The Afon Llyfni is a small river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in North Wales

North Wales

North Wales is the northernmost unofficial region of Wales. It is bordered to the south by the counties of Ceredigion and Powys in Mid Wales and to the east by the counties of Shropshire in the West Midlands and Cheshire in North West England...

 which arises as Nant Drws y Coed between Mynydd Mawr

Mynydd Mawr

Mynydd Mawr is a mountain in Snowdonia, North Wales, situated approximately west of Snowdon itself, overlooking Llyn Cwellyn and being the northern outlier of the Moel Hebog group. A popular ascent starts from Rhyd Ddu. On its western flank are the remains of ancient settlements and field systems...

 and Mynydd Drws-y-Coed

Mynydd Drws-y-Coed

Mynydd Drws-y-Coed is a peak on the Nantlle Ridge in Snowdonia, north Wales. It is located at the easterly end of the ridge, and is a subsidiary summit of Trum y Ddysgl. It has steep south facing cliffs, the summit area being the highest point on a thin ridge.The west side is very rocky and airy,...

 just to the west of Snowdon

Snowdon

Snowdon is the highest mountain in Wales, at an altitude of above sea level, and the highest point in the British Isles outside Scotland. It is located in Snowdonia National Park in Gwynedd, and has been described as "probably the busiest mountain in Britain"...

.

The main river forms at the outlet of the Llyn Nantlle Uchaf

Llyn Nantlle Uchaf

Llyn Nantlle Uchaf is a lake in the Nantlle Valley, Gwynedd, Wales. The lake forms the foreground in Richard Wilson's painting "Snowdon from Llyn Nantlle" ....

 and flows past the great slate

Slate industry in Wales

The slate industry in Wales began during the Roman period when slate was used to roof the fort at Segontium, now Caernarfon. The slate industry grew slowly until the early 18th century, then expanded rapidly until the late 19th century, at which time the most important slate producing areas were in...

 workings of Nantlle

Nantlle

Nantlle is a small village in the slate quarrying Nantlle Valley in Gwynedd, Wales. It lies on the north shore of Llyn Nantlle Uchaf....

 picking up drainage from the very deep flooded quarries.

It then passes to the south of Talysarn

Talysarn

Talysarn is a village in the slate quarrying Nantlle Valley in Gwynedd, Wales.The Welsh language poet Robert Williams Parry was born in 37, Station Road, Talysarn, where a plaque designed by R.L. Gapper commemorates the connection. Other persons connected with the village were Annant, quarryman,...

 and Penygroes before eventually discharging to the sea at Pontllyfni.

For a number of years there were concerns that the quality of the river might be polluted

Water pollution

Water pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....

 because of illegal and historical dumping of industrial

Industrialisation

Industrialization is the process of social and economic change that transforms a human group from an agrarian society into an industrial one...

 waste

Waste

Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...

in one of the deep, water-filled quarries. However, it appears that there has been little or no detectable quality shift.