A tidal bore is a wave that runs up a river against its current, and the largest, on China’s Qiantang River, can reach 9 meters and travel 40 kilometers per hour. The word comes through Old English from the Old Norse bara, meaning wave or swell. Unlike seismic tsunamis, bores are driven by the moon and sun, and they need two ingredients: a macrotidal range of more than about 6 meters and a funnel-shaped estuary where a broad bay narrows into a shallow river. Riverbed friction slows the bottom of the incoming flood tide, squeezing six hours of rising water into one sudden surge, a hydraulic jump that appears either as a steep breaking bore or a smooth undular bore trailed by secondary waves called whelps. Its low rumble comes from trapped air bubbles oscillating under pressure, plus scoured sediment, and carries miles inland.

Bores wreck scientific gear. On the River Dee in Wales an acoustic Doppler current profiler had to be dug out with shovels after one tide, and on Australia’s Daly River three minutes of macroturbulence toppled aluminum frames built for strong currents. They kill people too, on the Qiantang despite warning signs and on Nova Scotia’s Shubenacadie River, where visitors walk drained mudflats. Yet the churning oxygenates stagnant estuaries, feeding fish and shrimp on Indonesia’s Rokan River. Human engineering has erased bores: dams and irrigation nearly ended the Colorado River’s bore that once reached 47 miles inland, 1960s dredging eliminated the Seine’s La Barre, and Brazil’s Araguari bore was considered lost by 2015.

• A 1960s causeway between Moncton and Riverview shrank the Petitcodiac’s bore, once North America’s highest, to a ripple.
• Its gates opened permanently on April 14, 2010, and the river scoured out its silt and rebuilt the bore.
• In 2013 surfers rode a single Petitcodiac bore wave for 29 kilometers, a North American record.
• The Amazon’s Pororoca reaches 4 meters, and Indonesia’s Seven Ghosts bore on the Kampar River is feared for sinking boats.
• Diana Wynne Jones made a tidal bore the villain of her Dalemark Quartet.