As we can learn from the romans:
The Rhine, Danube and Nile

I think these rivers were in history used to ship whole armies, others like Euphrat&Tigris, Themse and Loire only in very few cases.

A thought about the trading:

Could it be implemented, that these rivers (and a very small area of coast) belong to Terhazza?
This would allow to use not trading ports but instead markers which give the regions a bonus of trading?

XSamatan