Tidal bores send a wall of seawater racing upriver against the current
On China's Qiantang River the incoming tide arrives as a wave up to 9 metres tall, charging upstream at around 40 kilometres an hour. It is the world's largest tidal bore, a rare phenomenon in which the leading edge of the tide reverses a river's flow, with a low rumble audible from far away.
Bores need special geography: a tidal range usually above 6 metres and a broad bay that funnels the flood into a shallow, narrowing channel. The funnel magnifies the tide and squeezes the rise into minutes, so the water level jumps suddenly. Bores come only with the incoming tide, never on the ebb. Some are a single breaking front with a churning roller; others are smooth waves followed by a train of smaller ones called whelps. The roar comes from turbulence, trapped air bubbles, scoured sediment and impacts on the banks, and its low frequencies travel long distances. The word itself traces back to the Old Norse bára, meaning wave.
They can be deadly. Despite warning signs, people die on the Qiantang each year after taking too many risks, and on Nova Scotia's Shubenacadie River tourists caught in drained riverbeds have drowned when the bore rolled in. Indonesia's Bono on the Kampar River is feared locally for sinking boats. Yet the churned, oxygen-rich water also nourishes fish and shrimp, and surfers seek out bores such as England's Severn Bore and Indonesia's Seven Ghosts.
Human engineering has erased some bores. Dredging practically eliminated the Seine's bore after the 1960s, irrigation diversions have almost silenced the one at the Colorado River's mouth, and Brazil's Araguari bore has been considered lost since 2015 because of dams, irrigation and buffalo farming.
Canada's Petitcodiac River shows the process can reverse. Once home to North America's highest bore, it shrank to a ripple after a causeway went up in the 1960s. When the gates were opened in April 2010 the wave returned, and in July 2013 professional surfers rode a metre-high bore 29 kilometres upstream to Moncton, a North American record for continuous surfing. The Amazon's pororoca, up to 4 metres high, is another famous example.
Source: Tidal bore
