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.