After watching this video two times there has to be a cable running under the upstream ends of the pontoons that keep the whole setup from floating downstream.
This reminds me of the primitive Roman ferry across the Snake River at the southern end of Grand Teton National Park.
"Bill Menor built the original ferryboat and cableworks. Today’s ferry and cableworks are replicas. The ferry is a simple platform set on two pontoons. The cable system across the river keeps the ferry from going downstream, while allowing it to move sideways. By turning the pilot wheel, the rope attaching the boat to the cable is tightened and points the pontoons toward the opposite bank. The pressure of the current against the pontoons pushes the ferryboat across the river in the direction the pontoons point. This type of ferry existed in ancient times and was used elsewhere in the United States." - from the National Park website.
http://www.nps.gov/grte/historyculture/menors.htm
Rob