The Gwydderig crosses the boundary between Powys and Carmarthenshire at the village of Halfway. It joins the Brân in Llandovery, Carmarthenshire, about 1+1⁄2 miles (2.4 km) upstream of the confluence of the Brân with the River Towy. The Gwydderig therefore forms part of the Towy's catchment basin and drains into the Bristol Channel at the Towy estuary.