On October 6, more than a hundred thousand tourists gathered on the banks of China’s Qiantang river to witness an extraordinary sight: a wall of water several feet tall racing its way upriver.
While this phenomenon may look like a tsunami, it’s actually a tidal bore, a wave that flows upstream in some rivers as high tide approaches. More than 80 rivers around the world have tidal bores, according to one widely-cited 1988 catalog.
What creates a tidal bore?
"Two principal factors affect the size and strength of bores … the magnitude of the tidal range and the shape of the river at its estuary,” said Victor Miguel Ponce, a professor in the civil and environmental engineering department at California's San Diego State University, in a 2005 interview with National Geographic.
Barring extreme weather conditions, bores form only when a river’s mouth is shallow, uniform, and resembles a funnel. Bore-bearing rivers also need to experience large swings in water level between high tide and low tide in their estuaries, where the river and sea meet and mix.
As high tide rolls into one of these bore-prone estuaries, the influx of water becomes more confined as it moves upriver. As it encounters shallow water, the crest of the wave moves faster than its trough, sharpening its peak. Eventually, the wave breaks—and a bore is born.
Some tidal bores have global reputations for their beauty and destructive power. Each year, the U.K.’s Severn River sees some 250 bores move upriver. The Lupar Benak bore in Sarawak, Malaysia, nearly drowned the British author Somerset Maugham during a 1949 visit. And as National Geographic reported in 2005, some bores in the Amazon have become tourist attractions, as thrill-seeking surfers try to ride them upriver.
The estuary’s structure is textbook bore material. The Hangzhou Bay, into which the Qiantang river empties, is more than 60 miles across at its mouth, but it narrows to a gap less than 13 miles across where it meets the Qiantang. As high tides enter the bay, they’re funneled in toward the river's mouth, spawning massive high tides and especially strong tidal bores.
These walls of water pack extraordinary power. Over the years, the Qiantang’s bores have constructed a massive complex of sandbars with an estimated total volume of 42.5 billion cubic meters. That’s enough sand to fill a box 2.6 miles to a side.
According to CCTV, China’s state-operated television network, the best time of year to see the Qiantang bore is midway through the eighth month on the Chinese lunar calendar, which happens to fall this year during mid-October. In general, tidal bores are at their strongest during spring tides, slightly more extreme tides that occur during full and new moons.