(typically more than between high and low water), and where incoming tides are funnelled into a shallow, narrowing river via a broad bay. Large bores can be particularly dangerous for shipping, but also present opportunities for river surfing. The funnel-like shape not only increases the tidal range, but it can also decrease the duration of the flood tide, down to a point where the flood appears as a sudden increase in the water level. The tidal bore occurs only during the flood tide, never the ebb tide. A tidal bore can create a powerful roar that combines the sounds caused by the turbulence in the bore front and whelps, entrained air bubbles in the bore roller, sediment erosion beneath the bore front and of the banks, scouring of shoals and bars, and impacts on obstacles. Tidal bores are being surfed along coastal rivers such as the pororoca on the Amazon River or England's River Severn. Severn bore Surfing the Severn bore has become a competitive sport with dozens of surfers vying to record the longest ride. The tidal surge also attracts canoeists and windsurfers. The present champion surfer is Dave Lawson from Hempsted, Gloucestershire, who has covered 5.7 miles on a surfboard. His record-breaking surf took more than 35 minutes and was logged by an official adjudicator from the British Surfing Association. Pororoca bore The pororoca is a tidal bore, with waves up to 4 metres high that travel as much as 13 km inland upstream the Amazon River. Petitcodiac bore Tidal bores are backwards-moving waves that travel upstream over forward-moving downstream waves. They occur twice a day on the Petitcodiac River in the Bay of Fundy, driven up the river by the world's highest tides. The North American record for surfing a single river wave was set by J.J. Wessels and Colin Whitbread of California, who rode the Petitcodiac River bore for 29 kilometres on 24 July 2013. Upstream river waves An upstream river wave is a phenomenon that looks similar to tidal bores, but is caused by ocean swells instead of tides. Similar to tidal bores, they form in the ocean and travel up the river. The Urumea river is an example of a well-known upstream river wave. Building river surf waves River surfing is gaining popularity around the world. Boise Whitewater Park's Wave Technician, Paul Primus, shares the dual benefit of installing recreational river features to replace