While waiting for a lock to fill on a canal holiday in the Midlands I heard an onlooker speaking to the occupant of the adjacent boat. "I can see how the water fills the lock and raises the boat so you come out at a higher level, but where does all the water come from?" A fair question, I thought. The guy on the other boat thought awhile then replied "Well, boats go the other way as well you know" and bring the water uphill with them presumeably