The Taf Fechan rises immediately to the south of Pen y Fan and runs southwards through three reservoirs and a former fourth, now abandoned. Whereas the Taf Fawr was the preserve of Cardiff Corporation, the Taf Fechan was used to supply water to Merthyr Tydfil Corporation. Of the Upper Neuadd and Lower Neuadd Reservoirs, the lower one was built first, authorised by an Act of Parliament obtained in 1876. Like those on the Taf Fawr, it had an earth embankment with a clay core. The clay had to be imported, and for this purpose a private branch line was built from the Brecon and Merthyr Tydfil Junction Railway near Torpantau railway station. The contract for the work was given to William Jones of Neath, and the project was completed in 1884. Now decommissioned, the water level when the reservoir was full was at above Ordnance Datum, and it held .