English: Garreg Ddu The hidden depths of Garreg-ddu dam

In the Elan Valley the second dam 'up-river' after Caban-coch doesn't really look like a dam at all. This is Garreg-ddu, which is sometimes called the 'submerged dam'. Garreg-ddu plays a more important part in providing a constant supply of water to Birmingham than you would guess. The part you can see above the normal water level, as in the photograph shown here, just looks like a narrow road bridge made of stone, with many arches. The water supply from the Elan Valley goes over 70 miles to Birmingham by a special pipeline called an 'aqueduct'. It travels all that way 'downhill' by gravity without being pumped, but to do that it has to start from a higher level than the reservoir at the far end.

The water enters the aqueduct through a special valve tower just above this submerged dam. The underwater dam does the job of keeping the Garreg-ddu Reservoir water level high enough to keep the flow going at all times.