Garreg Ddu Dam also known as the submerged dam is the second of the Elan Valley Dams.
The completely submerged dam which plays a vital role in maintaining a constant supply of water to Birmingham. Garreg Ddu holds water back on the upstream side so that water can always be extracted at the Foel Tower.
Height of weir - 82 ft above river bed
Total capacity - 4,585 million gallons
Height - 123 ft above river bed
Length - 417 ft 6 in.
Total capacity - 1,320,000,000 gallons