I was just playing around with Google Earth and found that the Prosna river, which is the river west of Kalisz on the travel map, is way narrower than I had expected. It looks to be no wider than a 20 or so meters at most points. Think more canal-width than major river. Might be worth considering in terms of implications for the game.
Put it this way - anyone who can swim could cross it easily. As for vehicles - the players could probably construct a raft or bridge with the right materials.