How the tides work

Tides are the periodic rise and fall of the sea caused by the gravitational pull of the moon and the sun on the Earth. Fundy’s tides are the highest in the world because of an unusual combination of factors: resonance and the shape of the bay.

The water in the Bay of Fundy has a natural resonance or rocking motion called 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. As the ocean tide rises and floods into the bay every 12 hours and 25 minutes, it 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 is required to keep the swing moving. Likewise the seiche in the bay is sustained by the natural resonance of the ocean tides.

The bay’s shape and bottom topography are secondary factors contributing to Fundy’s high tides. The bay becomes narrower and shallower — from 130 m (426′) to 40 m (131′) — toward the upper bay, forcing the water higher up onto the shores.

Source: Tides of Fundy by the Fundy Guild at Fundy National Park.

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