The Moon & the Bay of Fundy

The Bay of Fundy is an inlet of the Atlantic Ocean between New Brunswick and Nova Scotia, on Canada's east coast. I wanted to know why its tides are so large. The Moon pulls on oceans everywhere, so why does this particular stretch of coast respond so strongly?

Parks Canada reports a difference between high and low tide of up to 12 metres at Fundy National Park, and up to 16 metres at the head of the bay.¹ That difference is the tidal range. It measures how far the surface rises and falls, rather than how deep the water is.²

The map gives us part of the explanation. The broad Gulf of Maine opens into the Bay of Fundy, which gets narrower toward its upper end. But the shape of the bay also affects how long its water takes to rock back and forth.

The Sun and Moon have gravitational pulls that vary across Earth. The Moon pulls more strongly on Earth's near side than its far side. On the far side, it pulls Earth itself more strongly than it pulls the water; relative to Earth, that water is drawn outward too. That difference stretches the ocean along the Earth-Moon direction, with water tending to gather on both the facing and opposite sides.³ As Earth rotates, that tidal influence repeats. A place turns back toward the Moon roughly every 24 hours and 50 minutes; dividing that time into two cycles gives us about 12 hours and 25 minutes between high tides.⁴

That is the simple astronomical picture. Continents, water depth and coastlines shape how the tide actually moves through an ocean. Around Fundy, the incoming tide follows this roughly twice-daily, or semidiurnal, rhythm.

The bay has a rhythm of its own. Picture water displaced toward one end of a basin. Gravity draws it back toward a level surface, but its momentum carries it past that position. It rocks back again. This back-and-forth motion is a seiche, something that can happen in a bay or a bathtub.⁵ Parks Canada puts Fundy's natural cycle at about 13 hours.¹

The incoming tide keeps driving that motion. Because its rhythm is close to the bay's natural one, it can transfer energy efficiently into the oscillation, much as a well-timed push helps a swing climb higher. This is near-resonance. Friction and other losses limit the response; the water does not keep rising higher with every tide.⁶

The comparison below isolates that timing effect in a simplified oscillator. Both examples start at rest and receive the same strength of periodic forcing. Only the rhythm changes. Near the natural rhythm, a much larger movement builds up and settles into a repeating pattern.

Then there is the shape of the bay. Toward its head, the shores converge: incoming water has less room to spread sideways. Where more water flows into a stretch than flows out, it accumulates and the surface rises. A narrower stretch needs less added water for the same rise. The ebb reverses that exchange. This helps explain how a funnel-shaped coast can enlarge the difference between high and low water.⁷

Depth matters to the rhythm too. Long tidal waves travel more slowly in shallower water, so the basin's depth as well as its length affects the time it takes to oscillate.⁸ Shallower water also brings friction, so narrowing and shallowing are not a recipe for enormous tides everywhere. Fundy combines its shape with a natural rhythm close to the incoming tide's.

The three Maine gauges on the map show how different the response can be along the approach to Fundy. In NOAA's 1983-2001 reference statistics, Eastport's mean tidal range is about twice Portland's, with Bar Harbor between them.^{9, 10, 11} Mean range here is average high water minus average low water. The chart compares places; it cannot tell us how much of the difference comes from resonance rather than local geography.

Farther into the Canadian bay, Parks Canada reports the much larger ranges below. These describe how large the difference between high and low tide can become, rather than the long-term averages in the Maine chart.¹

At Alma Beach, Parks Canada says low tide can leave more than a kilometre of exposed flats between the high-tide line and the water's edge.¹ That is the horizontal reach of the tide: the same rise and fall spreads across a gently sloping shore. As the next tide comes in, that stretch of exposed seabed becomes part of the bay again.