The tug of the Moon and the Sun on our old planet causes tides - water sloshes back and forth across the oceans in a predictable if not straightforward way. Many if not most of the world's rivers entering the sea have noticeable tides, and some have extremely high tidal ranges. However, very few rivers have an obvious tidal bore. A tidal bore is the leading edge of the rising tide. It's a wave like phenomena that moves up the mouth of rivers, which are subject to exaggerated tides. Quite often where tides are impressive, tidal bores are as well.

In the Bay of Fundy (in Nova Scotia, Canada), which has the largest tidal fluctuation between high and low tides anywhere in the world, bores are well-known tidal features . Twice a day, approximately 100 billion tons of water move in and out of this 170 mile long inlet. While even a normal tide may rise and fall 40 feet, when the moon is full or new, the tide may be an astonishing 53 feet!

In contrast to the Bay of Fundy, on the Atlantic side of the Panama Canal, for example, the difference between the normal high tide and low tide is less than a foot. However, the fact that the Panama Canal lies fairly close to the Equator and the Bay of Fundy is about half way between the Equator and the North Pole has no bearing on the range of tidal fluctuations. On the Pacific end of the Panama Canal, the tidal range is 16 feet.

Tides tend to be retarded by friction when they reach shallower water, the tide therefore piles up and forms a low wall of water that can move upstream with considerable force as the tide continues to rise. The shape of the coastline, the configuration of the inlets and the depth of the water all play a role in determining tidal heights. In the Bay of Fundy, these factors conspire to make the tides special. The bay is funnel shaped, its shores more or less converge, and its bottom slopes upward continuously from the mouth (where the bay empties into the Atlantic) to the head (where the bay begins). The incoming tide is thus forced to rise.

Another thing that makes the Bay of Fundy unique is that its size and shape produce a definite rhythm to the tides. It's a geographic peculiarity that Fundy's depth and length seem to generate a tidal pulse nearly perfectly attuned to the Atlantic Ocean - water sloshes to and fro between the Bay of Fundy and the Atlantic like two people on a seesaw. This results in a kind of stationary wave. Essentially, the ocean tides and the wave action within the bay move in sync and urge the bay's waters to climb to heights greater than anywhere else on Earth.

Tidal bores are most pronounced where river channels narrow. The largest bore in the Bay of Fundy occurs on the Petitcodiac River. The bore here travels upriver at about 8 mph and is about 3 feet high during spring tides, when the Moon is full or new. However, this bore has been measured at 5 feet in some instances. At other times, the bore is hardly perceptible at all, as was the case when I visited. It was rather embarrassing to wait for this bore to roar by and then instead see a mere ripple move past by feet - the term "bore" was certainly appropriate.

Across the Atlantic from Nova Scotia, tidal bores occur on a number of rivers in northern Europe, including the Severn River in England and the Seine and Dordogne Rivers in France. A few rivers in the Cook Inlet area of Alaska are visited by tidal bores, but in the contiguous 48 states, no river has a regular tidal bore.

One of the most impressive of all bores is the pororoca (big roar) that effects the Amazon. As it approaches, it may be heard several miles in the distance, and under certain conditions it can be nearly 12 feet high as it moves up the mouth of the Amazon. Huge limbs and logs are easily carried by the bore, which crashes against the river's banks like an ocean wave breaking on a beach. Sometimes, logs are observed being hurled in the air by the frothing surf. In shallow water, it's very dangerous. In mid river, where the water is deeper, however, the bore is more like an ocean swell. Usually after the primary bore moves through, a series of smaller, less menacing bores will arrive about 150 m apart. As soon as the bore passes, the current flows, almost instantly, at full strength. Six feet of water can cover an area that was, ten minutes earlier, barely a foot deep.

Likely, the greatest of all tidal bores in the one that regularly churns up the Fuchun River (Qiantang Tidal Bore) in China. It speeds along at approximately 15 mph, and its crest may attain a height of 15 feet! The river outlet is extremely wide, forming the shape of a huge trumpet. When the sea tide comes up, it brings huge amount of water pushing inland, but the abrupt narrowing of the river together with the sandy river bed hinders its progress. The tide eventually surges higher and higher, forming a huge wall of water. Viewing this tidal bore has been a significant event for over 2,200 years. According to the Chinese calendar, the 18th day of the eighth lunar month is regarded as the best time viewing the bore (near the time of the Autumnal Equinox).

For more about this see the Earth Science Picture of the Day for Wednesday, December 19
Also see the February 1949, August 1957 and October 1972 issues of the National Geographic Magazine.