Why Are the Bay of Fundy Tides So Huge?

Situated between the Canadian provinces of Nova Scotia and New Brunswick, the Bay of Fundy has the largest tidal swing of any place in the world. It routinely sees a high tide of 56 feet (17m) above the low tide mark, more than the height of a five story building. And that’s just the routine tidal swing… when coupled with a storm surge, the tidal height has reached as much as 65 feet (20m)!

It takes just over six hours for the tide to change from low to high. Since the bay is 170 miles (270km) long, that’s an immense amount of water that flows into and out of the bay twice each day. In fact, it’s estimated that during each six hour period of each day, more water flows into or out of the bay than the combined total of all the water flowing in all the rivers of the world over the same period - about 110 billion tons of water.

A wharf at high tide Courtesy NOAA

The same wharf at low tide Courtesy NOAA

If you look at the tide tables for most places in the world, you’d see that the average tidal swing is about 3 feet or 1 meter. And just 16 miles (26km) away on the Atlantic coast of Nova Scotia, the tides average only about 10 feet (3m). So why, I wondered, are the tides so much higher in the Bay of Fundy?

Tides, as most of us know, are caused by the gravitational pull of the moon, and to a lesser extent, the sun. When the moon is directly overhead, its gravitational pull causes the water to bulge outward toward the moon, causing a high tide. This high tide follows the moon as it rotates around the earth and as the earth spins.

If the earth was a sphere, uniformly and totally immersed by water, the tide would be one long wave that moved around the planet across that vast sea. But it isn’t… there are innumerable land masses, and these land masses have innumerable coves, harbors and shoals that disrupt and interfere with the tidal wave. (The tidal wave here is different than the other, seismic generated tidal wave, BTW). The result is varying tidal ranges and periods all around the globe. Some places have tides rivaling those of Fundy while other places have virtually no tidal change at all - the Atlantic coast of Panama has an average tidal change of about 10” (0.25m), while the Pacific side, a mere 36 miles (60km) away, has an average tidal swing of more than 10 feet (3m), for example. Some places have two high and two low tides each day, while others have only one, or a varying number. The Pacific side of Panama regularly has its two highs and two lows each 24 hours and 50 minutes, while the Atlantic side varies between two highs and two lows, two highs and one low, and one high and one low every 24 hours.

The huge tidal swings in the Bay of Fundy occur because it has just the right shape, orientation and location. To begin with, the 170 mile length of the bay is just long enough to create what’s known as tidal resonance. In any body of water, whether a bathtub or Lake Superior, if you drop a rock into it on one side, it will take a set amount of time for the wave created by the rock to travel to the far side and return. The time for a wave to travel the length of the Bay of Fundy and back is between 12 and 13 hours. Since the tide there rises and floods into the bay every 12 hours and 25 minutes, it is in sync with the bay’s natural tidal resonance and reinforces the tidal wave.

Courtesy NOAA

An often used analogy is pushing a child in a swing. If you give the swing a slight push just as it starts its downward motion, it will swing higher and higher with each push - up to a limit of course. If you push the swing too soon or too late, the effect is much less or even counter to the motion of the swing.

While the  bay’s length is key to the large tidal swings, its shape is also important. It becomes narrower and shallower towards its head, forcing  the water higher up the sides.

The flow of the 110 billion tons of water flowing into and out of the bay is anything but smooth, however. The irregular coastline, headlands jutting out and streams flowing into the bay create numerous reversing falls, whirlpools and rapids, all well worth seeing. The erosion caused by the flow of water also creates some very interesting rock formations. And you also have the opportunity to walk on the ocean floor when the tide is out. Just be sure to get back ashore before the tide comes back in.

Burntcoat Head Courtesy Fundy Tourism

It would seem that with all that water flowing into and out of the bay, it would be a great place to build a hydroelectric plant. In actuality, the Annapolis Royal Generating Station was built in 1985 to take advantage of all that tidal power. It generated about 30 million kilowatt hours per year, with a peak output of 20 megawatts - enough to power 4500 houses. Unfortunately, it’s turbine also caused substantial fish mortality. For example, researchers found that almost a quarter of the American shad passing through were killed. When a critical component failed, it was shut down after 34 years of operation and never repaired, but it’s unclear whether this was because of the fish mortality or the cost of repairs.

Old Sow Whirlpool

The most famous whirlpool is Old Sow, located in Passamaquoddy Bay towards the mouth of the Bay of Fundy. According to Wikipedia, “it is the largest whirlpool in the western hemisphere, and the second largest in the world – second only to the Maelstrom Whirlpool of Norway”. With much trepidation, we traversed Old Sow during our second year aboard Nine of Cups when we were still quite new to the cruising life. To be safe, we timed it so we’d be crossing at its most benign, but even so, we were sure we’d be pummeled and dragged into the center of the huge vortex at any moment. I’m sure that if we had timed it differently, the day may have been more exciting, but for us, it turned out to be a non event - if we hadn’t verified our position with our GPS, we wouldn’t have even known we’d passed through it.

See you next week…