A site for a new reservoir was chosen at the head of the Grwyne Fawr valley because at 1725 ft above sea level it was high enough to allow water to be gravity fed to Abertillery and the surrounding areas.

Large boulders weighing up to 5 tons each were set in concrete to form the dam, which is 130 feet thick at its base, the walls were then faced with dressed stone.

A 16-inch diameter steel pipeline was tunnelled through Coity Mountain carrying water to a holding reservoir in Cwmtillery.

[3] The dam was finally completed in February 1928, after which the railway and structures that served its construction were quickly dismantled, including the workers’ village.

Poor management of the surrounding catchment area has allowed peat sediment to wash into the reservoir, causing water discolouration, and it has proved uneconomical to remedy the problem.