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Do the Great Lakes have tides?

PortandTerminal.com, October 16, 2020

HALIFAX, NOVA SCOTIA – Do the Great Lakes have tides? Yes, but they are not big enough to notice. More about that in a moment.

As every schoolchild learns, the tides refer to the rise and fall of our oceans’ surfaces. They are caused by the attractive forces of the Moon and Sun’s gravitational fields as well as the centrifugal force due to the Earth’s spin. 

Here on earth, because it is so much closer to us than the Sun, the Moon is the dominant force affecting our tides. The Sun’s tide-generating force is about half that of the Moon’s.

National Ocean Service (NOAA) does a great job explaining the tides on its website.

Tides are very long waves that move across the oceans. They are caused by the gravitational forces exerted on the earth by the moon, and to a lesser extent, the sun. When the highest point in the wave, or the crest, reaches a coast, the coast experiences a high tide. When the lowest point, or the trough, reaches a coast, the coast experiences a low tide.”

The point facing the Moon is formed because the gravitational pull of the moon is strongest on whichever side of the Earth faces it. Gravity pulls the ocean towards the Moon and high tide occurs.

As the Earth spins, different areas of the planet face the moon, and this rotation causes the tides to cycle around the planet.

Imagine the ocean is shaped like a football pointing at the moon. The football’s pointed ends represent the parts of the Earth experiencing high tide and the football’s flat sides are the parts of the earth experiencing low tide.

What about the Great Lakes?

Satellite image of the Great Lakes

True tides—changes in water level caused by the gravitational forces of the sun and moon—do occur in a twice-daily pattern on the Great Lakes. Studies indicate that the Great Lakes spring tide, the largest tides caused by the combined forces of the Sun and Moon, is less than 2 inches in height. These minor variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes. 

So the answer is yes, there are tides on the Great Lakes but not big enough to notice.

The same is true for the Mediterranean Sea – there are tides but they are not large enough for most people to notice averaging just an inch or two. That’s because the narrow outlet/inlet with the Atlantic ocean essentially dampens the tidal effect.

Bruce Parker, former Chief Scientist at NOAA writes in his book “The Power of the Sea” about the ancient Greeks’ lack of knowledge that tides existed until 450 BC when they travelled to the Red Sea and observed the phenomena there.

The very narrow outlet/inlet to the Mediterranean at the Strait of Gibraltar dampens the effect of the tides.

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