Garreg-ddu Dam is a submerged dam on the lowest level of the Elan Valley Water Scheme (NPRN 96459), carries a roadway across the top and separates Garreg-ddu Reservoir (NPRN 309541) to the north and Caban Coch (NPRN 32518) to the south.
The top sill level of the submerged dam or weir allows any water above that level in Garreg-ddu and Caban Coch reservoirs combined to be drawn off through the Foel Valve Tower (the extraction point for all water in the Scheme; NPRN 32527) and also allows water to be drawn off from the other reservoirs should the level in Caban Coch reservoir (which is used for river compensation supply only) fall below the top of the Garreg-ddu sill.

In appearance it is a rock faced brown stone viaduct with five narrow arches at the north-eastern end and seven broader elliptical arches in the centre. The parapet has massive rock hewn capstones at the ends. A commemorative stone at the south-western end is dated 19th June 1900.

B.A.Malaws, RCAHMW, 25 November 2003.