Turns out it wasn't a then/than mix-up. Still needed correcting, though.

data/core/encyclopedia/geography.cfg

@@ -124,7 +124,7 @@ Over the River Aethen, south of Fort Tahn, is a Wesnothian frontier region. It i
     • Heartfangs: the particularly forbidding stretch of high peaks southwest of Lake Vrug and north of the Forest of Wesmere. The most inhospitable and dangerous portion of the Heart Mountains; only hermits, madmen, and mages live there.
     • Swamp of Dread: a very large bog located between the Heart Mountains and the Great River. A notoriously dangerous place.
     • Lake Vrug: A large mountain lake whose river carves the only pathway through the Northern Mountains.
-    • Arkan-thoria: The river then comes out of Lake Vrug. This is the elvish name; among humans it is called Longlier.
+    • Arkan-thoria: The river that comes out of Lake Vrug. This is the elvish name; among humans it is called Longlier.
     • River Listra: The south-running tributary of the Great River into which the Arkan-thoria empties.
     • Lintanir Forest: The southernmost portion of the Great Northern Forest, a gigantic wood whose eastern and northern boundaries are known only the elves. Their capitol, Elensiria, has only seldom been visited by humans.
     • Great River: The origin of this river is somewhere in the east of the northern lands."