The Great Lakes do have real tides — the sun and the moon pull on them just as they pull on the ocean. But those tides are so small that the lakes are considered non-tidal. What moves the water in the Great Lakes, mostly, is the weather.
The water level and weather station at Holland, Michigan, at the entrance to Macatawa Bay. Water levels have been recorded near here since 1894. Credit: NOAA.
Tides too small to notice
True tides — changes in water level caused by the gravity of the sun and moon — do happen on the Great Lakes, in a semi-diurnal (twice daily) pattern. But studies show that even the Great Lakes spring tide, the largest tide produced when the sun and moon pull together, is less than five centimeters high. Changes in lake level caused by wind and barometric pressure are much larger and hide these small variations completely.
How lake levels really change
| Timescale | What drives it |
|---|---|
| Long-term | precipitation and the amount of water stored over many years |
| Annual | the seasons — a high in late spring and a low in winter, changing at a rate that can be measured in feet per month |
| Short-term | wind and weather |
The "tide" that is really a seiche
Wind and weather can set up a seiche, an oscillating wave that can be several feet high. On many of the Great Lakes, the time between a seiche's "high" and "low" is four to seven hours — very close to the six-hour period of ocean tides. That is why a seiche is often mistaken for a tide.
Sources
Based on "Do the Great Lakes have tides?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from oceanservice.noaa.gov
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