The Usymna River is one of the longest rivers in the world. At just under 5000 miles in length, it originates in the Great Scarp in southern Celobsil and flows north-west through the Coldfells and the Sjeivakh Lakelands until it reaches the Great Northern Sea at Icehell Bay. It has no major tributaries. The last thousand miles of its flow freeze over for four-six months of the year.
The only city or settlement of note anywhere on its length is Heddig. In the main, it flows past sparsely settled boreal country.