The dam at Pen-y-garreg seems like the ‘middle dam’ of the four dams in the Elan Valley. You can’t usually have a middle one out of four, but the ‘submerged dam’ of Careg-ddu normally has surface Pen-y-garreg dam in 2000water at the same level on either side, so it just looks like a bridge !
The row of openings you can see near the top of the dam wall are ‘windows’ to a tunnel which is used to get to the valve tower in the middle. When the reservoir is full the water pours over the top past these windows, and it is like looking out from inside a waterfall !

via Flickr http://flic.kr/p/D4tNwD