The river that narrowly avoided capture.
Rising as Blaen Senni in the deep glacial hollow north of Fan Nedd, it reaches the hamlet of Heol Senni by a meandering course through a broad flat-bottomed cwm. This great bowl attracted the attention of water engineers in the 1960s but the proposals for a dam here that would have flooded the village never came to fruition.
The river enters the Usk immediately downstream of the crossing which gives Sennybridge its name.