Many people associate large bodies of water with the rhythmic rise and fall of tides, a phenomenon readily observed along ocean coastlines. This raises a common question about the Great Lakes, vast freshwater seas. Despite their immense size, the Great Lakes do not exhibit noticeable tides in the way oceans do. This article explores the scientific reasons behind this difference, highlighting the distinct forces that govern water levels in these freshwater giants.
Understanding Oceanic Tides
Oceanic tides are primarily a result of the gravitational pull exerted by the Moon and, to a lesser extent, the Sun. The Moon’s gravity creates bulges of water on opposite sides of Earth. One bulge forms on the side facing the Moon, where the gravitational pull is strongest, drawing water towards it. A corresponding bulge forms on the opposite side because the Earth is pulled more strongly than the water on that distant side, causing the water to bulge outwards.
As Earth rotates, different locations pass through these tidal bulges, experiencing two high tides and two low tides approximately every 24 hours and 50 minutes. The vastness and depth of the oceans allow these gravitational forces to generate significant water level changes. For instance, the typical tidal range in the open ocean is about 1 meter (3 feet), while in confined areas like Canada’s Bay of Fundy, it can reach up to 16 meters (52 feet). This scale and connectivity are fundamental to the mechanics of ocean tides.
The Great Lakes: A Matter of Scale
The primary reason the Great Lakes do not experience significant tides is their relatively smaller scale and enclosed nature compared to oceans. While the same gravitational forces from the Moon and Sun act upon the Great Lakes, their limited surface area and volume prevent the formation of large-scale tidal bulges. The “wavelength” of the gravitational tidal force is simply too vast to manifest a noticeable effect within these basins.
Unlike oceans, which are globally interconnected and allow for the free propagation of tidal waves, the Great Lakes are inland bodies of water. Their restricted dimensions mean that the water cannot accumulate sufficiently to create the substantial rise and fall seen along coastlines. Consequently, the gravitational forces, though present, do not generate the pronounced oscillations characteristic of oceanic tides.
What Really Affects Great Lakes Water Levels?
Since astronomical tides are negligible, other meteorological and hydrological factors primarily influence Great Lakes water levels. One common phenomenon is a “seiche,” which is a standing wave that oscillates within a lake. Seiches are caused by strong winds pushing water to one end of a lake or by sudden changes in atmospheric pressure. When the wind subsides or pressure equalizes, the water “sloshes” back and forth across the basin.
These seiches can cause rapid and noticeable changes in water levels, sometimes reaching several feet. For example, seiches on Lake Erie have been observed to cause water level differences of up to 2.4 meters (8 feet) between its ends. Wind setup, where sustained winds pile water on the downwind shore, is another significant factor. Changes in barometric pressure can subtly affect water levels, with higher pressure depressing the water surface and lower pressure allowing it to rise slightly.
The Imperceptible Tidal Effects
While the Great Lakes are considered non-tidal for practical purposes, extremely minor tidal effects do exist due to the persistent gravitational forces of the Moon and Sun. These true tidal fluctuations in the Great Lakes are incredibly small, measuring less than 5 centimeters (2 inches).
These tiny astronomical tides are completely masked by the much larger and more frequent fluctuations caused by wind, barometric pressure, and seiches. The daily and seasonal variations from weather patterns far outweigh any gravitational tidal influence. While technically present, these imperceptible tidal effects have no practical impact on navigation, shoreline activities, or the overall dynamics of Great Lakes water levels.