Example: the Sanzu river from the Japanese myth, Hubur river from Egyptian myth, the river Styx and its rowman Charon, Vaitarna ruver in Hinduism.
Do you mean centralised root story that those cultures share, like the great flood?
A root story worth referencing is the Gilgamesh epic. For instance the story of Noa and the flood in the bible is taken from the character of Utnapishtim in the epic of Gilgamesh.