[1] As its name suggests, it is located in the Brecon Beacons National Park in the Powys unitary authority area and within the historic county boundaries of Breconshire.

Williams presented his reports in May and August 1881, suggesting that the best scheme was to impound the waters of the upper Taff Fawr.

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

For Cantref Reservoir, a standard gauge railway had been constructed from the main line at Cefn-coed-y-cymmer to the dam site, some 6 miles (9.7 km) to the north.

Most of the extension to Beacons passed through land owned by Lord Tredegar, with whom separate negotiations were taking place.

For Beacons, they decided to use direct labour for the whole project,[9] and work began in April 1893, in advance of obtaining the Act of Parliament.

One unusual item purchased at the time was 500 yards (460 m) of 15 in (381 mm)-gauge "Decauville railway", complete with turntables, points and wagons, but its use was never recorded.

The valve tower is offset towards the east side of the dam, and there is a spillway at its eastern edge, crossed by a stone bridge.