It is located in the Brecon Beacons National Park, mostly in the Powys unitary authority area and within the historic county boundaries of Breconshire.

There were 16 petitions against it and serious opposition from riparian landowners, but several influential civil engineers was called to give evidence, notably Bateman, Thomas Hawksley James Mansergh, and George H. Hill.

[4] Initially the contract for construction of the earth dam, which was about 600 feet (180 m) long with a puddle clay core, was awarded to William Jones of Neath.

Jones succeeded in cutting the embankment trench and filling the bottom with a concrete key, but he lacked funds, and did not have enough pumping plant to keep the workings dry.

The reservoir was formally opened on 14 September 1892 by Alderman David Jones, who was chairman of the Waterworks Committee, and impounding of water began on the same day.

At its maximum, the water is 73 feet (22 m) deep, and the dam created a reservoir which is 0.75 miles (1.21 km) long, holding 323 million imperial gallons (1,470 Ml).

Unlike the other two reservoirs in the chain, the size as built was that specified by the original Act of 1884, whereas the capacity of Beacons and Llwyn-on was doubled when construction began.

[5] The remote location of Cantref Reservoir and the huge amount of material needed to construct the dam resulted in plans for a standard gauge railway to be built.

This left the London and North Western Railway/Brecon and Merthyr Tydfil Junction Railway joint line near Cefn-coed-y-cymmer station, and passed through a rocky gorge, where a trestle bridge was built to carry it over the Taff Fawr.

[11] Puddle clay for the construction of the core of the dam was obtained from land owned by the Crawshay Brothers, part of their ironworks near Merthyr Tydfil.

[7] One narrow gauge locomotive is known to have worked at the clay pit, an 0-4-0 saddle tank named Darrell, which was obtained second-hand from the Bargoed Coal Company of Fochriw around 1885.

It was probably obtained by Mackay, but passed into Corporation ownership when his contract was terminated, and was sold to the Crawshay Brothers in early 1897 for £150, becoming their Cyfarthfa 14.