TORONTO—Hundreds of out-of-control

wildfires are burning in Canada, shrouding skies there and the U.S. in an eerie yellow-gray haze and forcing the evacuation of dozens of communities.

In some cases, the blazes have flared up so quickly that people in remote parts of northern Ontario have had only minutes to grab essentials before embarking on harrowing journeys to safety by boat as their communities burn to the ground.