Three months of rain fell in just 24 hours in some areas.
The flooding is the latest extreme weather event to hit northeast Canada - recent wildfires have burnt a record area, sending clouds of smoke south into the US.
There has also been extreme flooding in the US this month. The body of a two-year-old girl found along a river in Pennsylvania is believed to be one of two missing children swept away by flash floods last weekend. Her nine-month-old brother is still missing.
Scientists cannot say for certain that such extreme rainfall is caused by climate change, but the floods are consistent with the changes they expect in a warming world. This is because the warmer the earth becomes the more moisture the atmosphere can hold. This results in more droplets and heavier rainfall, sometimes in a shorter space of time and over a smaller area.