The Garreg Ddu dam in the lower Elan Valley serves a dual role. It is a low, completely submerged dam which plays a vitial role in maintaining a constant supply of water to Birmingham. It also supports masonry pillars carrying the access roadway to the neighbouring valley of the River Claerwen. Garreg Ddu holds water back on the upstream side so that water can always be extracted at the Foel Tower. The bottom of Caban Coch Reservoir is too low to allow water to be gravity fed to Birmingham. Extraction from here would require pumps.
The submerged dam played its intended role of maintaining the flow to Birmingham in September 1937 after a similar period of exceptional drought when the water levels in the Elan Valley dropped alarmingly.
The original road leading to this valley was to be lost, along with many original buildings, with the completion of the Caban Coch dam and the subsequent flooding of the two valleys.