Flowing entirely within Snowdonia National Park it rises beneath the peaks of Y Llethr and Diffwys within the Rhinogs mountain range and runs in a generally ESE direction towards Cardigan Bay.

Originally a natural lake, Llyn Bodlyn was enlarged in 1894 to provide Barmouth with water.

This old stone arch bridge ("probably 18th century") was once used by packhorses and drovers moving between Harlech and London.

[2] A further 3 km downstream the river enters a narrower wooded valley and is crossed by Pont Fadog.

An inscription on the bridge states that it was improved in 1762 by a mason called H. Edward, who was commissioned by William Vaughan, a member of the powerful owner family of nearby Cors-y-gedol hall (see Dyffryn Ardudwy).