I always wondered about how could people rely so heavily on rivers - for war, trade and communication - when the rivers themselves are pushing to one diraction. I assume that the exact method changed from place to place and from era to era, but I would like to know specifically about the medivial world, in Europe, the middle east and central Asia. Was the method of sailing different from river to river? And did it have implications for the people using the river?
Well the wind isn’t following the current, and near the shore you’re guaranteed of an onshore wind most mornings.
If the wind is against you, you can tack, usually in a zigzag, although it’s hard work and in a lowtech boat it’s often easier to just row. Rivers curve, so you’ll often alternate between sailing with the wind and against it.
If the wind is with you, you’re fine, barring obstructions. You’ll always have a clash between the deep keel needed for good ocean-going craft and the shallowness of some rivers.
In developed areas, oxen, horses, and other beasts of burden can be - and were - used to pull boats and ships upriver.