Afon Dulyn (Welsh : Black Lake river) is the outflow from Llyn Dulyn, a lake in the Carneddau mountains in north-west Wales. It is a tributary of the river Conwy. Just below Afon Dulyn the river is joined by Afon Melynllyn, a small river flowing from neighbouring Llyn Melynllyn. The river does not carry a great deal of water because water is transferred from Llyn Dulyn to the reservoirs of Llyn Eigiau and Llyn Cowlyd.