Rideau Canal Waterway - What is a Lock? (static)


		How a Lock Works

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How a Lock Works
(static version)

A lock is a fascinating piece of engineering that allows boats to travel either uphill or downhill. Many rivers posed navigation problems for early river travelers. Locks were the solution that made these waters navigable.

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Most rivers have rapids, shallow rocky areas with fast flowing water. In the early history of Canadian exploration, boats that could be lifted out of the water and carried (such as canoes) were used. To go around a rapid, you pulled your boat out of the water and lugged it around. Later, on more traveled waterways, steel pins were put into rock outcrops along the rapids and boats would winch themselves upstream. These boats had to have a shallow draft (didn't sink too deep in the water), and winching was a slow process.

On rivers that were deemed to be necessary for navigation, a more permanent solution was to build a canal around the rapids. A canal is a manmade waterway with a minimum depth of water in it. On the Rideau for instance, the minimum depth is 1.5 metres (5 feet). However if you just dig a ditch to the required depth and let the water flow through it you still have the other problem a rapid has, fast flowing water. In England, such canals were built, and boats were towed upstream by horses or oxen walking along trails built along the side of the canal just for this purpose.

For systems like the Rideau, canals with fast flowing water would not be practical. The solution to slowing down water flow is to build locks. A lock in a canal is essentially a dam, and it keeps the water at the top and bottom from flowing too fast. To get the boat up or down the required elevation it floats the boat in a tub of water. No power is required for a lock, to get a boat up, water is let into the "tub" from the upstream (high water) side. To get a boat down, water is simply let out of the "tub" until the boat is lowered to the level of the water on the downstream (low water) side. The following graphic illustrates the process:

In our example a the boat is "locking up":

A boat comes to the bottom of the lock and want to go to the upstream side.
The gates on the downstream side of the lock can easily be swung open because the water inside the lock is the same elevation as the water on the downstream side. So the lock staff turn the cranks that open the gates to let the boat in.
The boat floats into the lock, which is designed so that the water depth inside the lock meets the required minimum (in the case of the Rideau, each lock has at least 1.5 metres (5 feet) of water in it.)
Now the lower gate is closed.
Valves are opened at the top end of the lock, which lets water from the upstream side into the lock.
As the lock fills with water, the boat floats up.
Soon the lock is filled to the level of the upstream side, and the gates on the upstream side can be opened, and the boat floats out.

"Locking down" is a similar process except that now valves on the downstream side of the lock are used to let water out of the lock until the water is the same level as the downstream side.

Now that you know what a lock does, have a look at The Basics of a Rideau Lock for more detailed information about how a lock is built and how it works.

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