How Do Canal Locks Work? Lifting a Boat With Nothing but Water

Water finds its level, so a canal must be flat, and the countryside is not. Early canals dealt with slopes by flash locks, single gates in a weir that were opened to release a surge on which boats rode down and were hauled up against, wasting a reservoir of water each time and drowning people regularly. The pound lock, with two gates enclosing a chamber, or pound, appeared in China in 984 on the Grand Canal and in Europe in the Netherlands and Italy in the fourteenth and fifteenth centuries. It uses only one chamber's worth of water per passage, holds the boat still while the level changes, and can be built in flights, one after another, to lift a canal over a hill; the Caen Hill flight in Wiltshire has sixteen in a row.

Suppose a boat is going uphill:

• •The lock is emptied to the lower level, if it is not already, and the lower gates are opened
• •The boat enters the chamber and the lower gates are closed behind it
• •The paddles, sluices in the upper gates or in culverts around them, are opened, and water runs from the upper canal into the chamber
• •The boat rises with the water until the chamber is level with the upper canal, which takes a few minutes
• •The upper gates are opened and the boat leaves; the water that lifted it stays in the chamber until the next boat going down releases it

The gates are the clever part, and Leonardo's contribution. Rather than a single gate across the chamber, each end has a pair of gates hinged at the sides that meet in the middle at an angle, pointing towards the higher water, like a shallow V or a mitre joint in carpentry, which is why they are called mitre gates. The pressure of the deeper water pushes the two leaves against each other and seals them, so that the greater the difference in level, the tighter the gates shut, and they cannot be opened until the water on both sides is equal, which is a safety feature as much as a convenience. The gates are timber, oak traditionally, or steel on larger canals, and they are opened by a person leaning on a long balance beam or, on modern waterways, by hydraulic rams. The paddles that let water through are lifted with a windlass, a cranked handle that every canal boat carries.

Every boat that passes through a lock uses a lockful of water from the summit of the canal, which is why canals have reservoirs at their highest points and why a dry summer closes them. A narrow lock on an English canal holds about 250,000 litres; the locks on the Panama Canal use around 200 million litres per transit, drawn from an artificial lake fed by rainforest rivers, and the drought of 2023 cut the canal's traffic by a third. Engineers reduce the waste with side ponds, which store half the water from an emptying lock for the next filling, and the new Panama locks of 2016 save sixty percent that way. Where water is short altogether, the alternatives are lifts, such as the Anderton boat lift in Cheshire, which raises boats in a water-filled tank on a hydraulic ram, and inclined planes that carry the boat in a cradle on rails; the Falkirk Wheel in Scotland rotates two boat-filled gondolas 24 metres in a single turn, using the power of a few kettles, because the two sides always balance.

The largest locks in ordinary use lift ships over the spine of Central America. The Panama Canal is not a sea-level cut but a staircase: three locks on the Atlantic side raise a ship 26 metres into Gatun Lake, it steams across the lake and through a cutting, and three locks on the Pacific side lower it again. The original chambers of 1914 are 320 metres long and 33.5 wide, dimensions that fixed the size of ships for a century and gave the word Panamax to shipbuilding, and the third set opened in 2016 takes vessels 366 metres long. The newer, larger chambers opened in 2016 use rolling gates that slide into recesses, while the original chambers keep their mitre gates, and the ships are held in position by electric locomotives on the walls. Fourteen thousand ships a year make the passage, each taking eight to ten hours to be lifted, carried and lowered by water that came from the sky.

A lock raises a heavy boat without any extra effort because of Archimedes' principle. A floating boat pushes aside exactly its own weight of water, so a chamber holding a fully laden narrowboat weighs the same as the same chamber filled to the same level with no boat in it. The water flowing in from the upper level does not have to lift the boat as a separate load; it simply fills the chamber, and the boat rides up on it. The same principle is why the gondolas of the Falkirk Wheel always balance, whether they carry a large boat, a small one or none at all, provided their water levels are equal.

The commonest danger in a lock comes from its own structure. Below the upper gates, the floor of the chamber has a ledge of stone or concrete called the cill, on which those gates close, and it is hidden under water when the lock is full. A boat going down must stay well forward of it, because if the stern drifts back while the water drops, the hull can catch on the cill, tip steeply and flood within moments. Canal authorities mark the cill's position on the lock side, and boaters are taught to watch the stern throughout the descent and to drop the paddles at once if the boat begins to hang.

Many English locks were sized by money and water in the eighteenth century. When James Brindley and other engineers laid out the canals of the Midlands from the 1760s, they built locks only about seven feet wide and seventy-two feet long, roughly 2.1 by 22 metres, because a narrow chamber was cheaper to dig and used far less of the summit's water for each boat. Boats were built to fit, which is why the traditional narrowboat is long and slim and carried a cargo of around twenty-five to thirty tonnes, and why a boat built for those canals today still has to obey the same limits.

The choice made sense at the time and proved costly later. A single horse could pull a loaded narrowboat far more easily than a cart, so the canals slashed the price of moving coal, pottery and stone and helped feed early industry. But the narrow gauge meant that larger, more efficient barges could not use much of the network, and when railways arrived in the 1830s and 1840s the canals could not match them for speed or volume. Widening hundreds of locks was too expensive, so most commercial traffic dwindled, and much of the network survived because leisure boaters rediscovered it in the twentieth century.

Locks are not the only way to move a boat between different water levels. In Scotland, the Forth and Clyde Canal and the Union Canal were once linked by a flight of eleven locks that took most of a day to pass through. The locks were removed in the 1930s, and the link was restored in 2002 by the Falkirk Wheel, the world's first rotating boat lift.

The wheel carries boats in two water-filled gondolas at opposite ends of a pair of arms. Because a floating boat displaces its own weight in water, each gondola weighs almost the same whether it holds a boat or not, so the arms stay balanced and very little energy is needed to turn them. A crossing takes only a few minutes, and the wheel has become a tourist attraction in its own right.

A canal lock is a chamber with gates at both ends in which a boat is raised or lowered by letting water flow in from the higher level or out to the lower, so that it floats up or down to meet the next stretch. The mitre gates, which meet at an angle pointing upstream, are pressed shut by the water they hold back, the paddles control the flow, and every passage spends a lockful of water from the summit, which is why side ponds, lifts and wheels exist and why the Panama Canal, a staircase of locks fed by a rain-filled lake, slows in a drought.