The River Glaslyn is a river which forms much of the border between Caernarfonshire to the west and Merionethshire to the east. It is 16 miles long, and one of the main rivers of both of its counties. It carves a verdant valley, until it enters the bay at Porthmadog and there in partnership with the River Dwyryd it forms the broad estuary that is Porthmadog Bay.
The River Glaslyn rises in Caernarfonshire, in Glaslyn, a lake in a corrie on the flanks of Snowdon. It heads generally south amongst the mountains and is joined by Nant Traswnant which drains Pen y Pass and by Nant Cynnyd before entering the beautiful lake known as Llyn Gwynant, in the heart of Snowdonia.
From Llyn Gwynant the river flows south-west to Llyn Dinas and at the foot of Llyn Dinas where its waters escape it meets Merionethshire, and marks the border from there to its mouth.
A little below the bottom of Llyn Dinas the river enters the village of Beddgelert. In the middle of the village it is joined by a substantial tributary, the River Colwyn which drains the south western flank of Snowdon. The river now much enlarged flows down past Gelert's grave before tumbling down into the Pass of Aberglaslyn. This is a small rocky gorge close to the main road which provides easy public access to a most picturesque piece of Snowdonia scenery.
At Porthmadog is a broad, flat valley, now called Traeth Mawr, which land was once the estuary of the Glaslyn. However, in the nineteenth century the Porthmadog Cob was built which sealed the mouth of the estuary off and enabling the land to be reclaimed. Below the reclaimed land, the river slips beneath the railway line and the estuary is permitted to begin, where the waters of the Glaslyn meander in large pools and marshes before eventually passing through the tidal sluices on the Cob at the south-eastern end of the town of Porthmadog, and from there into Tremadog Bay; the joint estuary of the Glaslyn and the River Dwyryd coming from the heart of Merionethshire. Between them the rivers create a broad tidal estuary cutting deeply into the land.
The first pair of breeding ospreys in Wales nest at Pont Croesor about four miles upstream from Porthmadog where the RSPB have set up observation facilities. They first bred in 2005 and have returned every year since.
Fishing on the Glaslyn up to Beddgelert is controlled by the Glaslyn Angling Association.