Inland river trade should be a very significant factor. If a province is connected to an adjacent province by a navigable river, it should greatly enhance its trade with that province, much more so than a road. The economy within a province should also be greatly enhanced by the presence of a navigable river. Real poverty should only belong to provinces that have no water communications -- roads help, but they are no substitute for water transport.
So perhaps the answer to inland trade is powerful river ports buildings where there is a navigable river. The river ports would provide increased trade and tax revenue.