The picture below shows a late stage in the building of the dam wall in the Vyrnwy valley. Large blocks of stone were being used for the first time in Britain to build a dam on this scale. You can see how large the dam is by the tiny figures of the people in the picture.
The row of arches along the top of the dam wall are being finished at this point, and huge steam-driven cranes are being used to lower the blocks into place.
The openings at the top of the dam are to allow surplus water to spill over the top and pour down the other side like a waterfall when the lake is full.

The dam is 26m high (84ft) from the bed of the lake to the sill for the overflow, but almost twice that if measured from the buried foundations to the top of the final structure. The dam is 357m long (1172ft), and the base is 36.5m thick (120ft).
The stepped openings at the bottom of the wall in the large picture above contain tunnels which can be controlled to allow enough water to carry on down the river so that it continues to flow normally. The machinery for operating the valves is located inside the two stone towers on the top of the dam wall above the two tunnels. This water supply is called 'compensation' water. One of these outlets is shown in the bottom picture.

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