A term coined by seamen around 1600, tidal bore describes a violent wall of water rushing up a shallow, narrowing river, estuary, or bay. Bores (also called bore tides) form when an incoming tide meets a particular geography, a resistance—sand, silt bars, narrowing channels—and heaps water up to ﬁfteen feet high, moving inland at ten to ﬁfteen miles per hour. Most bores build after low water of a spring tide—the year’s biggest tidal ﬂux. Famous bore tides occur in the Truro River in Canada’s Bay of Fundy, and in Turnagain Arm in Alaska’s Cook Inlet, the latter driven by a tidal rise of thirty feet in six hours. In summer, travelers driving along Turnagain Arm can witness windsurfers bundled in drysuits dancing along the wave’s face, their brightly colored sails a dramatic contrast to the gray, silty inlet. When it surges up the lower Amazon, the bore—pororoca—can shave forests and destroy homes. It is greatly feared by locals, yet ridden by elite surfers from around the world.