The Karth River is located mostly within The Reach. It flows northwards from the Jerall Mountains, passing to the east of Markarth, before flowing through Karthwasten, Dragon Bridge and beneath the great arch of Solitude.
It merges with the Hjaal River at its estuary and finally terminates in the Sea of Ghosts. Along the way it picks up several tributaries flowing down from the mountains to the west, including that passing through Markarth. For much of its course, the river flows through a deeply cut canyon that bisects the rocky hills of the Reach.
The Karth River's source can be traced as far back as Bard's Leap Summit, specifically to the large waterfall towering over the ruin. The waterfall itself can be traced back to somewhere within the Jerall Mountains.