Longest of the rivers in the Geopark, the Usk is especially important as a haven for wildlife.
The entire length of the river (Welsh: Afon Wysg) downstream of Usk Reservoir has been designated a site of special scientific interest (SSSI) and special area of conservation (SAC) . The designation is in recognition of its importance for various fish species – three species of lamprey, the twaite shad, the bullhead and the Atlantic salmon. It is also important for its otter population.
With little development along its length, the quality of the Usk’s water is high and there is relatively little disturbance from man. The designation extends to most of its tributaries making the entire catchment a very valuable one for Welsh wildlife.
The river rises as a small stream at Waun Lwyd some 500m up on the northern flanks of the Black Mountain. Flowing north it soon enters the Usk Reservoir, dammed in 1955 to supply water to Swansea. The emergent river flows east through Cwmwysg, passing Trecastle, Sennybridge and Llanspyddid before reaching Brecon. Its course turns to the southeast as it leaves Fforest Fawr Geopark bound for Abergavenny, Usk and eventually the sea at Newport.
For part of its course the river has a rocky bed of St. Maughan’s Formation sandstones and mudstones. Other sections of the Usk valley are filled with river sands and gravels and with glacial deposits.
In fact the ice ages have caused the river to dramatically alter course at least once or twice. At Brecon it is thought that earlier versions of the Usk flowed to the north of the present route from Aberyscir via Cradoc and both north and south of Pen-y-crug at different stages.
A south-bank tributary of the Usk
The Afon Crai rises as Blaen-crai at the head of the major valley pirecing the uplands of Fforest Fawr west of Fan Gyhirych. Its upper reaches were impounded in 1907 by Swansea Corporation as Cray Reservoir in order to supply the populations to the south with water. Some of its water is accordingly diverted via tunnel into the Swansea Valley.
Downstream of the reservoir the Afon Crai flows by Felin-crai through Cwm Crai to join the Usk a couple of miles west of Sennybridge.
Draining the northern slopes of the Central Beacons.
The waters of the Nant Sere and Nant Cynwyn combine to form the Cynrig. The river flows north to join the Usk immediately to the east of Brecon. At Abercynrig the Environment Agency operate a salmon hatchery beside the river.
The river that narrowly avoided capture.
Rising as Blaen Senni in the deep glacial hollow north of Fan Nedd, it reaches the hamlet of Heol Senni by a meandering course through a broad flat-bottomed cwm. This great bowl attracted the attention of water engineers in the 1960s but the proposals for a dam here that would have flooded the village never came to fruition.
The river enters the Usk immediately downstream of the crossing which gives Sennybridge its name.
Greatest of the Usk’s south-bank tributaries.
Rising near Storey Arms, the river flows north and east through Glyn Tarell, past the village of Libanus, and gaining the waters of Nant Cwm Llwch and Nant Gwdi before joining the Usk on the western edge of Brecon.