Coteau-du-Lac is just outside Valleyfield, but it belongs naturally in a day here. The site preserves the remains of an early lock canal and military works at a difficult St Lawrence River passage, where people had been portaging around rapids for thousands of years.
The visible remains are low-key rather than monumental. Go for the layered river history: Indigenous travel routes, British military supply, canal engineering, and the later shipping systems that helped shape this part of Quebec.