While Lake Michigan does not experience the dramatic, regular astronomical tides found in oceans, its water levels constantly fluctuate. The lake is subject to the same gravitational forces from the Moon and Sun that govern oceanic tides, but its enclosed nature significantly limits the resulting effect. Instead of true tides, the most noticeable short-term changes in water level are caused by weather-related phenomena. These changes often mimic the timing of ocean tides, leading to frequent confusion among observers.
Defining Astronomical Tides
Astronomical tides are the periodic, predictable rise and fall of sea level created by the gravitational pull of the Moon and the Sun on Earth’s oceans. The Moon is the primary driver because its proximity gives it a greater tide-generating force than the Sun. This force creates a bulge of water on the side of the Earth facing the Moon and a corresponding bulge on the opposite side.
As the Earth rotates beneath these two bulges, a coastal location experiences two high tides and two low tides during a cycle that lasts approximately 25 hours. The massive scale and depth of the oceans are necessary for these forces to generate the large, visible changes in water level typically associated with the word “tide.”
Gravitational Influence on Lake Michigan
The Moon and Sun exert their gravitational pull on Lake Michigan, meaning a true astronomical tide does exist, but it is extremely small. The limited size and depth of the lake basin prevent the formation of the large-scale tidal bulges seen in the open ocean. The Great Lakes are too small for the gravitational force to create the substantial horizontal flow of water necessary for a noticeable vertical change. Studies indicate that the largest possible gravitational tide is less than five centimeters, or about two inches, in height. This minor, twice-daily variation is almost completely masked by much larger fluctuations caused by weather, leading the Great Lakes to be officially considered non-tidal.
The Phenomenon of Seiches
The most significant short-term changes in Lake Michigan’s water levels are caused by a phenomenon called a seiche (pronounced “saysh”), which is a standing wave oscillation in an enclosed body of water. A seiche occurs when strong winds or sudden shifts in atmospheric pressure push water from one end of the lake basin to the other, essentially tilting the water’s surface. This initial water displacement, sometimes called a storm surge, can pile water several feet high on the downwind shore, while the water level drops on the opposite shore.
Once the wind or pressure disturbance passes, the piled-up water oscillates back across the lake as a standing wave, much like water sloshing back and forth in a bathtub. This back-and-forth motion can continue for hours or even days, with the period between a high and low water peak often falling between four and seven hours. These time intervals are similar to the six-hour period between high and low ocean tides, which is the primary reason seiches are often mistaken for tides.
These meteorological events can cause water level changes much larger than any gravitational tide, sometimes exceeding three feet in height on Lake Michigan. For instance, a fast-moving line of thunderstorms, known as a derecho, can rapidly create a storm surge that generates a particularly dangerous seiche. In 1998, a squall line crossing the lake created a seiche that caused the water level in Calumet Harbor, Illinois, to rise five feet in less than half an hour.
The sudden and significant changes in water level associated with large seiches can create dangerous conditions, including strong rip currents and localized flooding. While small seiches are an almost daily occurrence, the more intense events can be hazardous to boaters and people on the shoreline. The observable “tides” on Lake Michigan are not driven by the predictable forces of the Moon and Sun, but by the unpredictable, powerful forces of the weather.