Llyn Celyn is a large reservoir in Gwynedd, Wales, situated within Eryri (Snowdonia) National Park. Constructed between 1960 and 1965 through the controversial flooding of the Tryweryn valley — which required the forcible removal of residents from the village of Capel Celyn — the reservoir carries significant historical and cultural weight in Wales. It covers approximately 320 hectares (790 acres), measures roughly 2.5 miles long by 1 mile wide, and reaches a maximum depth of 140 feet (43 metres), giving it a capacity of around 71.2 million cubic metres.

The reservoir is managed by Dŵr Cymru (Welsh Water) as a water supply facility, with its primary function being to support low river flows in the River Dee. Water released from Llyn Celyn flows into the River Tryweryn, passing through a small hydro-electric plant before entering a stilling basin and then the narrow, rocky Tryweryn valley below. That downstream section — not the reservoir itself — is the primary paddling destination in the area, hosting the Canolfan Tryweryn National White-water Centre, which provides facilities for international-level white-water canoeing and rafting.

Paddlers interested in white-water should note that water releases from the reservoir into the Tryweryn are managed, and some water is held specifically in reserve for scheduled white-water events. The best conditions for such events tend to occur during extended dry spells in summer, when the reservoir's role in augmenting the River Dee's low flows is most critical and controlled releases are most likely to be scheduled.

Ongoing infrastructure work at the dam site — the construction of an additional spillway — is expected to be completed by the end of 2025, with the car park at the dam entrance closed during this period. Once complete, additional recreational facilities including picnic tables and information boards are planned for the dam entrance area.

The reservoir itself holds wild brown trout and perch, and is controlled by Welsh Water. Paddlers planning to use the reservoir directly rather than the downstream Tryweryn should contact Dŵr Cymru regarding any access permissions or restrictions, as the site is primarily a water supply infrastructure.