Lakes, unlike vast oceans, do not experience astronomical tides. While the gravitational forces from the Moon and Sun act upon all bodies of water, the effects are typically negligible in lakes. Despite the absence of true tides, lakes can exhibit various other water movements that sometimes lead to noticeable fluctuations in water levels.
The Mechanics of Ocean Tides
Ocean tides are primarily driven by the gravitational pull of the Moon and, to a lesser extent, the Sun. The Moon’s gravitational force attracts the Earth’s water, creating bulges on both the side of Earth facing the Moon and the side directly opposite. On the near side, water is pulled towards the Moon, while on the far side, inertia causes the water to bulge away from the Moon as the Earth itself is pulled more strongly towards the Moon.
These gravitational forces, known as tidal forces, are differential, meaning their strength varies across the Earth’s vast oceans. This difference in pull creates the noticeable rise and fall of ocean water levels. As the Earth rotates, different coastal areas pass through these bulges, resulting in the regular cycle of high and low tides. The immense scale and interconnectedness of the global ocean system allow these forces to generate substantial tidal movements.
Why Lakes Differ from Oceans
Lakes do not experience significant astronomical tides due to their relatively small size and enclosed nature compared to oceans. Although the gravitational pull of the Moon and Sun still acts on lake water, the limited surface area prevents the differential gravitational forces from creating substantial water bulges. The effect is too minor to be readily observed or cause significant displacement.
Even in the largest lakes, such as the Great Lakes, true astronomical tides are extremely small, measuring less than five centimeters (about two inches). These minuscule tidal effects are often masked by other, more powerful environmental factors.
Lake Water Movements That Aren’t Tides
Lakes experience various water movements that can cause noticeable changes in water levels, often mistaken for tides. One common phenomenon is a seiche, which is a standing wave that oscillates back and forth within an enclosed body of water, similar to water sloshing in a bathtub. Seiches are typically caused by strong, sustained winds pushing water towards one end of a lake or by sudden changes in atmospheric pressure.
When the wind subsides or pressure equalizes, the piled-up water rebounds, creating an oscillation that can last for hours or even days. These oscillations can lead to significant differences in water levels between opposite ends of a lake, with some extreme seiches on Lake Erie, for example, causing water level differences of more than five meters. Strong winds can also directly cause wind-driven setup, where water is pushed towards the leeward shore, resulting in a temporary rise in water level there and a corresponding drop on the windward shore. This effect is particularly pronounced in elongated lakes when winds align with their longest axis.
Changes in atmospheric pressure over a lake can also influence water levels. A drop in barometric pressure can cause the water level to rise slightly, while an increase in pressure can cause it to fall. While these barometric effects are less dramatic than seiches or wind-driven setup, they contribute to the complex and dynamic nature of water level fluctuations observed in lakes.