Usk Reservoir is a remote upland reservoir of approximately 280 acres located at the western end of the Brecon Beacons National Park (now Bannau Brycheiniog National Park), near Trecastle in Powys, Wales. Situated a few miles from the source of the River Usk, it lies beneath the Black Mountain and is surrounded by Glasfynnedd Forest and open moorland. The nearest postcode is LD3 8YF, approximately 1km east of the dam, and the reservoir is reached via the Llanddeusant road, signposted from Trecastle about 6 miles away. It appears on Explorer Map OL12 or Landranger Map 160 at grid reference SN 834 286.

Paddling access is restricted to organised groups only, through the Brecon Beacons Reservoir Passport Scheme, administered jointly by Bannau Brycheiniog National Park Authority and Dŵr Cymru Welsh Water, in association with the South Wales Outdoor Activity Providers Group (SWOAPG). Individual paddlers are not permitted to launch independently. The scheme covers canoeing, kayaking, and stand-up paddleboarding. Groups wishing to apply should contact the Bannau Brycheiniog National Park Authority headquarters in Brecon. Usk is one of four reservoirs included in the scheme alongside Pontsticill, Pentwyn, and Beacons Reservoirs.

The reservoir is owned and managed by Dŵr Cymru Welsh Water. Given its remote upland setting, paddlers should be prepared for exposed conditions typical of moorland reservoirs. The surrounding forest and moorland landscape give the water a quiet, isolated character, and the site is recognised as one of the National Park's darkest locations for stargazing, reflecting its distance from significant light pollution sources. A 5.4-mile waymarked walking and cycling route runs from the car park around the reservoir and across the dam, which may be of interest to groups combining paddling with other activities.

The reservoir is also a well-regarded trout fishery, so paddling groups should be aware that anglers are likely to be present on the water. Fishing rules prohibit activity near the dam face and in a conservation area on the north bank, which may be relevant to paddling routes as well.