The upper Afon Giedd drains the western slopes of Fan Hir, its source just south of Bwlch Giedd which divides Fan Hir from Fan Brycheiniog.  The river then flows south to enter a section of valley cut through the Carboniferous Limestone outcrop which is often dry.  In wetter weather the waters finally enter the ground at three or four spots at Sinc y Giedd / Sinc Giedd. 

It was once thought that they re-emerged further down the Giedd valley but it is now known that they make their way south-eastwards to emerge at Dan-yr-Ogof.

The lower Afon Giedd occupies the same valley as the upper Giedd but is not hydrologically connected with it. The river is joined by the Nant Cyw and then shortly joins the Afon Tawe at Ystradgynlais.

Virtually the entire length of the river can be followed by road, track or access land - though certain sections are difficult of access and best skirted around.