The Rhondda Fawr has its source on the eastern side of Craig y Llyn south of Llyn Fawr in a spring called Ffynnon y Gwalciau at an elevation of about 544m OD [REF].
The Rhondda Fach rises about a mile to the east of the source of the Rhondda Fawr on the hills above Blaenrhondda in a marshy area between Mynydd Beili Glas and Mynydd Bwllfa at an elevation of about 489m OD [REF].