Liftlock

Liftlocks are mechanisms that enable vessels to transit waterways, like canals, or canalized riverqs, when those waterways have a large change of elevation. Liftlocks are complicated than the much more common canal locks.

Canal locks have chambers that have water-tight doors at each end. At least one door is always closed. When a vessel wants to transit a lock, if the water level is already at the correct level, the closest door will be opened, the vessel will proceed into the lock, and the door will be closed. When both doors are closed then valves will be opened to let gravity adjust the level of the water in the chamber.

If the vessel is traveling upstream, valves open to left water in the upper portion of the waterway fill the lock. Once the level of water in the lock chamber matches that on the upper course of the waterway the upper doors can be opened, and the vessel can then proceed upstream.

Liftlocks, on the other hand, have vessels enter a basin, or "caisson", once its doors are sealed both the basin and the transiting vessel are raised and lowered together. The basin's rise and fall is always balanced by a counterweight. In some systems the counterweight is a twin basin that rises when it falls.

Liftlocks save water. An entire lock chamber full of water flows downstream every time a convention canal lock is used. But the only water flow with a liftlock is that displaced by the boats or ships it carries. And, if the displacement of the ships going upstream is the same as the displacement of the ships going downstream, no water is lost from the upper reaches of the canal system.

Liftlocks can also lift vessels more quickly over a large vertical distance. Three Gorges Dam on the Yangtze River, was completed with two pathways of five canal locks each, and it takes a vessel over four hours to transit through all five locks. The Three Gorges liftlock was completed almost a decade late, but once completed it could transit vessels in half an hour.

Marine railways and canal inclined planes

Related technologies are the marine railway, and canal inclined plane.

Marine railways

With a marine railway a vehicle like a very large railroad flatcar is rolled underwater. The vessel proceeds over the flatcar, and, once it is secured, the flatcar is towed out of the water to the other reach of the waterway. Like a liftlock, no water is wasted. They have the added advantage that, when the vessel is out of the water, its hull can be washed, to prevent invasive flora and fauna from traveling from one reach of the waterway to the next.

Canal inclined planes

Russia employs a few canal inclined planes on its large Siberian rivers. English mining companies employed canal inclined planes to lift small boats from one level of a mine to another. However modern mines employ narrow gauge railways for internal transportsion.