The River Usk rises on the northern slopes of the Black Mountain, Wales, in the westernmost part of the Brecon Beacons National Park. Initially forming the boundary between Carmarthenshire and Powys, it flows north into Usk Reservoir, then east through Sennybridge to Brecon before turning southeast to flow by Talybont-on-Usk, Crickhowell and Abergavenny, after which it takes a more southerly course.