Llwyn-On Reservoir is a 150-acre body of water located in the Taff Fawr Valleys area of Wales, within Bannau Brycheiniog (Brecon Beacons) National Park. Built in 1926, it is the largest of the three Taff reservoirs and sits at coordinates approximately 51.799842, -3.441765. The reservoir is set amid mixed woodland and conifer forest, with a shoreline walk of approximately 3.4 kilometres available to visitors. The surrounding landscape supports notable wildlife including otters, great crested grebes, goldeneye, goosander, and coot.

Paddling is permitted on the reservoir, though access is restricted to approximately one third of the lake. Within that accessible area, there are reported to be some interesting coves and inlets to explore. Paddlers should be aware of this restriction before planning a trip and should not expect to circumnavigate the full perimeter by water.

The Garwnant Visitor Centre is located nearby and provides facilities including toilet access. The Mountain Centre also has toilet facilities. These represent the primary amenities available to visitors in the area.

The reservoir is managed by Dŵr Cymru (Welsh Water) and is primarily known as a trout fishery, with fishing permitted using fly, spinner, and worm methods. Paddlers sharing the water with anglers should be mindful of this activity. The reservoir is described as having good access and is set in scenic surroundings.

The trail around the reservoir features a mix of flat and gently inclined terrain with both paved and unpaved sections. Some areas, particularly near the weir, can become muddy following wet weather, which may be relevant for paddlers accessing the water from the shore. The reservoir can be visited year-round, though wet conditions are common in this part of Wales, particularly in autumn and winter.