I'm aware of the historically importance of rivers for earlier civilizations. Some of the most significant economies of the past were built on the bank of a river.
I would like to know if:
Was it possible to navigate a river against the current on a medieval boat?
If it depends on the river, I'm more interested on the Douro river, starting at Oporto city (Portugal) and going to Spain (Valladolid). What would be the requirements of a medieval ship to get two people to make this trip?
If that was not possible, I would like to know if there was any value on walking along with the flow of the river, and if it was a common practice at the medieval time.