At 2:46 PM on March 11, 2011, at a spot 60 kilometres off the coast of Japan, the Pacific tectonic plate suddenly slipped and plunged under the Eurasian plate. The resulting Tohoku earthquake, lasting six minutes and measuring 9.0 on the Moment Magnitude Scale, was the most powerful in Japanese history and the fourth largest ever recorded, causing thousands of deaths and injuries and inflicting trillions of Yen in damage. But worse was yet to come. Ten minutes after the shaking finally ended, residents along Japan’s northeast coast watched in horror as a wall of water 40 metres tall came roaring out of the sea towards them. The giant wave was even more destructive than the earthquake that preceded it, wiping away dozens of cities and towns and bringing the final death toll to 19,759. It also triggered the meltdown of three of the reactors at the Fukushima Daiichi nuclear power plant, creating a radiological disaster whose effects may take 30 years or more to clean up. Thanks to this and a similar disaster in the Indian Ocean on December 26, 2004, the Japanese word for monster waves became firmly planted in the world’s vocabulary: tsunami. Many English speakers, however, still commonly refer to this phenomenon as a “tidal wave.” Yet the terms are not interchangeable, the latter referring to an entirely different type of hydraulic phenomenon – one that makes rivers run backwards and creates waves that can be surfed for dozens of kilometres. So grab your surfboard as we dive into the fascinating science and history of real tidal waves and how they differ from tsunamis.

“Tidal waves” are more properly known as tidal bores – a term derived from the old Norse bára, meaning “wave” or “swell.” Whereas tsunamis – Japanese for “harbour wave” – form in the open ocean and are caused by underwater geological displacements like earthquakes, volcanic eruptions, and landslides, tidal bores – true to their name – are caused by rapidly-rising tides and form in narrow bays and rivers. Tidal bores are also significantly smaller than tsunamis, with the largest – observed on the Qiantang River in Hangzhou, China – measuring 9 metres tall.

Scientifically speaking, a tidal bore is a kind of hydraulic jump, a phenomenon whereby a fluid flow abruptly slows down and changes its elevation, exchanging part of its kinetic energy for potential energy. You’ve probably observed this very phenomenon every time you turn on your kitchen sink: when the stream of water impacts the bottom of the sink, it flows outward in a thin layer; but just a few centimetres outward from the point of impact, this layer suddenly jumps upwards, forming a shallow, circular ring. In the case of tidal bores, the jump and resulting wave front are caused by a rapid change in water level at the river mouth. While the water level can change rapidly during both flood tide and ebb tide, tidal bores only form during flood tide. The most common type of tidal bore consists of a single wavefront or roller flowing upstream against the river current. This breaker is usually very turbulent, churning up large amounts of sediment and air as it sweeps along. This turbulence in turn causes many bores to produce a loud, low-frequency roaring or rumbling sound that can be heard for many kilometres. However, a second, less violent type of bore called an undular bore is sometimes observed, consisting of a larger, smooth wave front followed by multiple secondary waves called whelps.

The conditions needed to create a tidal bore are very specific, meaning the phenomenon occurs in only around 60 places in the world. The river must be relatively shallow, its mouth narrow, the estuary where it meets the ocean wide and flat, and the tidal range at the river mouth must be very large – on the order of 6 metres or more. Indeed, the largest tidal range in the world – 16 metres – occurs in the Bay of Fundy in Atlantic Canada, and many rivers in the region including the Peticodiac, Salmon, and Shubenacadie experience regular tidal bores twice a day. These large tides also cause certain rivers to temporarily flow backwards, a phenomenon famously observable at Reversing Falls, a popular tourist attraction on the Saint John River in New Brunswick.

However, there are rare exceptions to these rules. For example, perhaps the most famous tidal bore of all, the pororoca, forms on the Amazon River, whose mouth is relatively large. However, this estuary is very shallow and dotted with numerous islands and sandbars, creating the same hydraulic effect as a narrow mouth. Indeed, the pororoca is so powerful that unlike most rivers its size, the Amazon doesn’t have a delta – the wide, triangular drainage plain that forms when a river slows near its mouth, causing sediment to build up and the river to break up into multiple shallow channels. Instead, the tidal surge carries the sediment down a single channel directly into the Atlantic Ocean.

