The River Rhythallt is a 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 Gwynedd

Gwynedd

Gwynedd is a county in north-west Wales, named after the old Kingdom of Gwynedd. Although the second biggest in terms of geographical area, it is also one of the most sparsely populated...

, 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...

 whose source is Llyn Padarn

Llyn Padarn

Llyn Padarn is a glacially formed lake in Snowdonia, Gwynedd, north Wales, and is an example of a moraine dammed lake. The lake is approximately long and at its deepest point is deep. At its south-eastern end it is linked to the neighbouring Llyn Peris...

. It flows in a northwesterly direction past the village of Brynrefail, Gwynedd

Brynrefail, Gwynedd

Brynrefail is a small village in Gwynedd, Wales that straddles the A4244 road roughly between Deiniolen and Llanberis.The village is in Wales the Caernarfon and the Gwynedd Council Ward of Penisarwaen.Brynrefail stands a short walk from the northern shore of Llyn Padarn lake...

 and changes its name to Afon Seiont downstream of Pont Rhythallt in Llanrug

Llanrug

Llanrug is a large village in Gwynedd, north-west Wales. It lies about 4 miles to the east of Caernarfon, 7 miles south of Bangor and 3 miles to the west of Llanberis. It is the largest populated village in the Arfon area of Gwynedd, Wales. The old name term of the village was initially...

. The Afon Seiont eventually reaches the sea at Caernarfon

Caernarfon

Caernarfon is a Royal town, community and port in Gwynedd, Wales, with a population of 9,611. It lies along the A487 road, on the east banks of the Menai Straits, opposite the Isle of Anglesey. The city of Bangor is to the northeast, while Snowdonia fringes Caernarfon to the east and southeast...

. The river has a broad flood plain which occasionally threatens homes in the village of Cwm-y-Glo.