Garreg-ddu Reservoir is part of the Elan Valley Water Scheme (nprn 96459). Bounded on the south by Garreg-ddu submerged dam (nprn 752). In the reservoir to the west and north of the dam are, respectively, the easten outlet of the Dol-y-mynach Tunnel (nprn 261172) carrying water from the Claerwen River, and the Foel Valve Tower (nprn 32527) which is the extraction point for all water in the Scheme.
Garreg-ddu Dam separates Garreg-ddu Reservoir 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 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.
B.A.Malaws, 25 November 2003.