What if canals had no locks?

Water is flat on top. It always wants to be level.

But land has hills. How can a boat go up a hill? And what if canals had no locks?

Pour water into a tray and tip it: the water runs to the low end. Water always settles flat and level. That's a problem for canals, because the land they cross rises and falls.

Locks are the clever answer. They work like steps made of water, lifting a boat up or lowering it down one step at a time. So what if canals had no locks? In this lab we explore how the water steps work, what canals would have to do without them, and the giant locks of the Panama Canal.

A canal's water surface must be level along each reach, or water would simply flow downhill out of it. Where the ground changes height, a canal needs a way to move vessels between reaches at different elevations.

This thought experiment removes the lock and follows three threads: how a lock moves a vessel between levels, what a lock-free canal can and cannot cross, and how the Panama Canal's locks lift ocean ships over the Isthmus of Panama. Engineering facts come from the Panama Canal Authority, the Smithsonian and Britannica; the lock-free world is a set of reasoned ideas.

A lock is a big box of water with gates at both ends.

A boat goes in. The gates close. Water flows in to lift the boat up, or out to let it down!

The Smithsonian describes canal locks as water-filled stairs that move ships across sloping land. A boat sails into a chamber, and the gates close behind it, so the chamber is shut off from the water around it. Then the chamber is filled to raise the boat, or emptied to lower it.

When the water inside matches the level ahead, the front gates open and the boat sails out, one step higher or lower. A long climb uses a row of locks, one after another, like a staircase.

In a lock, a vessel enters a chamber and the gates close, isolating it from the surrounding water; the chamber is then filled or emptied to raise or lower the water and the vessel with it (Smithsonian Libraries). Once the levels match on the far side, the gates open.

A flight of locks chains chambers together for larger rises. The Erie Canal, completed in 1825, needed 83 stone locks along its 363 miles, Britannica notes, each designed to be worked by a single person. At Lockport, a staircase of 10 locks in five levels, two side by side, carried boats over the Niagara Escarpment, a 75-foot (23 m) rock ridge.

Without locks, a canal must stay flat all the way.

It could cross flat land. But it could not climb a hill!

Without locks, a canal's water would have to stay level along its whole length. That works where a canal can be dug at one level all the way. Egypt's Suez Canal, opened in 1869, connects the Mediterranean and Red seas with no locks at all: it is a sea-level canal, dug down to the level of the two seas it joins.

But where land rises, a lock-free canal would be stuck. Its builders might have to dig a very deep cut through the hill, or wind a long way around it. Those are our ideas: in reality, people built locks.

A lock-free canal is restricted to a single water level. Britannica describes the Suez Canal, completed in 1869, as a sea-level waterway, an open cut without locks, running 193 km (120 miles) and using several lakes along its route between the Mediterranean and the Red Sea.

Where the terrain rises, the alternatives to locks would be deep excavation down to a common level or long detours along the land's contours. Both are reasoned ideas here; their cost and practicality would depend on each route's geology and height difference. History's usual answer was the lock.

The Panama Canal lifts big ships up to a lake on a hill.

It uses no pumps! Water just flows down from the lake to fill the locks.

The Panama Canal crosses a hilly strip of land between the Atlantic and Pacific oceans. Its builders dammed a river to make Gatun Lake, 85 feet above sea level. Locks lift ships up to the lake, they sail across, and other locks lower them back to the sea on the far side.

Amazingly, the Panama Canal Authority says no pumps are used. Water simply flows downhill from the lake, through giant tunnels 18 feet wide inside the lock walls, and up through holes in the chamber floor.

Panama's builders dammed the Chagres River to form Gatun Lake. According to the Panama Canal Authority, water lifts ships 85 feet above sea level to the lake surface, floats them across the Continental Divide and lowers them to sea level on the other side.

No pumps are used: gravity alone does the work. Water enters or leaves through culverts 18 feet in diameter running inside the walls, then through 20 cross culverts under each chamber floor, each with five openings, 100 holes per chamber, spreading the flow evenly to control turbulence. To fill a chamber, valves at its lower end close and those at the upper end open.

The Panama Canal's first locks are huge. Each one is 1,000 feet long!

Their big gates swing like double doors.

The Panama Canal opened on August 15, 1914. Its original locks are enormous: every chamber is 110 feet wide and 1,000 feet long, and they come in pairs, so ships can travel in two lanes. There are 12 chambers in all.

The gates close in a wide V and swing like double doors. Their lower halves are hollow and watertight, so they partly float, which takes weight off their hinges. Small electric locomotives running on tracks along the lock walls guide ships through. In 2016 a third lane opened with even bigger locks, 1,400 feet long, for today's giant ships.

The Panama Canal Authority lists identical original chambers of 110 by 1,000 feet, built in pairs for two lanes: three steps at Gatun, one at Pedro Miguel and two at Miraflores, 12 chambers in all. Construction of the locks began with the first concrete at Gatun on August 24, 1909, and took four years; the canal opened on August 15, 1914 (Smithsonian Libraries).

The miter gates close in a wide V. Their hollow, watertight lower halves are buoyant, reducing the load on the hinges; leaves are 64 feet wide, 7 feet thick and 47 to 82 feet tall. Electric towing locomotives run on tracks atop the walls at about 2 miles per hour. These figures describe the original locks; since June 26, 2016, the Agua Clara and Cocolí locks have added a third lane for NeoPanamax ships, with chambers 1,400 feet (427 m) long and 180 feet (55 m) wide (Panama Canal Authority).