Information from an Archaeological Assessment of Penmaenmawr Reservoir Discontinuance, Gwynedd, conducted by Gwynedd Archaeological Trust in 2019:

'As specified in data supplied by Stillwater Associates Limited Penmaenmawr Mountain reservoir consists of two bodies of water, the upper an enlarged natural lake and the lower artificially created. The lake has an uncontrolled inlet from an unnamed stream and an 8m wide spillway leading to the Afon Gyrach. The lower reservoir has an earthfill embankment that is 4.5m high and 40m in length to the north, a 75m embankment to the west and a 30m embankment separating the reservoir from the enlarged natural lake to the south. It has a diversion channel from the upper reservoir overflow channel and a 450mm diameter pipe spillway to the Afon Gyrach. The construction date of the reservoir is unknown but must date to the period between 1889 and 1900 as is does not feature on the 1st Edition 25 inch map but does on the 2nd and 3rd Editions. The reservoir measures c. 30m long East to West and 75m wide North to South.'

RCAHMW, 15th January 2020