The Usk Reservoir is located in the upper Usk Valley, at 1,050 feet (320 m) above sea level, in Carmarthenshire, southern Wales. The reservoir is situated in part of the Brecon Beacons National Park. It is situated to the western end of the park, sitting below the Black Mountain (range), and most of the summits in the range are visible from the reservoir, especially (from left to right in the image) Fan Hir, Fan Brycheiniog, Fan Foel, Picws Du and Waun Lefrith. It lies about six miles to the north of the centre of the escarpment at Fan Foel, and is an important landmark for walkers on the mountain range.
Completed in 1955 by Swansea City Council, the dam is approximately 100 ft (30m) in height and 1600 ft (480m) in width. It was the first example in the UK of an earth dam with horizontal drainage blankets. The reservoir covers some 280 acres (110 ha) of land. It can be located just off the minor road between Trecastle and Llanddeusant, 7 miles (11 km) west of Sennybridge. The reservoir is now owned by Welsh Water and the surrounding woods by the Forestry Commission. Most of the area is open to walkers and cyclists via forest tracks and other footpaths. There are several small car parks for visitors. The reservoir is stocked with fresh water fish such as trout and can be fished with an appropriate licence.