At a glance
Lower reservoir of the Dinorwig pumped-storage hydro scheme — a narrow lake below the vast Dinorwig quarry terraces, adjacent to Llyn Padarn in Llanberis. Dramatic industrial-mountain views. Not suitable for swimming (variable water levels). National Slate Museum nearby (closed for redevelopment until ~2027). LL55 4UB.
About Llyn Peris
Llyn Peris is a narrow ribbon lake at the foot of Dinorwig quarry in Llanberis — the lower reservoir of the Dinorwig pumped-storage hydroelectric scheme that operates inside the mountain above. The scheme uses the elevation difference between Llyn Peris and the upper reservoir, Marchlyn Mawr (about 500 metres above), to store energy: water is pumped up overnight during low demand and released back through turbines in the mountain to generate up to 1,728 MW at peak demand, within 12 seconds of a signal. The National Slate Museum at the quarry workshops and Electric Mountain's underground tours both sit at the lake's eastern shore, making this the most industrially layered lake in north Wales.
The quarry terraces above the lake — carved between the early 18th century and 1969 in purple-grey slate — form one of the most extraordinary industrial landscapes in Wales, rising in level steps from the water to over 400 metres. The National Slate Museum preserves the original quarry workshops, water wheel, and infrastructure. The combination of Victorian industrial heritage, Cold War-era engineering inside the mountain, and the Snowdon massif on the skyline gives Llyn Peris a concentration of interest unusual in a small lake.
The lake is separated from Llyn Padarn (to the north-west, where Llanberis village sits and watersports operate) by a low causeway. The A4086 runs alongside Llyn Peris with viewpoints and lay-bys. Llanberis is the departure point for the Snowdon Mountain Railway. The Snowdon Ranger Path starts from the valley beyond the lake.
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Frequently asked questions
Llyn Peris is the lower reservoir of the Dinorwig pumped-storage hydroelectric scheme — one of the fastest-responding power stations in Europe, built inside the mountain above the old Dinorwig slate quarry (officially Dinorwig Power Station, and once open to visitors as "Electric Mountain" until that visitor centre closed). The system works by pumping water from Llyn Peris up through tunnels bored in the mountain to the upper reservoir, Marchlyn Mawr, during periods of low electricity demand (typically overnight). When demand peaks — famously at the end of major television broadcasts when millions of kettles are switched on simultaneously — water is released from Marchlyn Mawr back down through the turbines in the mountain to Llyn Peris, generating up to 1,728 MW of electricity within approximately 12 seconds. The scheme uses the natural elevation difference between the two lakes (about 500 metres) to store energy on a massive scale.
Llyn Peris is not recommended for swimming. As the lower reservoir of the Dinorwig pumped-storage scheme, the water level can change rapidly and unpredictably when the power station operates — currents can form and water levels can rise or fall significantly in short periods. The lake is managed as part of the power generation infrastructure. Llyn Padarn, immediately to the west (the village lake at Llanberis), is the designated water sports and swimming lake in the area — Llyn Padarn Watersports operates kayaking, canoeing, and paddleboarding from the Llanberis shore. Llyn Padarn is also used for triathlons and open water swimming events.
The view from Llyn Peris is one of the most dramatic industrial-mountain landscapes in Wales — the vast slate quarry terraces of Dinorwig rise in purple-grey steps from the lake shore to over 400 metres above the water, covering the entire eastern hillside. The terraces were carved by generations of quarrymen between the early 18th century and 1969, when the quarry closed. The National Slate Museum occupies the quarry's main workshops at the lake shore. Beyond the quarry, the Snowdon massif closes the skyline. The narrow lake reflects the quarry terraces in still conditions. The A4086 runs along the north shore of the lake with viewpoints and lay-bys giving access.
Llyn Peris and Llyn Padarn are adjacent lakes separated by a low causeway — from the A4086, the two lakes appear almost continuous. Llyn Padarn (1.5 miles long) is the more northerly lake and the one on which Llanberis village directly sits — it is the setting for the Llanberis Lake Railway, Llyn Padarn Country Park, and the main watersports activities. Llyn Peris (1 mile long) is the more southerly lake, hemmed between the quarry terraces and the road, and is the Dinorwig power station's lower reservoir. Llyn Padarn is historically a natural lake; Llyn Peris has been modified as part of the pumped-storage infrastructure. Both are visually dramatic and both are overlooked by the quarry.
Walking around Llyn Peris is possible but limited compared to Llyn Padarn (which has the Padarn Country Park trail system). The A4086 runs along the north shore of Llyn Peris, with footpaths giving access to the lake edge. The National Slate Museum at the eastern end of the lake has paths through the quarry site. A longer walk follows the valley road beyond the lake towards the Snowdon foothills at Nant Peris (the original village, before Llanberis grew around the quarry). The Snowdon Ranger Path starts from the western shore and gives the most direct walking connection between the lake and the mountain. Most walkers use Llyn Peris as a view rather than a destination in itself, pausing alongside it en route to the quarry museum or Snowdon.