Careg-Ddu is in the lower Elan valley. It serves a dual purpose, it is a low, completely submerged dam and it also carries the road to the neighbouring valley when the original road was flooded. Cwm Elan House and its Ten thousand acres where the Poet Shelley was a house guest now lies at the bottom of the reservoir
The Foel Tower is the starting point of the 73 mile journey of the water from the Elan Valley to Birmingham. It was built in a Baroque style .
The Tower houses a system of valves and cylinders which can be raised and lowered hydraulically to draw water off from the reservoir, this water is then directed to pipes and is carried by gravity, pumping is not required because the network drops 52 metres (171 ft) along its 73 miles (117) km, a gradient of 1:2300 maintains a flow of less than 2 miles per hour (3.2 km/h); water takes one-and-a-half days to reach Birmingham.