Dwyfan and Dwyfach, sometimes also called Dwyvan and Dwyvach, in Welsh mythology, were the equivalents of Noah or Deucalion who take their names from small rivers, as told in a flood legend from the Welsh Triads. A great flood was caused by the monster Afanc, who dwelt in Llyn Llion (possibly Bala Lake). All humans were drowned except Dwyfan and Dwyfach, who escaped in a mastless boat. They built an imposing ship (or ark) called Nefyd Naf Neifion, on which they carried two of every living kind. From Dwyfan and Dwyfach all of the island of Prydain (Britain) was repeopled. Dwyfach appears to take her name from the small Dwyfach (Welsh: little Dwy) River of Gwynedd (until 2018, Caernarvonshire) that flows into Cardigan Bay; Dwyfan would then derive from the river it enters, the Dwyfawr or Dwyfor (Welsh: great Dwy).
Source: Dwyfan and Dwyfach – Wikipedia