The Afon Crai rises as Blaen-crai at the head of the major valley pirecing the uplands of Fforest Fawr west of Fan Gyhirych.  Its upper reaches were impounded in 1907 by Swansea Corporation as Cray Reservoir in order to supply the populations to the south with water.  Some of its water is accordingly diverted via tunnel into the Swansea Valley.

Downstream of the reservoir the Afon Crai flows by Felin-crai through Cwm Crai to join the Usk a couple of miles west of Sennybridge.