True tides, changes in water level caused by the gravitational forces of the sun and moon, do occur in a semi-diurnal pattern on the Great Lakes. The investigations of the U.S. Coast and Geodetic Survey indicate that the spring tide, the largest tides caused by the combined forces of the sun and moon, is less than 2 inches (5cm) in height. These minor variations are masked by the greater fluctuations in lake levels produced by wind and barometric pressure changes. Consequently, the Great Lakes are considered to be essentially non-tidal.
The moons lunar force is nine million times weaker than the force of Earth's gravity at the Earth's surface.
If the Earth's surfaces were completely submerged by an ocean of unvarying depth and if the Earth were to face the Moon at all times, there would be tides two feet high.
In addition to rivers, lakes are also affected by the lunar tide action. These are effects usually easier to detail. Lake high tides are opposite the Moon when the Moon is below the horizon. If the Moon is up, the water of the lake follows the Moon around the shore. Low tide goes along the opposing shore. Lake Michigan, for example, has a tidal difference of 1 and 3/4 inches.
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