If you were to stand by the ocean for a full day, you’d see the water rise and recede twice each, the high tide periods swallowing the beach, and the low tides revealing it again. The same, however, isn’t true of lakes.
Or, at least, it doesn’t appear to be true. The forces in nature that cause tides affect more than just the sea, including lakes. Although we can clearly see the results of these forces acting on the Earth’s oceans, it’s much harder to spot in other bodies of water like ponds, rivers, and lakes.
That doesn’t mean nothing is going on, however. Let’s take a look at what actually causes tides—and why we experience them differently, depending on the body of water.
How Tides Work
Two primary influences create tides: The gravitational pull of the Moon on the Earth and the Earth’s rotational force. (The Sun can also affect tides, but the Moon is the dominant player here.) The Moon’s gravitational pull on the Earth creates a tidal force, which, in turn, causes the ocean to bulge out on both the side of the Earth-facing the Moon and the side facing away from it.
The water on the side facing the Moon bulges due to the strength of the Moon’s gravitational pull—that is, the pull, which is always stronger on the side of the Earth nearest the Moon, tugs the ocean toward it, creating the first of the two bulges. The water on the opposite side, meanwhile, bulges due to inertia. Because the Moon’s gravitational pull is weaker on this side of the Earth, the water here is better able to resist that pull. This, combined with the strength of the Earth’s rotational force, results in the second bulge. We experience these two bulges as high tide.
The water that makes up these high tide bulges must come from somewhere, of course, which is where the low tide comes into the picture. When the bulges are at their height, the ocean has receded elsewhere on Earth, which we experience as low tide.
Meanwhile, as the Earth spins on its axis, it continually changes what part of the planet faces the Moon. This, in turn, causes the tides to move around the planet as well. Although there are exceptions, most shorelines see two high tides and two low tides each lunar day—that is, every 24 hours and 50 minutes.
Tides and Lakes
Here’s the thing: Lakes technically do experience tides, but they’re just too small for us to notice.
While standing on the shore of, say, Lake Superior, might feel about the same to us as standing on the coast of the Atlantic Ocean, in reality, oceans and lakes are vastly different from each other. For one thing, all of the oceans in the world are connected—that is, they’re really just one ocean, uncontained and so large that it covers about 71 percent of the Earth’s surface. The Earth’s lakes, however, are generally separate bodies of water, much smaller, and typically self-contained. These differences matter a great deal: Whereas the sheer volume of water held by the Earth’s giant global ocean results in tidal ranges of many feet on the planet’s shorelines, the volume of water held by any one of the Earth’s lakes is so minuscule that the tidal range becomes negligible.
Consider, for example, the Great Lakes. Made up of Lakes Superior, Michigan, Huron, Erie, and Ontario, this group of interconnected lakes is the world’s largest system of freshwater; together, they contain a whopping 21 percent of the Earth’s surface freshwater by volume. Due to some of their ocean-like characteristics, they’re sometimes even referred to as “inland seas.” Even so, the Great Lakes’ largest tides still only measure less than five centimeters in height. As such, they’re considered non-tidal.
If lakes as big as the Great Lakes see such a minuscule change between high and low tide, it’s only to be expected that smaller lakes would see even less change still.
The Illusion of Tides
Even though lakes don’t see enough action to be considered tidal, they can experience phenomena that look a bit like tides. The water level of a lake may change based on the season. The Great Lakes, for example, are at their highest in the summer and their lowest in the winter. Wind and atmospheric pressure can also create something called a seiche, causing water to slosh around in the lake basin for hours at a time.
While these occurrences can be mistaken for tides on a lake, they’re not the real thing. The real thing may still be occurring, though—even if you can’t see it.