Llyn Trawsfynydd is a man-made reservoir covering approximately 1,180 acres, situated at around 700 feet elevation within Snowdonia National Park in North Wales. At this size it is slightly larger than Llyn Tegid (Bala Lake) and is considered Wales' largest man-made lake. The reservoir was originally constructed to supply water for the Maentwrog hydro-electric power station, a process completed in 1928. It was later used as a cooling water source for the Trawsfynydd nuclear power station, Britain's first inland nuclear facility, which operated until decommissioning began in 1991. Since closure, water temperatures have returned to natural levels, supporting regenerated plant and fish life. The lake averages around 12 feet in depth and features islands, large rocks, and submerged stone walls — features worth noting for paddlers navigating the water.

Access for paddlers requires a launch fee of £5 per craft, payable at the on-site café. Only rigid canoes, kayaks, and paddleboards are permitted; inflatables are not allowed. The postcode for navigation is LL41 4DT. Facilities at the site include parking, a visitor centre with café, and toilets and showers. For enquiries, the site can be reached by phone at 01766 540313 or 07501080788, or by email at llyntraws@gmail.com.

The reservoir sits within a landscape of mountain views characteristic of Snowdonia, with walking and cycling trails around its perimeter. A footbridge crosses the lake near the village of Trawsfynydd, and the surrounding area includes the Coed y Brenin Forest Park trails. The decommissioned nuclear power station is visible from the water and forms a distinctive, if unusual, part of the landscape. The lake is also a well-regarded fishing venue, regularly hosting Welsh fly fishing team trials, so paddlers should be aware of anglers — both bank and boat — sharing the water. Fish species present include brown trout, rainbow trout, pike, perch, and rudd.

The walk and cycling route around the lake is described as well-marked and popular with local users, suggesting moderate visitor activity, particularly during warmer months. The recommended walking season runs from February through November, which likely reflects the more accessible paddling window as well.