It flows from its source near Sanquhar in the Southern Uplands and joins the River Nith two miles southwest of Thornhill. The total length is 30 km (19 mi) . During its course it descends from 500m to 55m altitude, and forms part of the boundary between Tynron and Keir Parishes.
The river is renowned for troutfishing and canoeing, especially at the Glenmarlin Falls near Penpont. A local legend tells of the ghosts of a horse and rider who drowned in a deep pool, known colloquially as the Black Hole, at the bottom of the falls.