But while tides are regular and predictable, this is not always the case with tidal bores. Some bores, like the so-called benak on the Batang Lupar River in Malaysia, occur regularly every day. Others, like the pororoca, occur only during the strongest spring tides. Many other factors can affect the formation of bores, including rainfall, wind, and shipping traffic. Indeed, large-scale human activity on a river can disrupt bore formation. For example, the River Seine in France used to experience a powerful, 7-metre tidal bore called the mascaret, but extensive dredging, canal construction, and other hydraulic engineering projects in the 19th century caused it to periodically disappear. Finally, dredging of the riverbed in 1963 eliminated the mascaret altogether. Similarly, the Petitcodiac River in New Brunswick, Canada, used to have the largest tidal bore in North America at 2 metres high. But the construction of a causeway between the cities of Moncton and Riverview in the 1960s caused the bore to disappear. In 2010, however, as part of the Petitcodiac River Restoration Project, the causeway gates were reopened and the tidal bore returned.

Despite being significantly less powerful than tsunamis, tidal bores can still be extremely dangerous. Reaching heights of up to 9 metres and speeds of up to 40 kilometres an hour, tidal bores can capsize ships, destroy docks and other riverside infrastructure, and sweep people off riverbanks into the water. Indeed, prior to its elimination in the 1960s, the Seine River bore or mascaret had a particularly sinister reputation, destroying some 217 ships and damaging countless more between 1789 and 1850. Many bores, such as on Malaysia’s Batang Lupar or India’s Hooghly River are well-known navigation hazards for local shipping, while every year the Qiantang River bore in China carries off a handful of people who stand too close to the riverbank.

Yet tidal bores can also be a valuable part of the local ecosystem, stirring up sediment and oxygenating water as they sweep along. They can also stun or kill large numbers of river animals, providing a bonanza for predators and scavengers. For example, piranha follow the Amazonian pororoca to gather up fish, crabs, and birds; saltwater crocodiles swim behind the Styx River bore in Australia, while eagles and grizzly bears around Alaska’s Cook Inlet will often gather after a tidal bore to pick up dead fish along the riverbank.

Tidal bores can also be popular tourist attractions, especially among rafters, kayakers, and surfers. For example, certain regions such as the Amazon not only feature large, powerful bores, but also long, uninterrupted stretches of river, creating the unique conditions for long-distance endurance surfing. Indeed, since 1999, an annual championship has been held in the Brazilian municipality of São Domingos do Capim to see who can ride the famous pororoca the greatest distance. Currently, the record belongs to Brazilian surfer Picuruta Salazar, who in 2003 rode the tidal bore for 12.5 kilometres in 37 minutes. However, the absolute record for continuous surfing is held by California surfers Colin Whitbread and JJ Wessels, who in July 2014 rode the Petitcodiac River bore in New Brunswick, Canada for an incredible 29 kilometres from Belliveau Village to Moncton. The record-breaking ride took nearly two and a half hours and raised more than a few eyebrows – specially, as Wessels recalls, among customs agents at the Canadian-American border:

“We pull up and [the customs agents] look at us like what is on the top of your car. Then they look at our passports and, ‘where are you going?’ ‘We’re going to go surf a river wave in Canada.’ ‘Seriously boys what are you going to go do? Don’t lie to me.’ ‘I’m telling you we’re going to go surf this river wave.’”

Yet before you grab your surfboard and set off for the Amazon or Canada, know that river surfing is not without its unique dangers. Not only are certain rivers like the Amazon full of dangerous wildlife like crocodiles and venomous snakes, but tidal bores often carry large amounts of debris including entire trees that can easily entangle and injure unwary surfers. Consequently, this is a sport only for the most experienced, and YOLO personality types. Which is always a bit of an ironic term, given the fact that “you only live once” could just as easily be used as a major reason NOT to do dangerous or foolish things, despite most using it as motivation for such shenanigans in their lives. As ever, much like when on a surfboard, an optimal life is all about balance.

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