The Upper Neuadd Reservoir is rather larger, and was authorised by an Act of Parliament obtained in 1895. The dam which is high was constructed of masonry, and some 100,000 tons were carried along an extended railway to the construction site. Although the contractors Holme and King of Liverpool were employed to do much of the preparatory work, the building of the dam was carried out by direct labour, with Merthyr Corporation's surveyor, Thomas F Harvey, acting as resident engineer. Work began in 1896 and finished in 1902. It holds , and with a water level at , it is one of a very small number of artificial lakes built above the contour. The dam has a small tower to the west, and a larger one to the east, below which a tunnel outlet built in Piranesi style sits at the base of the dam. Neuadd House, built as part of the reservoir project in 1896, is just downstream from the dam. An octagonal building at the east side of the Lower Neuadd dam is also a listed structure.