They were shipped in special ships, build only to sail on these specific rivers. But it would be a waste to create a ship for every river, maybe one for all? And then a blockade at the delta, so other ships cannot enter the river? In this case we could represent that only residents of the river could built some ships.
A ford can only be used to cross a river (this should be like it is) but these ships could be used to travel alongside the river to carry troops from the delta to the origin e.g., and to blockade some fords.
Forget it, a crazy idea, i even can't express to myself
XSamatan
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