The search for the perfect wave has been transformed in recent years from beautifully shaped ocean swells to include all manner of breaking waves. Some, such as slabs, are short and fast with all their energy compacted into a small area, while at the other end of the temporal spectrum are tidal bores.

Around the world there are a handful of rivers that offer the chance to ride kilometre long, thigh-burning waves at particular times of the year. The waves are not a result of wind blowing over water, however, but a combination of large tidal ranges and ideal river contours.

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The Earth's surface is made up of 71% water and the rest land masses. As the Sun and Moon orbit their large gravitational pulls move the oceans from one side of the Earth to the other, causing the Earth's tides.

These tides make their way into bays and estuaries with some flowing up rivers and streams. Sometimes a tide running in from a broad bay into a flat, converging rivermouth can cause the height between high and low water – called the tidal range – to be greatly amplified. If the height between high and low water reaches above six metres a moving wall of water known as a tidal bore can result.

While the large tidal range is a significant factor in the formation of a bore, the shape of the river is equally important. To occur the river needs to funnel the incoming water so that it reduces the duration of the flood tide (incoming tide) to such a point where the tide actually moves in as a sudden increase in water level. This means that a wave can only be produced on the incoming flood tide and never the outgoing ebb tide.

The most well known tidal bore to surfers is the Pororoca in the Amazon River which can be surfed for 13 kilometres with wave faces ranging up to 4 metres. The Pororoca has been regularly surfed since 1999 in the form of an annual surfing championship but was actually first discovered by explorers Vicente Pinzon and Charles-Marie de La Condamine way back in the 16th and 18th centuries. La Condamine's first account of the bore is fascinating:

"...one can hear at 4 to 8 kilometres a terrifying roar that announces the Pororoca. One can see a wall of water of 4 to 5 metres in height, then another, then a third one and sometimes a fourth one, that comes close together, and that occupies all the width of the channel; this bore advanced very rapidly, breaks and destroy everything."

"The canoes, and dinghies can only escape the tidal bore by moving in the deep water section of the channel."

A more recent and perfect discovery is the Bono on the Kampar River in Indonesia where Rip Curl's Seven Ghosts was filmed. The tidal ranges through the Kampar fall short of the commonly accepted minimum of 6 metres for the bore to form, coming in at 4 metres, but this looks to be offset by the rivers bottom configuration. The Bono offers up steeper and more hollow sections compared to any other tidal bore with the shallow banks it breaks along helping to shape the waves.

Other well known bores include the Silver Dragon in the Qiantang River, China, the Hooghly on the Ganges River, the River Severn in Wales, and the Seine River in France, but the question for local readers is: what about Australia?

There are three known tidal bores around the country, one of which doesn't break anymore. The Daly River in the Northern Territory offers a small undular bore which takes the form of a standing wave instead of a breaking surfable wave rolling down the river. The Styx River in Central Queensland, however, offers breaking waves up to 1 metre due to its large 11 metre tidal range. The only catch is the sharks that are reported to follow closely behind the back of the bore, feeding off the upturned water.

The bore that looked to offer the most potential was located in the Ord River in the Northern Territory. It formed downstream of the damming of the river which suppressed flooding and allowed silt to build though the river channel. Image 6 shows a perfectly peeling wave through the middle of the river. Unfortunately this tidal bore has since disappeared after the river was flushed by a large flood in 2001. Given enough time between major floods, we may see the Ord River tidal bore reappear.

There may be more tidal bores scattered around Australia for the intrepid to discover, but they'll be limited to the crocodile infested regions of North West Western Australia, Northern Territory, and jellyfish riddled Queensland coast where the tidal ranges are considered large enough. All you need is good river navigational charts, a boat, and a strong set of quads and you'll be fine. //CRAIG BROKENSHA

Image 1: Pororoca, Toninho Javely Image 2: Pororoca, Bruno Alves Image 3: Pororoca, Unknown Image 4: Pororoca, Toninho Javely Image 5: Pororoca, Unknown Image 6: Ord River, Dr Eric Wolanski Image 7: Styx River, Google Earth Image 8: Daly River, Google Earth