Nowadays you see trucks moving massive logs on paved roads. It seems impossible to move logs that big without paved roads. How did they do this in the Middle Ages? Wouldn't the wheels get stuck in the mud? This wouldn't work well if you dragged them with horses or oxen either.
(Added): Tl;dr: The significance of river transport in pre-modern Europe is sometimes very underestimated.
Do you have any idea where medieval Europeans cut trees down before the import from other countries became common in the 20th century?
Probably somewhere in the forest near mountainous area.
Then, what connects the mountain with the densely populated lowland?
So, the simplest way to transport the log from the mountain was by the river networks.
The basic process (since the 12th or the 13th centuries) is as following:
Major cities in southern Germany (Bavaria) or Switzerland sometimes either purchased the land of the forest area in the upstream area of the river that went down to their city, or concluded a contract with the lumberjacks in the upstream mountains to achieve the stable supply of wood resources to the city that was important not only as building materials, but also as firewood. They also appointed special city officials to take care of these supplies.
(Added): This is an illustration of the raft discharging in Upper Rhein in ca. 1600.
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