River Crossings

As we cycled alongside the Vltava towards Prague we enjoyed the river crossings. There were several ferries which all operated on the power of the stream, with the boat attached by pulley to a wire that went across the river. I could understand how the stream might pull the ferry across, but am still trying to work out how it managed to pull it back as well. There were also several examples of white water courses created for canoeists where the river lost height.