In the lock

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Two narrowboats are using the lock together, which is good practice because it saves not only time but water. Locks are simple devices that allow boats to proceed "upstream" and "downstream". Water can only flow in one direction, so every time a lock is used hundreds of gallons of water go "downriver". Water has therefore to be supplied at the high points of canal systems, either from reservoirs or by pumping. The Grand Union Canal is a "broad" canal, which means that it is able to take boats that are double the width of the narrowboats you can see here. There are not many broadboats around today, but formerly there were plenty - taking tons of coal, iron, clay and finished goods to factories and ports. A broad lock like this might be expected to take twice as long to fill as a narrow lock, but that is not necessarily so, as is proved here, where the "fall" between the top and bottom of the lock is relatively small. This stretch of the canal has a series of these locks spread across a mile or so, which would have made the traffic flow as fast as possible.