The Karth River is located mostly within The Reach. The 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.
The river passes close to Old Hroldan Inn, before going over a series of waterfalls and splitting in two. Several tributaries empty into the river, the biggest where Reachwater Rock is located. A canal also joins shortly downstream, as the river flows east of Markarth, before a smaller branch reemerges to the main river. Going through Karthwasten, the river enters a lake formation and continues towards Dragon Bridge, where it reaches Solitude.
It merges with the Hjaal River at its estuary and finally terminates in the Sea of Ghosts. For much of its course, the river flows through a deeply cut canyon that bisects the rocky hills of the Reach.