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Afon Melynllyn is the outflow from Llyn Melynllyn, a lake in the Carneddau mountains in north-west Wales. It is a tributary of Afon Dulyn, itself a tributary of the river Conwy.

53°11′00″N 3°55′42″W / 53.18336°N 3.92847°W / 53.18336; -3.92847