Rivers also run east and west, in case he forgot to mention that
In fact, rivers run downhill, always, forever, never up. So you can actually run your river wherever you want it to run, as long as you put some highlands (mountains, hills, etc) around it to keep it flowing downhill.
Even in flatlands, rivers are running downhill, but on very gentle slopes, such in a plain, they also meander around a lot. Rivers in high country, where the slope is steeper, tend to be less meandering.
When I have a client who wants a river in a particular place, I build the mountains and hills around to make the river run naturally. An example here:
http://www.cartographersguild.com/at...0&d=1414787311
When that map was first posted, someone remarked that the river from Big Tern Lake should flow to the nearest seacoast instead of running all the way south to Wheatly Bay. I pointed out that I had added hills and a region name "Golden Highlands" precisely to justify that flow.
Hope this helps