At Maentwrog the Dwyryd becomes a long and sandy tidal estuary, flowing under the road and railway line at Pont Briwet, before joining with the estuary of the River Glaslyn and then entering into Porthmadog Bay. The Afon y Glyn which drains the south west 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.