Lake Superior, like other large inland bodies of water, does not experience significant astronomical tides. While the Moon and Sun exert gravitational forces, the resulting water level changes are minuscule. Instead, Lake Superior’s water levels fluctuate due to atmospheric conditions and the lake’s physical characteristics, creating dynamic changes often mistaken for oceanic tides.
What Causes Ocean Tides?
Ocean tides primarily result from the gravitational forces of the Moon and, to a lesser extent, the Sun on Earth’s waters. The Moon’s closer proximity makes its gravitational pull the dominant factor in tide generation. This attraction causes the oceans to bulge outwards on both the side facing the Moon and the side directly opposite. As Earth rotates, different regions pass through these bulges, leading to the rhythmic rise and fall of water levels observed as high and low tides.
The Sun also contributes to tidal forces, its influence apparent when celestial bodies align. When the Earth, Moon, and Sun align (during new or full moons), their combined gravitational pull creates larger spring tides. Conversely, when the Sun and Moon are at right angles, their gravitational forces partially counteract, resulting in smaller neap tides. This interplay of gravitational forces and Earth’s rotation dictates ocean tide patterns.
Why Lakes Don’t Experience Astronomical Tides
Lakes, including Lake Superior, do not exhibit noticeable astronomical tides due to their relatively small size and enclosed nature compared to oceans. While the Moon and Sun’s gravitational pull affects lake water, the force is nearly uniform across a lake’s surface. This uniformity prevents the differential pull necessary to create the significant bulges characteristic of ocean tides. Unlike oceans, lakes lack the volume and open connections for large-scale water redistribution.
Minute tidal changes that occur in large lakes like Lake Superior are typically less than 5 centimeters (about 2 inches). These minor fluctuations are often masked by more powerful forces, such as wind and changes in atmospheric pressure. For practical purposes, large freshwater bodies like the Great Lakes are considered non-tidal. Unlike oceans, where tidal cycles are a prominent daily feature, lakes lack substantial tidal ranges.
Real Water Level Changes in Lake Superior
While Lake Superior does not experience astronomical tides, its water levels frequently change due to meteorological factors, primarily through a phenomenon known as a seiche. A seiche is a standing wave that causes water in an enclosed basin to oscillate back and forth, similar to water sloshing in a bathtub. These oscillations are triggered by strong winds pushing water across the lake or rapid changes in atmospheric pressure. When wind subsides or pressure stabilizes, the piled-up water surges back, setting off continuous motion lasting hours or days.
Seiches can cause water levels to rise significantly on one side of the lake while dropping on the opposite side. For instance, a notable seiche event on Lake Superior caused water levels in Sault Ste. Marie to fluctuate by 65 inches (1.65 meters) within hours. Smaller seiches are common, but intense events can leave boats stranded or cause collisions in harbors. Lake Superior has a characteristic seiche period of approximately eight hours.
Beyond seiches, other weather-driven effects also influence Lake Superior’s water levels. Sustained strong winds can lead to “wind set-up,” where water is blown to one end of the lake, causing elevation on the downwind shore and a decrease on the upwind side. Atmospheric pressure differences also impact water levels; lower pressure results in higher levels, while higher pressure causes water to depress. These short-term fluctuations, ranging from a few inches to over a meter, account for the observable water level changes on Lake Superior.