The Great Lakes’ vastness often prompts questions about whether they experience ocean-like tides. This article clarifies the nature of tides and the distinct water dynamics shaping these freshwater giants’ levels.
Understanding True Tides
True tides result from the gravitational forces exerted primarily by the Moon and, to a lesser extent, the Sun. The Moon’s gravity pulls on the Earth’s waters, creating bulges on both the side facing the Moon and the side opposite it. This occurs because the gravitational pull is stronger on the near side of Earth and weaker on the far side, effectively stretching the planet and its water.
As the Earth rotates, these bulges create the rhythmic rise and fall, resulting in high and low tides. The Sun also contributes to these forces, and when the Sun, Moon, and Earth align during new and full moons, their combined gravitational pull produces higher-than-average tides known as spring tides. Conversely, when the Sun and Moon are at right angles to each other, their gravitational forces partially cancel out, leading to more moderate tides called neap tides.
The Great Lakes and Gravitational Pull
The Great Lakes are subject to the same gravitational forces from the Moon and Sun. Despite this, gravitational tidal effects on the Great Lakes are exceptionally small and largely imperceptible. Studies indicate that even the largest gravitational tides, known as spring tides, are less than five centimeters (about two inches) in height.
The relatively small size of the Great Lakes, combined with their landlocked nature and rigid basins, means any gravitational water movement is minimal. This minor variation is typically masked by other, much stronger influences on lake levels. Consequently, for practical purposes, the Great Lakes are considered non-tidal bodies of water.
What Really Affects Great Lakes Water Levels
While astronomical tides are negligible, Great Lakes water levels fluctuate due to meteorological and hydrological factors. One significant phenomenon is a seiche, a standing wave that oscillates within a confined body of water, similar to water sloshing in a bathtub. These oscillations are often triggered by strong winds or sudden shifts in atmospheric pressure, causing rapid and temporary changes in water levels that can be several feet high. For example, a seiche in Lake Erie has been observed to cause water level differences of more than 5 meters (15 feet) between its ends.
Wind setup plays a role, where strong, sustained winds push water toward one end of a lake, causing levels to rise on the downwind side and drop on the upwind side. Atmospheric pressure changes also influence water levels; lower pressure can cause a slight rise, while higher pressure can lead to a slight drop. These short-term fluctuations can last from a few hours to several days.
Beyond these short-term events, Great Lakes water levels experience seasonal and long-term variations. Seasonal changes are driven by precipitation, evaporation, and runoff from snowmelt. Water levels rise in spring and early summer due to increased precipitation and snowmelt, then decline through fall and winter as evaporation rates increase. Long-term fluctuations, spanning years or decades, are influenced by persistent wet or dry periods, affecting overall water supply to the lakes.