Except they /do/ flow from highlands to lowlands. If I may:
The Serpent River's starts out in the highlands to the north and west, passing through the valleys and collecting mountain runoff and emptying into Lake Ban, from where it splits off to the Aras and Scarlet Seas. Granted, it would be easier to see if the map was in full relief, but its apparent from reading the other sourcebooks. A better question might be the presence of
jungle at those latitudes (better seen if you have a map of the whole Selestrean Basin, as in the
Theran Empire sourcebook), but the answer is simply the presence of Death's Sea creating a tropical climate in the area.