In Early May, Quebec’s Mistassini River is still full of ice. Muscled up with spring runoff and stained almost black by tannins from tundra far to the north, the eddies are swirling, acre-wide slurries. Underneath a highway bridge in the town of Dolbeau-Mistassini, 40,000 cubic feet per second—almost half the flow of Niagara Falls—rush through a narrow gap and then plunge over a jagged line of granite bedrock ribs. Oceanic waves, some more than ten feet high and 70 feet wide, rise and break, and the river implodes into churning pits of whitewater known simply as Bridge Rapid. Normally, no one here pays the rapid much mind—it’s just another thunderous falls in this broad, waterlogged province—but today there is a spectacle brewing.