The Afon Llyfni is a small

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

North Wales North Wales ( ) is a Regions of Wales, region of Wales, encompassing its northernmost areas. It borders mid Wales to the south, England to the east, and the Irish Sea to the north and west. The area is highly mountainous and rural, with Snowdon ...

which arises as Nant Drws y Coed between

Mynydd Mawr is a mountain in Snowdonia, North Wales, 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 a ...

and Mynydd Drws-y-Coed just to the west of

Snowdon Snowdon (), or (), is a mountain in Snowdonia in North Wales. It has an elevation of above sea level, which makes it both the highest mountain in Wales and the highest in the British Isles south of the Scottish Highlands. Snowdon i ...

. The main river forms at the outlet of the

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". Llyn Nantlle Isaf (Lower Nantlle Lake) no longer exists as it was drained to protect the slate works.''The ...

and flows past the great

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

workings of

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 and is part of the community A community is a social unit (a group of people) with a shared socially- ...

picking up drainage from the very deep flooded quarries. It then passes to the south of

Talysarn Tal-y-sarn () is a village in the slate quarrying Nantlle Valley in Gwynedd, Wales, next to Penygroes. It is part of the community of Llanllyfni and includes some of Llandwrog. The ward had a population of 1,930 at the 2011 census, the bui ...

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 Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...

because of illegal and historical dumping of industrial

waste Waste are unwanted or unusable materials. Waste is any substance discarded after primary use, or is worthless, defective and of no use. A by-product, by contrast is a joint product of relatively minor Value (economics), economic value. A wast ...

in one of the deep, water-filled quarries.Excursions in North Wales: Including Aberystwith and the Devil's Bridge
William Bingley (1839) However, it appears that there has been little or no detectable quality shift.

References

Llyfni Llyfni {{Wales-river-stub