The Sellen River is one of the most heavily traveled waterways in eastern Avistan and plays an incalculably important role in the trade and culture of all the nations of central and northeastern Avistan. Originating in the Lake of Mists and Veils north of Brevoy, the Sellen flows south through Numeria and the River Kingdoms (where it branches into countless fingers winding through the marshy countryside), and along the borders of Kyonin, Galt, and Andoran, finally pouring into Star Bay and the Inner Sea at the Taldan port of Cassomir. It is easily one of the longest rivers on Golarion.
In Numeria, the Sellen River borders the capital city of Chesed, which was constructed on the Eastern side of its mouth. The river branches near the border of Numeria with the River Kingdoms, and the city of Hajoth Hakados is constructed at the border.
Fed by dozens of tributary rivers, high mountain lakes and thousands of streams, the Sellen moves more water than all the other rivers that touch the Inner Sea combined. For this reason it doesn't show typical flooding and dry seasons like most rivers, but flows as faithfully as the sun rises.
In Taldor, the Sellen that flows south along the borders between Taldor and Andoran, goes through the Verduran Forest, meets a large tributary, which is called the Verduran Fork River, at the center of the forest, and ends its 1,200-mile journey into Inner Sea.
Trade along the countless branches of the Sellen River provides the lifeblood of most of the nations of central and northeastern Avistan. Plying the waterways of what is known as the Sellen Passage, the river traders connect such inland nations as Ustalav and Brevoy with the warmer, more populous nations surrounding the Inner Sea.