I'm fairly certain that rivers are all the same width that cannot be altered to accomodate ships and ports.

However, the OiM team (Ogniem i Miezcem) released several screens of their campaign map for the RTW version they were planning, where they used the 'sea' to make the wider parts of the rivers. (from the deltas upwards some way inland). Offcourse, this would mean that the rivers wont 'flow' and generally wont look as good. This would, however, let fleets sail some way up rivers and enable trade along rivers to some extent.