Anybody have an answer to Jack's question posted above?
I think the wording 'including the supplied space' means 'including the space you are coming from' this ensures a river line is being followed. That's how I interpret it as to do otherwise would fundamentally change the game. Using the words 'supplied space' suggests the original space (which is after all the one being supplied) which is where the confusion occurs as the first part says it does not need to be supplied. In fact it wont be in many instances before a fortification is built. And you can't build a fort in a non supplied space of course. This would mean you can't build a fort on a space connected via land only to another space with a river in it but you could if it was connected to any other type of supplied space, such as a fort. I do not think is the intention.
Also it might be useful to stress the rule that the supplied space and it's connection to the next space is not considered at all under any circumstances.
EDIT: To avoid any wrong impressions, this is one of my favourite games and the rules are well written, I just got stuck on that one