Our route has been following the Danube river, often right along the river banks. We'd been puzzling over why this ideal waterfront property hadn't been developed. Even when we passed through towns, the buildings are set back from the river bank with a wide promenade.
This type of boat seems to be the answer. A long time ago, it was the main cargo ship along the river, able to carry up to 20 tons. Downstream was (relatively) easy, but going upstream the ship was towed by horses. It took up to 60 horses to get a speed of 10 km per day!
Our riverside bike path is the old horse trail.