It is 28 miles (45 km) in length, and is much branched; many of the significant tributaries are of a similar size to the main river.

The catchment area is bounded to the east by the Aran Fawddwy massif and to the west and north by the Harlech dome which forms a watershed just south of Llyn Trawsfynydd.

Rainfall can also be very heavy and it falls on very base-poor soils leading to episodes of strongly depressed pH.

Despite this, the river sustains an important salmon and trout fishery and the countryside through which it flows is some of the most spectacular and scenic in the UK.

The main tributaries starting in the west and working clockwise are: Below Dolgellau the river enters a wide and sandy estuary of great beauty.