River Vyrnwy

River Vyrnwy (Welsh: Afon Efyrnwy) runs from the Welsh mountains; its sources are from many and varied brooks and tributary rivers from around the lake. However, since it was flooded, the river starts at the foot of the dam and flows east towards England, eventually finding its way to Shropshire where it converges with the River Severn near the village of Melverley on the Welsh border. The river runs for 63.9 kilometres (39.7 mi); the last 13 kilometres (8 mi) forms a natural boundary between England and Wales. The River Severn then takes its course though England to the Bristol Channel.

311 brooks, waterfalls and rivers flow into the lake and are named after the mountains or hillsides they flow from. Some are no more than a trickle, while others cascade down the mountains. The main ones, clockwise from the west side of the dam, are named Afon Hirddu, Eunant, Afon Eiddew, Afon Naedroedd, Afon Cedig and Afon Y Dolau Gwynionew.

On the northern edge of the lake is a small hamlet called Rhiwargor where the rivers Afon Eiddew and Afon Naedroedd meet. Up the valley of Afon Eiddew is an impressive waterfall, one of the largest surrounding the lake. Known locally as Pistyll