Water levels in the Great Lakes have long-term, annual, and short-term variations. Long-term variations depend on precipitation and water storage over many years. Annual variations occur with the changing seasons. There is an annual high in the late spring and low in the winter. These changes occur at a rate that can be measured in feet per month.
True tides, changes in water level caused by the gravitational forces of the sun and moon, do occur in a semi-diurnal (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 five centimeters (two inches) in height. These minor variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes.
Consequently, the Great Lakes are considered to be essentially non-tidal.
For more information:
Center for Operational Oceanographic Products and Services
Great Lakes Water Level Data
Great Lakes Online
Tide Predictions and Data Frequently Asked Questions
Tides Tutorial, NOS Education
What are Tides? - Diving Deeper audio podcast