The Great Lakes, vast freshwater bodies, exhibit unique characteristics that differentiate their water level fluctuations from oceanic tides. Although extremely minor, imperceptible tidal effects do exist, the significant water level changes observed on the Great Lakes are driven by other powerful natural phenomena.
The Nature of Ocean Tides
Ocean tides are primarily generated by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun. The Moon’s gravitational pull creates bulges of water on the side of Earth facing it and on the opposite side. As Earth rotates, these bulges result in two high tides and two low tides each day in most oceanic locations. The Moon’s closer proximity makes its gravitational influence on Earth’s oceans approximately twice as strong as the Sun’s.
Why the Great Lakes Are Different
The Great Lakes, while immense, are relatively small and enclosed freshwater basins. Their limited size and depth mean that the gravitational forces from the Moon and Sun cannot create noticeable tidal bulges like those found in oceans. A minuscule, millimetric tidal effect, typically less than five centimeters (two inches) in height, does technically occur on the Great Lakes. However, these minor variations are imperceptible and are completely masked by other, much larger water level changes. For practical purposes, the Great Lakes are considered non-tidal.
Real Water Level Changes on the Great Lakes
Water levels on the Great Lakes are dynamic, moving with weather patterns, seasons, and long-term climatic shifts. These fluctuations are primarily due to meteorological effects rather than gravitational tides.
One significant phenomenon is a “seiche,” a standing wave caused by strong winds or rapid atmospheric pressure changes. Seiches can cause water levels to fluctuate dramatically, sometimes by several feet, over hours or days. For instance, Lake Erie, being shallow and oriented east-west, is particularly susceptible to large seiches, where water level differences of more than 5 meters (16 feet) have been observed across the lake.
“Wind set-up” or “storm surge” occurs where persistent strong winds push water towards one end of a lake. This action causes water levels to rise on the downwind shore while simultaneously lowering them on the upwind side. These surges can range from one to eight feet in height. Changes in atmospheric pressure also contribute to water level fluctuations, with high-pressure systems causing slight drops and low-pressure systems leading to slight rises.
Great Lakes water levels experience seasonal and long-term fluctuations. Seasonal changes, driven by factors like spring snowmelt, precipitation, and evaporation, typically see levels rise in late spring and early summer and fall in winter. The average seasonal range is about 0.4 to 0.6 meters (1.3 to 2 feet) across the lakes. Long-term fluctuations result from sustained periods of high or low net basin water supplies. These multi-year changes can lead to differences of several feet between extreme high and low water levels, with Lake Superior varying by 1.2 meters (3.9 feet) and other lakes by over 1.8 meters (5.9 feet).