Llyn Peris has a circumference of approximately 5 kilometers, being roughly 1.8km long and 0.5km wide. It was once a natural lake, 'but was transformed into a curved reservoir 600m long and 40m high as part of the construction of the Dinorwig Power Station (NPRN 408885). The reservoir has a storage capacity of 7 million cubic metres with the Natural Bottom Water Level at 600m above sea level. During full operation of the pumped storage scheme (which powers the turbines within Elidir Mountain), the water level of the lake will rise and fall 34m daily.' Work began on converting the old Dinorwic slate quarry (NPRN 40538) into a hydroelectric power station in 1975, and it was fully comissioned in 1984.
During the construction of Dinorwig Power station, Llyn Peris lake was 'drained and lined with slate waste.' During the process two boats were uncovered (NPRN 240681 and NPRN 240682).
External source: 'Dinorwig Power Station: Marchlyn Mawr Reservoir' Archaeological Assessment prepared for Capita Symonds by Jphn Roberts of the Gwynedd Archaeological Trust, July 2006.
M. Ryder, RCAHMW, 4th October 2018.