The whale rises from the river without so much as a suggestive swirl of water, a few dozen shimmering tons of black skin and pearl-white flukes so close it blocks our view of the horizon. “Markie, freeze!” I whisper to my 15-year-old daughter. She stops her kayak paddle in mid-stroke, as if the slightest movement might spook a giant humpback. For two days Markie and I have paddled a sea kayak through rain, fog, wind, and the choppy slop of the St. Lawrence River, near the small village of Tadoussac, Quebec. A quarter of the world’s freshwater resources—including all the flow from the Great Lakes—meets the tides of the Atlantic under our kayak, a massive mixing bowl that during the summer months draws whales by the hundreds. There are fins, humpbacks, blues, minkes, and more, 13 species in all, including a year-round population of endangered beluga whales. The whales feed from the tip of the Gaspé Peninsula to just a few miles up river from Tadoussac, where they heavily...