The Llyn Brianne dam was built as part of the River Towy Scheme, about six miles downstream from the river's source. The reservoir holds over 13,400 million gallons when full, and supplies drinking water to much of south-east Wales.
The dam has a core formed of nearly 14 million cubic feet of clay, protected by rockfill to both faces, nearly 60 million cubic feet extracted from immediately local sources. It is the largest clay core dam in the world.
The dam crest is about 950 feet long, and about 300 feet high, the tallest in the United Kingdom. It is accessed by an unusual bridge (above) that crosses a monumental spillway, the largest in Europe, and a favourite with extreme kayakers. A second bridge (below) crosses the lower end of the spillway.
The reservoir has a maximum depth of approximately 275 feet, and a surface area of circa 530 acres. It is fed from a catchment area of roughly 22,000 acres. Just one unoccupied farm was inundated when the reservoir was filled.
The principal contractor was Wimpey Construction, and the dam was formally opened on 15 May 1973. The spillway was raised by about three feet in 1996 as part of construction of a hydroelectric facility at the foot of the dam. 4.6 megawatts of electricity are produced from the three turbines.
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