Discover more about tidal bores, the unusual phenomenon where rivers run backwards around the world
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Most rivers flow in one direction. For a few hours during each tidal cycle, however, a small number around the world do the opposite. This can form a tidal bore, a wave that travels upstream as the incoming tide pushes inland against the current of a river.
Instead of the gradual rise in water level normally associated with the tides, the flood tide arrives as a distinct front, sometimes little more than a ripple, sometimes a breaking wave several metres high. Fewer than 100 rivers worldwide experience tidal bores, making them one of the rarest tidal phenomena.
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Their formation depends as much on geography as astronomy. A tidal bore develops where a river enters an estuary with an exceptionally large tidal range and the channel narrows inland. As the incoming tide is squeezed into a smaller space, the water has nowhere to go but upwards, creating a wave that continues upstream against the river’s flow.
One of the best-known examples is the Severn Bore, which forms when tides from the Bristol Channel move into the River Severn. Although it’s rarely more than a metre high, it regularly attracts surfers and spectators hoping to catch the wave.
The bore itself passes a point relatively quickly; its effects last much longer. As it passes, the river briefly reverses direction before continuing to rise for several hours. The turbulence churns up sediment from the river bed and mixes oxygen through the water, redistributing nutrients across the estuary. Over time, these repeated surges help shape channels and sandbanks, making tidal bores an important part of how some estuaries evolve.
Although the timing of the tides can be predicted with remarkable accuracy, the size of a tidal bore is much less certain. River discharge, recent rainfall and the shape of the channel all influence how the wave develops. The largest bores usually occur during spring tides, when the Sun, Moon and Earth are aligned. On China’s Qiantang River, these conditions can produce the world’s largest tidal bore, with waves reaching up to nine metres high and travelling upstream at speeds of around 40 kilometres per hour. The approaching wave is often heard before it’s seen, producing a deep roar that has earned it the nickname ‘Silver Dragon’.
Not all tidal bores have survived. The Mascaret of France’s River Seine was once a familiar sight, but dredging and other engineering works altered the shape of the channel, weakening the wave until it almost disappeared. Elsewhere, the Amazon’s impressive Pororoca remains powerful enough to travel hundreds of kilometres inland, carrying huge amounts of energy and sediment with it.
Tidal bores are a reminder that rivers are shaped not only by the landscapes they drain but also by the seas into which they flow. They occur only where tides, river channels and coastal landforms combine in just the right way, turning an ordinary high tide into a river that, however briefly, runs backwards.
