Fundy's tides are the highest in the world because of an unusual combination of resonance (or seiche) and the shape of the bay.

Like water in any basin, the water in the Bay of Fundy has a natural rocking motion called a seiche. You could compare this to the movement of water in a bathtub. Although the water in a bathtub sloshes from one end to the other and back again in a few seconds, it takes about 13 hours for the water in the bay to rock from the mouth of the bay to the head of the bay and back again. The Atlantic Ocean tide rising and flooding into the bay every 12 hours and 25 minutes reinforces the rocking motion. To imagine this, picture an adult giving a gentle push to a child on a swing. Just a very small push, at the right time, is enough to make it go higher and higher. A pulse from the ocean tides sustains the seiche in the bay.

The Bay of Fundy's length is important. That's what makes the seiche frequency match the pulse from the Atlantic Ocean tides.

The bay's shape is of secondary importance although still significant. The bay becomes narrower and shallower towards its head, forcing the water higher up the shores.

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