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Off the coast of Ste-Anne-de-Bellevue in Montreal’s West Island, shore watchers have been witness this summer to a scene almost biblical in scope: the waters have turned from brown to green and the rapids have changed direction.

“It is very unusual,” said Patrick Tomasino, lockmaster for the Ste-Anne-de-Bellevue lock system and a Parks Canada worker for 21 years. “It’s the first time I’ve even seen it. I would say it’s a once-in-a-generation thing.”

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The origins of the phenomena can be traced back to the Great Lakes and this season’s high water levels.

Normally, the waters flowing by Ste-Anne-de-Bellevue on the western tip of the island go from north to south, from the Lake of Two Mountains to Lac St. Louis, which is about two feet lower in elevation, creating the rapids that flow beneath the Galipeault Bridge on Highway 20.

Lake of Two Mountains is fed by the Ottawa River, which is known as a “forest river” because the leaves and branches of the forest fall into it and the rivers and streams that feed it, and help to turn the water brown, along with the churned-up sediment of the river bottom. Lac St. Louis is fed by the mighty St. Lawrence, which is in turn supplied by the Great Lakes, whose waters have deep basins to filter out the sediment, so it runs green and clear. Most years, there is a demarcation between where the brown waters of the Ottawa River meet the Mediterranean green of the St. Lawrence, a line of green and brown clearly visible for those flying overhead or boating through. Generally, the line is situated in Lac St. Louis, sometimes close to Beaconsfield. Swimmers and boaters often gravitate toward the green water because it looks cleaner and clearer.