Several important rivers rise in Fforest Fawr’s mountains as fast flowing streams.
Many of the rivers have complex histories, their flows having been interrupted by glaciers on more than one occasion, the ice deepening their valleys and diverting their courses. The Geopark’s rivers include:
- In the Towy catchment: Afon Sawdde, Afon Gwydderig, Afon Cennen
- In the Tawe catchment: Afon Twrch, Afon Giedd, Nant Llech
- In the west of the Geopark: Afon Llwchwr, Afon Aman
- In the Usk catchment: Afon Crai, Afon Senni, Afon Tarell, Afon Cynrig
- Those of the Neath catchment: Nedd Fechan, Afon Mellte, Afon Hepste, Afon Pyrddin, Sychryd
- The Taf Fawr & Taf Fechan in the east of the area
The upland streams are fast flowing and birds such as the common sandpiper and the dipper are relatively commonplace.
Other water features include a number of reservoirs and small glacial lakes such as
- Llyn y Fan Fach,
- Llyn y Fan Fawr
- Llyn Cwm Llwch.
These rivers and lakes also provide habitats for brown trout, spawning Atlantic salmon, the white clawed crayfish and otters.
Small lakes and ponds occur at places like Mynydd Illtud and on Cefn Llechid, sometimes occupying rock hollows though many have filled over the millennia to become bogs – equally valuable for wildlife but hosting a different mix of plant and animal species.