In general, a tidal bore is a wave that runs UP-river because the rising tide (or other surge) at the mouth has overwhelmed the river's flow, rising past the normal surface level of the river. It is not so unusual for the lower reaches of rivers to be tidal, but when the difference in level is significant (large and sudden) the wave forms and the river appears to flow backward for a time behind the bore, following the wave upstream for miles. The event will have ended when the water level has equalized and normal river flow re-established.
There are several of these around the world; the Bay of Fundy's tidal swing is enormous--the world's largest, at 40 feet or so--but the bore on the Petitcodiac River at Moncton is not the biggest. (That might be an annual high-tide, sun-and-moon alignment, Chinese bore, which kills people every year. See the Youtube video[s].)
Even so, Moncton's is an impressive and wonderful thing as it hisses and foams along its way upstream, moving past the town at about a swift walking pace. It's such an important tourist attraction that they built a viewing area complete with bleachers. I made a pretty good video from there, but the file size is extremely large; it is (for the time being) trapped on my computer. In place of that, here are links to information, explanations, and videos:
• Tidal Bore
• Bay of Fundy
• Petitcodiac River: "Before the construction of a causeway in 1968, the river had one of the world's largest tidal bores, which ranged from 1 to 2 metres (3.3–6.6 ft) in height and moved at 5 to 13 kilometres per hour (3.1–8.1 mph). With the opening of the causeway gates in April 2010, the river is flushing itself of ocean silts, and the Bore is returning to its former glory." (Wikipedia)
• New Brunswick Tourism
• Moncton vids @ Youtube
• Other (all) Youtube examples
Mark_
30 October 2016
