For a boat to pass downstream or upstream through the Lock, the water inside the chamber must be at the same level as the top weir pool, to allow the gates to open. The top gates are opened by hydraulic powered arms to allow the boat to enter.
The gates are closed behind the boat and the water in the lock chamber is released by opening large hydraulically operated butterfly valves. The butterfly valves are only slightly smaller than an average house door.
Water flows from the lock chamber, via tunnels, to the weir pool below. As the water level in the chamber drops, the boat is lowered with it, until equal to the level of the lower weir pool. The bottom gates are then opened and the boat continues on its way. The water levels in locks are raised and lowered entirely by gravity; no pumps are used.
To travel upstream the reverse occurs. After the boat enters the lock chamber and the gates are closed behind it, valves are opened above the upstream gates to fill the chamber, via tunnels from the top weir pool.
Water enters the chamber from the tunnels through ports spaced along the bottom of each wall. This is to distribute the inflowing water to minimise turbulence in the lock chamber as it fills.
When the water level in the lock chamber is equal to the weir pool, the gates can be opened and the boat can proceed. It takes only 7 minutes to empty or fill the lock chamber but it normally takes 15 to 20 minutes to pass a boat through the lock. Six to eight medium houseboats can be accommodated in one lockage.