Pen y Fan reservoir is a well preserved example of a canal feeder reservoir, built around 1794-6 as part of the engineering works for the Crumlin arm of the Monmouthshire Canal. Along with the reservoirs at Abercarn and Hafodyrynys, these reservoirs were used to maintain the water level in the canal which was heavily used and had a large number of locks on it.

The reservoir is formed by a large earth dam with stone facings, forming banks on three sides of a gentle slope. The outlet to the canal feeder forms a stone-lined, oval tunnel and a valve chamber is set into the bank above.
Source: Cadw scheduling description. RCAHMW 06.06.2008