Lake Michigan does not experience traditional tides; the water level fluctuations observed are caused by entirely different physical mechanisms. The Great Lakes are considered non-tidal because the gravitational forces that drive ocean tides are overwhelmed by other factors in this enclosed freshwater system. The drivers of Lake Michigan’s short-term water level changes are meteorological events, often mistaken for the rhythmic rise and fall of a tide. Understanding the distinction between these forces is key to grasping the unique hydrology of this body of water.
What Exactly Are Tides?
Tides are the predictable, periodic rise and fall of a body of water, primarily caused by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun. The Moon’s gravitational pull creates a bulge of water on the side of the Earth facing it, and an inertial bulge forms on the opposite side as the Earth is pulled toward the Moon. As the Earth rotates, coastal areas pass through these two bulges, resulting in two high tides and two low tides each day. The Moon’s gravitational influence is approximately 2.2 times stronger than the Sun’s due to its closer proximity to Earth. When the Sun, Moon, and Earth align during new and full moons, their combined gravitational force produces the highest high tides, known as spring tides.
The Physical Limits of Inland Seas
Although the gravitational forces of the Moon and Sun act on Lake Michigan’s water, the lake’s size and enclosed nature prevent the formation of substantial tides. The maximum gravitational tide on the Great Lakes, even during a spring tide alignment, is less than five centimeters (about two inches) in height. This negligible variation is easily masked by far greater fluctuations caused by weather. The limited size of the lake basin does not allow the volume of water movement necessary to generate the large tidal bulges seen in the open ocean.
The natural period of oscillation for the Lake Michigan basin is also not in sync with the 12.42-hour lunar tidal cycle. The physical dimensions of the basin mean that any minor gravitational tidal effect is too small to be practically relevant. Consequently, the massive, daily fluctuations that define ocean tides are absent.
Lake Michigan’s True Water Movers: Seiches
The dramatic, short-term water level changes on Lake Michigan are caused by a phenomenon called a seiche (pronounced saysh), which is an oscillating standing wave in an enclosed body of water. Seiches are primarily initiated by meteorological disturbances, such as sustained, strong winds or rapid changes in atmospheric pressure. When strong winds blow across the lake’s surface, they push water toward the downwind shore, causing a temporary pile-up known as a storm surge.
Once the wind stops or the atmospheric pressure rapidly shifts, the piled-up water rebounds, sloshing back toward the opposite shore, similar to water oscillating in a bathtub. This back-and-forth movement can continue for hours or even days, with the period between the peak high and low often lasting between four and seven hours. Seiches can cause water levels to change by several feet in moments, creating rip currents and localized flooding. A severe seiche event in 1998, caused by a fast-moving line of thunderstorms called a derecho, resulted in significant water level changes.
Longer-Term Water Level Changes
Water levels in Lake Michigan also fluctuate significantly over longer periods, driven by changes in the hydrological cycle rather than short-term weather or gravity. Seasonal variations occur annually, with water levels typically rising an average of 12 to 18 inches from winter to early summer. This pattern is primarily due to winter precipitation accumulating as snow and ice, followed by increased spring runoff and rain contributing to the lake’s volume.
Long-term fluctuations, which can last for years or decades, are governed by the overall water budget of the entire Great Lakes basin. This budget includes over-lake precipitation, runoff from the surrounding land, and evaporation from the lake surface. For example, a sequence of years with above-average precipitation and high ice cover contributed to the record high water levels experienced between 2017 and 2020. Conversely, prolonged periods of low precipitation and high evaporation rates can lead to multi-year declines in the lake’s average water level.