 FACT: When John is fleeing up the canal, he is obviously going  "upstream". He takes a narrowing, converging fork and the canal gets  smaller. When Arnie jumps the bike into the canal, he does it from a  point that would be dividing the rushing water rather than  converging it, hence, they are now inexplicably going "downstream".  A flood control system NEVER diverges the water, it only collects it  into bigger and bigger canals toward a common destination.