I think you'll have to name them yourself Emirikol.
Can I offer a suggestion: River Aarnau, and river Buuren (or Manaan's river)
according to the 1st edition Marienburg: Sold Down the River, the northern river would pass through or close by the frontier town of Aarnau. What's interesting of that town (pop. 4 000) is that its the site of the last remaining Old Noble house of Wastelands, the van Buurens (I suspect you know this allready Emirikol, planning an adventure and everything). There Baron Martinus van Buuren lives his days in past glories with his 150 men strong militia, hunting, and taking his self-appointed role as the guardian of Wasteland's independence quite seriously.
the other river Buuren, would run some miles north of Fort Solace (pop. 300 owned by Wasteland Import-Export Exchange). Ships reaching the Manaanspoort Sea at nightfall often stop there before making the final journey to Marienburg. So another suggestion of that "W" river would be Manaan's river.
Emirikol If you don't possess or are able to get the book I'm referring to, I be more than happy to email a selection of the book's contents to you.