Tides are usually explained by the moon, and that explanation is correct and incomplete.
The moon’s pull produces a tidal range in the open ocean of roughly a metre. That is the raw signal, and it is the same order of magnitude everywhere, because the moon does not favour particular coastlines.
Yet tidal ranges around the world vary enormously. Some coasts barely notice the tide. Others see the sea retreat several hundred metres and return twice a day. And in one bay between two Canadian provinces, the difference between high and low water reaches something in the region of sixteen metres — the height of a four-storey building.
That amplification has nothing to do with astronomy and everything to do with the shape of a particular hole in the coastline. Here is the mechanism.
Water in a Container Has a Natural Rhythm
The physics is the same as a bathtub, which is the analogy everyone uses because it is exact rather than approximate.
Push the water in a bath toward one end and let go. It travels down, rises at the far end, comes back, rises at your end, and continues sloshing at a specific rate. That rate is not arbitrary — it is determined by the length and depth of the bath, and it will be the same every time.
Any body of water in a basin behaves this way. The technical term for the rocking motion is a seiche, and every bay, lake and gulf has a natural period at which it does it.
Larger and deeper basins slosh more slowly. A bath takes a few seconds. A large lake might take hours. And a bay of the right proportions can take half a day.
In the case in question, the bay is roughly 270 kilometres long, and the time for the water to rock from the mouth to the head and back again is somewhere between twelve and thirteen hours.
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The Push Arriving at the Right Moment
Now bring in the ocean tide, and the whole thing resolves.
The Atlantic tide floods into the bay every twelve hours and twenty-five minutes — the interval between successive high tides, which is slightly longer than twelve hours because the moon has moved on in its orbit and the Earth has to rotate a little further to catch up.
Compare that to the bay’s natural rocking period of twelve to thirteen hours. They are almost identical.
Which means every incoming ocean tide arrives at almost exactly the moment the bay’s own oscillation is ready to receive it. The push and the rhythm are synchronised.
The universal analogy is a child on a swing. A swing has a natural period, and pushing at random achieves very little. Push at the right point in every cycle — even gently — and the swing goes higher and higher, because each small input adds to what is already there rather than fighting it.
That is resonance, and it is the entire explanation. The ocean tide is a modest push applied with excellent timing, and the bay accumulates it.
Note what this implies: the enormous tides are not caused directly by the moon at all. They are caused by the ordinary Atlantic tide, which is itself caused by the moon, reinforcing an oscillation that belongs to the bay.
Why the Shape Matters Too
Resonance is the primary mechanism, and the geometry contributes.
The bay narrows and shallows toward its head, dropping from something like 130 metres deep to around 40. When a given volume of water is pushed into a progressively smaller cross-section, it has nowhere to go but up.
This is why the tidal range increases along the length of the bay rather than being uniform. Near the mouth it is a few metres. At the head, where the funnel is tightest, it reaches its maximum.
The sources are consistent about the ranking: the length of the bay is what matters most, because that is what sets the natural period and produces the resonance. The funnel shape is a secondary factor that is nonetheless significant.
Get the length wrong and no amount of funnelling produces this effect. A bay half as long would have a natural period out of step with the tide, the pushes would arrive at the wrong moments, and the oscillation would be damped rather than amplified.
What Sixteen Metres Actually Looks Like
The scale is difficult to convey and worth attempting.
Something in the region of a hundred billion tonnes of water enters and leaves the bay twice a day. A frequently cited comparison holds that during each six-hour tidal period, more water flows into or out of the bay than flows in all the rivers of the world combined over the same interval.
At low water, mud flats extend for a kilometre or more from the high-tide line, and it is possible to walk out across ground that will shortly be under many metres of seawater. Six hours later the same area is submerged.
There is a further phenomenon in the rivers that feed the bay. Where a river has a very gently sloping bed and the incoming tide is large enough, the leading edge of the tide travels upstream as a visible wave against the river current. This is a tidal bore, and several rivers in the area produce them.
A note of correction is worth making here, because it is a common confusion. The bore is not sixteen metres high. Tidal bores in that region are typically on the order of a couple of metres, which is still remarkable — the sixteen-metre figure is the total range between high and low water over six hours, not a wall of water arriving at once.
Why This Happens So Rarely
The conditions required are specific enough that very few places manage it.
You need a basin whose natural period is close to the tidal period, which means a particular combination of length and depth. You need it open to an ocean that supplies the driving tide. And you need it oriented so that the tide enters cleanly rather than at an unhelpful angle.
Change any of those and the resonance fails. Most coastlines have basins of the wrong dimensions, and the tide simply rises and falls by the ambient amount.
A small number of other locations around the world do achieve partial resonance and consequently have very large tidal ranges, which supports the explanation rather than undermining it — the effect appears wherever the numbers happen to line up.
There is one further consequence worth knowing: the effect is not static. Basin dimensions change over geological time as sea level and sediment shift, which means a bay can drift into or out of resonance across thousands of years. The tides there have not always been this extreme and will not always be.
Why the Tide Is Not the Same Everywhere Anyway
It is worth stepping back, because the resonance case is an extreme example of something that applies to every coastline.
The moon produces a tidal bulge in the open ocean of roughly a metre. What arrives at any particular shore is that signal after it has crossed an ocean basin, encountered a continental shelf, entered a gulf or a channel, and interacted with the shape of the seabed and the coast.
By the time it reaches land it has been amplified, damped, delayed, split or reflected, and the local range can be anything from almost nothing to many metres.
Enclosed seas with narrow connections to the ocean frequently have very small tides, because the driving signal cannot get in fast enough to raise the whole basin before it reverses. Some have a tidal range of a few centimetres.
Broad continental shelves tend to amplify, because a tidal wave entering shallower water slows and grows. Narrow channels between land masses can produce fierce tidal streams even where the vertical range is modest, since the same volume of water has to pass through a smaller gap.
And in a few places the basin dimensions happen to be right for resonance, which is the case described above.
Which means the tide table for any coast is not a moon calculation. It is a moon calculation put through the local geography, and the geography frequently matters more than the astronomy.
An Accident of Dimensions
What makes this satisfying is how contingent it is.
There is nothing special about the water, nothing unusual about the moon’s behaviour over that part of the world, and no local force doing anything remarkable. There is a bay that happens to be about the right length and depth to rock at almost precisely the rate the Atlantic pushes it.
Had it been shorter, longer, deeper or shallower by enough, it would be an ordinary bay with an ordinary tide and nobody would have heard of it.
Instead it is a place where you can walk out across the seabed in the morning, and where the same ground is under fifty feet of water by the afternoon — because a basin and an ocean happen to share a rhythm.
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