 1884 on 7 August. It allowed the Corporation to build three reservoirs.