River Styx

As one of the major planar pathways, the River Styx is a mighty river that flows through the Lower Planes, connecting them and providing a more conventional means of travel other than magic portals between these planes. It meanders through the top layer of each plane, as well as some tributaries and bends that touch on additional layers. 

However, beware the waters, for a single touch can erase memories.

River Styx

Planescape Canonical Campaign kenmarable