 To help a boat get out of a lock (going downstream) the lockkeeper sometimes would give a "swell", that is, opening the paddle valves (wickets) on the upstream gates, and the water would flush the boat out.Some wily lock keepers would demand money from the boatmen for this "service".If a boat ran aground between locks, they would sometimes ask a passing boat (going upstream) to tell the next lockkeeper to give an extra heavy swell, by opening all the wickets on the upstream lock thus raising the water level temporarily, so that they could get unstuck.