I can see both sides in the debate of geological accuracy vs 'it's fantasy and it doesn't matter'. It's true that one small river isn't going to ruin your suspension of disbelief, but in these days of in-depth discussions on internet forums, someone will notice and they will surely mention it, even if it's just so they can be a smartypants. Why start out with a problematic map if it can be avoided? And sometimes, even if you don't have great knowledge about geography, you can look at something and it just feels wrong. You may not be able to put your finger on the reason why, but you know something's off and it niggles...