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. Each chamber has rolling gates that slide into recesses rather than mitres, 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 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.