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 ...

North Wales North Wales ( cy, Gogledd Cymru) 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, ...

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 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 (histori ...

. The main river forms at the outlet of the

Llyn Nantlle Uchaf Llyn Nantlle Uchaf (Upper Nantlle Lake) 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 ...

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 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 buil ...

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 because of illegal and historical dumping of industrial

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 pr ...

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