Fact or Fiction: Are there tides on the Great Lakes like the ocean?

Weather Blog

Sleeping Bear Dunes, North Bar Lake. Lake Michigan. July 2009. Photo courtesy of K. McConnelly.

ROCHESTER, N.Y. (WROC) — The rising and falling of the ocean tides are as reliable as the changing weather. Tides occur as a result of the gravitational force of both the sun and moon that causes the ocean levels to change on a semi-diurnal, or twice a day cycle.

They are essentially large tidal waves that slowly emanate from the ocean to the shoreline where the highest part of the wave (crest) is known as high tide, and the lowest part of the wave (trough) is known as low tide.

What causes tides?

Ocean tides are the changing of water levels of the ocean caused by the gravitational force of both the sun and moon.

The moon’s gravitational pull plays the biggest role and creates what’s known as the tidal force. The tidal force creates a squeezing effect on the entire planet, and forms a “tidal bulge” on both opposite sides of the earth.

These tidal bulges make up tides that occur on earth every day, and because earth rotates through both of these bulges once a day, you get two high tides and two low tides.

Image courtesy: noaa.gov

The tidal bulge causes the oceans to bulge out like a football on the side that’s closest to the moon and the side that’s farthest. This happens because the tidal force is a differential force that’s equally distributed across the whole planet.

Did you know? High tides can range from 2 feet in open waters up to 40+ feet near the coastline.

Do lakes such as our Great Lakes have tides?

The answer is yes, our Great Lakes do have tides that occur twice each day, but they are much smaller in scale and barely noticeable unlike the ocean.

The largest “lake tide” that happens is called the Great Lakes spring tide, and is less than 5 centimeters, or 2 inches in height. In reality, it’s the wind and pressure changes that have a larger effect on lakes and can usually overcome the effects of this “mini tide.” Because of this, the Great Lakes are technically considered to be non-tidal.

Strong winds can create large waves that are pushed on to shore, and large changes in pressure over these lake bodies can produce what’s known as seiches.

A source from NOAA states, “Seiches are typically caused when strong winds and rapid changes in atmospheric pressure push water from one end of a body of water to the other. When the wind stops, the water rebounds to the other side of the enclosed area. The water then continues to oscillate back and forth for hours or even days. In a similar fashion, earthquakes, tsunamis, or severe storm fronts may also cause seiches along ocean shelves and ocean harbors.”

Lake Erie is known for seiches as southwesterly winds often blow across these waters. Back in 1844, a 22-foot seiche was strong enough to breach a 14-foot high sea wall that caused 78 fatalities, and caused Niagara Falls to stop flowing temporarily as ice blocked the water flow. Seiches can also cause lakeshore flooding that affects places like Buffalo.

Precipitation, evaporation and runoff can also affect lake levels on a much greater scale too.

Why should we care about tides?

As ocean levels rise, coastal flooding becomes a growing concern as these heightened ocean levels mean higher tides than ever before. Scientists use satellites that Meteorologists use for tracking weather to monitor how the tides are changing every day to ensure to safety and protection of our communities as well as wildlife along the coast.

~Meteorologist Christine Gregory

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