The moon’s influence on lakes is often questioned, given the dramatic rise and fall of ocean tides. While the moon’s gravity pulls on all water, the resulting change in lake water level is vastly different from the large-scale tides observed in oceans. This difference is primarily due to the physical characteristics and size constraints of inland bodies of water.
The Mechanism of Tidal Forces
Ocean tides are a consequence of the gravitational pull exerted by the moon and, to a lesser extent, the sun on the Earth’s water. This force is not uniform across the planet’s surface; instead, it is a differential force. The side of the Earth facing the moon experiences a stronger pull, which draws the water toward it, creating a tidal bulge.
A second bulge forms on the side of the Earth farthest from the moon, where the gravitational pull is weakest. This occurs because the moon’s gravity pulls the solid Earth away from the water on the far side, which lags behind due to inertia. As the Earth rotates through these two bulges, an observer experiences two high tides and two low tides approximately every 24 hours and 50 minutes. This mechanism requires a continuous, massive body of water, like the ocean, to allow the water to move freely and redistribute itself.
Why Lakes Behave Differently Than Oceans
The mechanics that generate noticeable ocean tides are rendered negligible when applied to lakes, mainly because of their limited size and enclosure. The tidal force depends on the difference in gravitational attraction across the body of water. In an ocean that spans thousands of miles, this gravitational differential is significant enough to move enormous volumes of water.
Lakes, even the largest ones, do not possess the necessary surface area for the differential force to create a substantial bulge. The water body is simply too small for the change in gravitational pull from one side to the other to generate a noticeable effect. Furthermore, lakes are confined by rigid land boundaries that prevent the continuous, unrestricted flow of water needed to form the large, daily tidal bulges seen in the open ocean.
The relatively shallow depth of most lakes, compared to the deep ocean basins, also limits the water’s ability to respond to small gravitational forces. For example, the Great Lakes have tides measured in inches, which is insignificant compared to the typical ocean tidal range of many feet. The lake basin’s physical characteristics prevent the water from accumulating and resonating with astronomical forces like the massive, interconnected ocean system does.
The Presence of Micro-Tides
Despite the lack of visible movement, the moon’s gravitational force does produce a measurable effect on lakes, referred to as “micro-tides.” These are genuine, gravitationally induced tides, but their amplitude is extremely small, typically measured in millimeters or fractions of an inch. In a lake as large as Lake Superior, for example, the lunar tide is often less than 5 centimeters (about 2 inches).
Measuring these micro-tides is challenging because they are easily masked by other powerful meteorological factors. Changes in barometric pressure, wind stress, and river inflows can cause much greater and more immediate fluctuations in lake level than the gravitational pull. Specialized, highly sensitive water level gauges are required to isolate the tiny, predictable gravitational signal from the background noise of weather-related water movements.
A separate phenomenon that causes noticeable water level changes in lakes is a seiche, which is a standing wave oscillation. Seiches are typically triggered by sudden changes in atmospheric pressure or strong winds pushing water to one side of the basin. While a seiche causes significant water level change, it is an inertial response to weather, not a true gravitational tide.