 It flows from Llyn Celyn in the Snowdonia National Park and after 19 km joins the river Dee at Bala.It is one of the main tributaries of the Dee and has been dammed to form Llyn Celyn.Water is stored in winter when flows are high, and released over the summer to maintain the flow in the Dee, meaning that the river often has suitable water levels for kayakers and rafters at time of year when natural rivers throughout Wales may be too low.