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Penygarreg* (Cronfa Ddŵr) is one of series of dams in Elan Valley built to supply water to the city of Birmingham between 1893 and 1904. The scheme is regarded as being amongst the greatest civic achievements of the late nineteenth century. The height of the reservoirs enabled water to reach the outskirts of Birmingham by gravity alone, without the expense of pumping (the top water level in the reservoir is 945 feet above sea level).

The dam and valve tower are jointly listed as Grade II* “as part of one of the greatest civil engineering projects of nineteenth century” (Cadw Record ID 16199 Link Historic Wales).

* The spelling “Penygarreg” is used on the OS map and by Historic Wales. A plaque (originally on the dam but now at the visitor centre) refers to it as “Pen-y-Careg Reservoir”.