The Thundering River rises in a northern branch of the Kodar Mountains just west of the Mines of Hope Lost in Irrisen. From there it meanders some 220 miles west gradually turning north-west and then north. It acts as the border between Irrisen and the Lands of the Linnorm Kings for the first half of its course. Twenty miles after the river leaves the border and enters the Linnorm Kingdom of Southmoor it meets its tributary, the Summermelt River within the Grungir Forest at Delmon's Glen. After travelling through the forest, the river becomes the border between the Linnorm Kingdoms of Grungir Forest and Hagreach for about twenty-five miles. Finally, for its last thirty-five miles, the river is the border between Hagreach and the Thanelands before it flows into the mighty Rimeflow River as a tributary. On its journey the river passes the settlements of Whiterook, Delmon's Glen, Losthome and Eldentre.123
For additional as-yet unincorporated sources about this subject, see the Meta page.