A river that marked the boundary between the kingdoms of Surluse and Logres; or separating Arthur’s Britain from Galehaut’s land of Sorelois in the Prose Lancelot. It could only be crossed by two bridges: the North Wales Bridge and the Irish Bridge, both of which were long, dangerous, and partially submerged. In the midst of the waterway was the Lost Island, where Galehaut had a residence.

According to the Livre d’Artus, the Assurne separated Britain from Gorre (Gore), which has similarities with Sorelois. R. S. Loomis thought that it was the river Severn.