The waterwheel was seen as the most important power source in the world, from the Middle Ages to the end of the nineteenth century. When smaller streams became saturated, medieval engineers turned their attention to larger rivers, eventually leading to the development of the hydropower dams that still exists today. Lesser known are the intermediate steps toward that technology: boat mills, bridge mills and hanging mills. Boat mills had already appeared in 6th century Italy and spread all over the world. Most of them remained in use up until the end of the 1800s, with some of them surviving well into the 1900s.