The Eridanos River is a minor river in the Southern Vashnar Mountains that flows from the mountains north of Mount Piraeus down across the peak and through the valley of Actar. At the north end of Actar, the river is interrupted by a massive waterfall. The Eridanos supplies fresh water for both communities as it runs its course; it also serves the additional purpose of turning the water wheel that powers the cyclopean blade mill.
Fisherman ply the Eridanos's southern length around the Actar half, and it is here that the water spirit known as Naiad makes her home. In addition, beavers and king salmon populate the more mountainous northern half.