The locks canal of Coteau-du-Lac (1779-1780)

Coteau-du-Lac National Historic Site

One of the oldest in North America!

A Strategic Canal

The military invasion manoeuvres threatening Canada during the American War of Independence (1775-1783) revealed a flaw in the colony’s defence system. In those days, the St. Lawrence River was the only communication route supplying military outposts on the Great Lakes, which protected Canada’s western border. To get there, the British had to navigate up the river’s rapids upstream from Montreal: the need to skirt these rapids by going inland slowed the troops and merchandise transport, making these outposts more vulnerable.

The Coteau-du-Lac rapids were the most difficult to cross. Upon the suggestion of Captain William Twiss, Sir Frederick Haldimand had a canal dug to facilitate transport on the waterway. With a length of over 100 m, the Coteau-du-Lac canal had three locks, which were 1.8 m wide and a draught measuring approximately 80 cm.

The Coteau-du-Lac canal went into full use from the moment it opened. However, it was unable to solve all the problems involved in river navigation, upstream from Montreal. In 1783, three additional canals were built in this area: the Faucille, Trou-du-Moulin, and Rocher-Fendu canals. These canal infrastructures represented a valuable aid to navigation, as they bypassed the River’s most difficult section.  

A Brilliant Engineer

At the age of 15, William Twiss (1745-1827), began a brilliant military career in the British Army. He completed his military engineer training as part of his first posting, at the Fortifications of Gibraltar. Aside from building the Coteau-du-Lac Canal on the St. Lawrence River, he worked on several fortification projects in Quebec City, Sorel, Saint-Jean, and Île-aux-Noix.

An Arduous Construction

The Canal’s entire construction project was carried out by hand. The Coteau-du-Lac Point was composed of dolomite (a type of rock containing lime and magnesium oxide) arranged in layers. British soldiers used different excavation techniques to build a canal there. The top soil was removed with shovels and pickaxes. Blasting holes were bored in the bedrock with a jumper and a sledgehammer. To blast the rock, the holes were filled with gunpowder which was then lit. Earth and rock fragments were transported with wheelbarrows. The cut stones used to build the canal walls were fashioned with chisels and a small sledgehammer and then laid by a mason. Hoisting shears (a manual crane) were used to lift heavier rocks. It is assumed that the freeze-thaw cycle contributed to the fracturing of the rock, facilitating the work.

The materials used to build the lock gates came from different places: the flat iron was from England, the cast iron from the Forges du Saint-Maurice, and the wood from the Coteau-du-Lac area.

Did you know that the lock canal used double leaf gates equipped with sluice valves to control water flow? This technology was invented by Leonardo da Vinci in the XVI century.

In addition, a toll was charged on boats using the St. Lawrence locks.

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