As I haven´t played a full game from beginning to the end, that was exactly the thing I wondered about, i.e. if there is a technology that allows you to build bridges (i.e. "real" bridges, not only fallen tree longs over the river).
I agree, if this ability isn´t in the game (yet) it should definitely be introduced.
Maybe as a construction project, that can be initiated for river tiles within the borders of your empire and then be executed by the nearest town