The dam is 358m long, with a maximum height in the centre of 44.2m from foundation to crest. The body of the structure consists of irregularly shaped stone blocks, each up to 10 tonnes in weight. A network of drainage tunnels built into the foundations, and two transverse discharge tunnels through the base of the dam, relieve pressure using hydraulically operated valves. The discharge tunnels release compensation water into the Afon Vyrnwy so that the river does not dry up. Only one of the discahrge tunnels was working on that day. The reservoir has been depleted during the drought in the summer, but is now filling up almost daily.
About two thirds of the way along the dam you may spot some High vis jackets. Workmen were repairing the cracks in the dam. A pair of them can be seen in the PiP.