The Tumblestone is a major river in the northern Westerlands, and a tributary of the Red Fork of The Trident river system. It is somewhat north of Golden Tooth, which guards the only major pass through the mountains that form the border between the Westerlands and the Riverlands. The Tumbelstone flows east out of the Westerlands into the Riverlands, where it meets the Red Fork which flows up from the south. The great castle of Riverrun, seat of House Tully, is located at the point where both rivers meet. After the Tumblestone feeds the Red Fork at Riverrun, the Red Fork flows east to meet with the other two forks of the Trident.
In the books
The Tumblestone is the major river of the northern Westerlands. It is deeper than the Red Fork of the Trident. Riverrun is located where both rivers meet, between the fork of both rivers. The third side of Riverrun is fronted by a massive man-made ditch, which using sluice gates can be filled with water from the Tumblestone and Red Fork to create a moat during sieges, effectively turning Riverrun into an island.