Afon Dysynni

The Afon Dysynni flows from the western end of Tal-y-llyn Lake in a southwesterly direction to its confluence with the Nant Gwernol at Abergynolwyn. From here it flows in a northwesterly direction to its confluence with the Afon Cader, from where it resumes its southwesterly direction, meandering through a broad valley. It is joined here by its tributary, the Afon Fathew, which flows from near Dolgoch and joins it to the east of Tywyn, near Bryncrug. The river enters Cardigan Bay through the Broad Water lagoon to the north of Tywyn, near Tonfanau.

The river is interesting geologically because it exhibits river capture. This is likely to be as a result of river damming during a glacial period. The upper course of the river follows a fault line but at Abergynolwyn must have been blocked by a major landslide, the scar from which can still be seen on the side of the valley. The valley below here is blocked by debris on a rock base. The Afon Dysynni was thus forced to adopt a new course, and pushed its way through the hills northwestwards to find a new course in the parallel valley to the north.