The Afon Mawddach (English: River Mawddach) is a river in Gwynedd, Wales, which has its source in a wide area SH820300 north of Dduallt in Snowdonia. 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.
The Mawddach has been the site of significant industrialisation and land management. Gold mining and subsequently gold panning have had major impacts but forestry, the preparation of animal skins, the storage of old munitions and the use of hill-sides as artillery ranges have all added to the legacy of pollution. The river is also very flashy - prone to very rapid rise and fall in level depending on rainfall. 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.