In my last game session one of my players was surprised to discover the major river on the map flowed north. I'm baffled as to why he thought it was unique and why it's a popular misconception that river flow southerly...because they don't rivers flow downhill.
There are plenty of examples to smack the geographically and geophysically illiterate in the head with like THE NILE, it's over 4,000 mile long and flows from the south to the north and I'm sure people have heard about the Nile river before. How about the Amazon? It's pretty darned big and it flows from the west to the east. I'll say it again: rivers flow downhill.
Yup a pet peeve of mine. I just had to rant. Don't even get me started on the "rivers flow towards the equator" meme.
So how to use this in an RPG game? Make sure your rivers flow downhill unless they have a darned good reason to do otherwise.