Many wonder if the Great Lakes experience tides similar to the ocean. While technically yes, these tidal movements are incredibly small and generally imperceptible. Agencies like NOAA consider the Great Lakes non-tidal, as other forces overshadow minor gravitational effects. The largest tides, known as spring tides, measure less than five centimeters (about two inches).
Understanding Tides
Tides result primarily from the gravitational forces of the Moon and, to a lesser extent, the Sun, on oceans. The Moon’s gravity pulls on the water closest to it, creating a bulge. On the side of Earth farthest from the Moon, inertia causes water to bulge outwards as Earth is pulled towards the Moon. As Earth rotates, different regions pass through these two bulges, resulting in the rhythmic rise and fall of high and low tides.
Though the Sun is far more massive, the Moon’s closer proximity means its gravitational influence is more significant. These combined effects create complex tidal patterns.
Why Great Lakes Tides Are Different
Despite universal gravitational forces, the Great Lakes exhibit negligible astronomical tides, classified as non-tidal. Their small, enclosed nature, compared to vast oceans, explains this difference. Limited water volume and restricted space prevent significant bulk movement for noticeable tidal fluctuations.
This minute change is easily overshadowed by stronger environmental factors. While tides technically occur, their magnitude is too small to impact navigation or shoreline activities.
Other Factors Affecting Great Lakes Water Levels
Great Lakes water levels are influenced by various natural phenomena that create far more significant fluctuations than astronomical tides. These phenomena include seiches, wind set-up, atmospheric pressure changes, and seasonal variations.
One dramatic phenomenon is a “seiche,” a standing wave oscillating within a confined body of water, like water sloshing in a bathtub. Seiches are primarily caused by strong winds pushing water to one side of a lake or by rapid changes in atmospheric pressure. These events cause rapid, substantial water level changes, sometimes several feet high, over four to seven hours, often mistaken for tides due to similar timing. Lake Erie, shallow and oriented along prevailing wind directions, is particularly susceptible to large seiches, with historical events causing water level differences of 13 to 15 feet between its ends.
Strong, sustained winds also cause “wind set-up” or “storm surge.” This occurs when wind blows consistently across water, piling it up at the downwind end and lowering levels at the upwind end. These wind-driven surges can temporarily raise water levels by one to eight feet. Changes in atmospheric pressure also directly affect water levels; high-pressure systems depress surfaces, while low-pressure systems allow them to rise. These pressure changes often combine with wind effects to amplify water level fluctuations.
Beyond short-term weather, Great Lakes water levels vary seasonally and long-term, driven by water entering and leaving the system. Precipitation (rain and snow) adds water and contributes to runoff, increasing levels. Conversely, evaporation from lake surfaces removes water, reducing levels, especially when warm lake waters meet cold, dry air in fall and winter. These processes create an annual cycle: levels rise in spring and early summer due to snowmelt and rainfall, then decline through late summer, fall, and winter as evaporation increases.
Annual seasonal fluctuations average 0.4 to 0.6 meters, or about 1.3 to 2 feet. Over decades, long-term changes in precipitation and evaporation can cause water levels to fluctuate by several feet, with variations of four feet for Lake Superior and six to seven feet for the other Great Lakes having been recorded.