The reservoir which is seven miles west of Sennybridge was completed in 1955 to supply water to Swansea. It is 1050 feet above sea level and covers 280 acres. The site of Dol Hywel Chapel (NPRN 417756), thought to have been attached to Talley Abbey, now lies beneath the reservoir. A number of farms and smallholdings were also subsumed by the reservoir, including Pentre-bach, Pentre-bach-Gwern-wyddog, blaen-wysg, Pwll-y-gerwn and Blaen-y-ddol.

Sources include:
Ordnance Survey, 1887, first edition 25in
www.brecon-beacons.com/usk-reservoir.htm

Claire Parry, RCAHMW, 17 March 2011.