The dam was intentionally designed to stay mostly submerged to avoid looking like a traditional dam wall, maintaining the aesthetic of a natural lake.
It was constructed as part of a massive project to supply fresh water to Birmingham, located over 70 miles away.
The structure carries the B4518 road, which is one of the few places in the UK where a public highway crosses directly over a reservoir dam crest.
The design incorporates 16 arches to allow the flow of water while maintaining the road above.
Construction required a dedicated railway line to transport materials and workforce to this remote Welsh valley.
Garreg Ddu Dam is a unique Victorian-era submerged dam located in the Elan Valley, Wales, designed to support the Caban-coch Reservoir while carrying a public road across its crest. It features a distinctive multi-arched design built from local stone, specifically engineered to remain partially submerged to create the illusion of a natural lake level. Completed in 1904, the structure serves as a critical component of the Birmingham Corporation Waterworks scheme, which supplies water to the West Midlands. The dam acts as a bridge, allowing vehicular and pedestrian access between the valley sides. Its architecture utilizes a combination of masonry and concrete, reflecting the civil engineering aesthetics of the early 20th century.
From the water's edge on the downstream side during high water levels to capture the arches.
Park at the nearby designated roadside pull-offs to walk safely across the dam.
Bring binoculars as the reservoir is a frequent site for red kite sightings.
Check local weather forecasts, as the valley can experience sudden changes in visibility and wind levels.
Attempting to drive across during peak tourist weekends when narrow road sections can become congested.
Stick to designated paths; do not climb on the masonry walls or dam parapets.
