The Dwyryd rises in the hills to the north of Ffestiniog. At its most northern extent, water draining from Moelwyn Mawr drains into the Tanygrisiau Reservoir, the outflow of which forms the source of the Afon Goedol. This is joined by the River Bowydd at grid reference: . Below Rhyd y Sarn, the river is joined by the Afon Cynfal which flows from the east down a deep wooded gorge which includes the spectacular Rhaeadr Cynfal (Cynfal waterfall) south of Ffestiniog. The main river then flows through a wide valley formed by glaciation, with a broad flat base formed from glacial moraines and riverine gravel deposits. The valley, the Vale of Ffestiniog, has much agriculture but is subject to routine winter flooding.