For those asking for proof of whether warships ever actually sailed on these rivers, I wouldnt know for anywhere in the East, but certainly there were battles on the Nile in ancient times. Thucydides gives a decent account of such an event re the Athenian expedition in the mid 4th century BC. In the North, then the Vikings can provide sufficient evidence of which rivers were navigable by ocean going ships - although of course their ships were relatively "flat" in comparison to others.

This would add another interesting dimension to the game but it does raise a few questions for me.

1. How would the AI handle it?
2. Can a river be navigable and still have bridges/fords?
3. Are bridges over certain rivers meant to be realistic or actually just representative of ferries?
4. Can river access be limited to certain types of ships?
5. If this would require a lot of work, would it be worth it?