The Afon Llyfni is a small

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

North Wales , area_land_km2 = 6,172 , postal_code_type = Postcode , postal_code = LL, CH, SY , image_map1 = Wales North Wales locator map.svg , map_caption1 = Six principal areas of Wales common ...

which arises as Nant Drws y Coed between

Mynydd Mawr Mynydd Mawr ( Welsh for ''big mountain'') 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 ...

and

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 west-facing cliffs, the summit area being the highest poi ...

just to the west of

Snowdon Snowdon () or (), is the highest mountain in Wales, at an elevation of above sea level, and the highest point in the British Isles outside the Scottish Highlands. It is located in Snowdonia National Park (') in Gwynedd (historic ...

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

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 of Llanllyfni Llanllyfni () is a village and a community in Gwynedd, Wales. ...

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 built ...

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 adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...

because of illegal and historical dumping of

industrial Industrial may refer to: Industry * Industrial archaeology, the study of the history of the industry * Industrial engineering, engineering dealing with the optimization of complex industrial processes or systems * Industrial city, a city dominate ...

waste Waste (or wastes) 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 economic value. A waste prod ...

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