A stone revetted dam retaining the north-east end of Llyn Arenig Fach. The dam is intact and measures over 200m long by 5m wide and is up to 1.5m high from the current water level. There is a rectangular stone sluice housing in the center of the dam with the winding mechanism surviving on top. The sluice housing is 4m long by 3m wide around the shaft. A metal superstructure to the winding mechanism has fallen off and lies adjacent to it. There is an overflow channel one quarter of the way along the dam from the southern end. It is shown on both the 1st Edition and current OS mapping. P.J.Schofield, OANorth, 8th January 2009.