Flowing out of the eastern forest, the Egelsee River marks most of Mendev’s southern border with Numeria, although the frequent raids across the river in Mendev’s past have ebbed since the crusaders’ arrival. For a relatively short river, its strategic importance to Mendev is immense, and not solely as a source of pure water and fisheries for the capital at Nerosyan. More importantly, the Egelsee flows through hills infused with nexavar, and this unique mineral actually helps to purify the Sellen River as their waters commingle. In fact, it was through observing this strange phenomenon that the Magisterium of the Second Crusade discovered the existence of nexavar and its uniquely salubrious effects, incorporating it into the Wardstones that help to contain the spread of the Worldwound. Because of the constantly corrupting presence of the taint, the Sellen as far as Storasta remains an unhealthy flow prowled by aquatic horrors. The nexavar-laden silt carried along by the river mediates the taint somewhat, and more importantly actually eradicates unnatural residents once the river veers south, beyond the reach of the taint — a fact for which southern nations should be eternally grateful.
Sources: Pathfinder Chronicles Campaign Setting