River Dwyryd

There are a number of small lakes and reservoirs in the woodlands north of Plas Tan y Bwlch which also drain south into the river.

The Afon y Glyn which drains the southwest catchment from Llyn Tecwyn Uchaf and Llyn Tecwyn Isaf enters the southern side of the Glaslyn estuary at the south end of a large extent of salt marsh known as Glastraeth (green beach) on its south bank and opposite the village of Portmeirion.

The Afon Prysor is the largest tributary of the Dwyryd, entering its left bank in the tidal section downstream of Maentwrog.

Prior to construction of the dam in the 1920s for hydroelectric power, the river had wandered across a broad upland marsh here known as Cors Goch.

The Afon Prysor resumes its course below the dam, to flow down to the Dwyryd through the steeply wooded valley of Ceunant Llennyrch which is at the core of a national nature reserve.

At no point is the river deep enough to accommodate sea-going ships, but in the second half of the 18th century a number of quays were constructed west of Maentwrog from which small vessels took cargoes of timber and, increasingly, slate to be transferred to sea-going ships[2] in deeper water southwest of what would become Porthmadog, transferring to Porthmadog itself when its harbour was opened in 1824.