Photo: pericomart (flickr)
When talk about tides, we usually mean the way the level of the sea rises and falls on a favorite beach, but at a lake shore, we never see the shore line creeping closer. Why don’t lakes have tides as well?
To answer this question, we need first to explore what causes ocean tides. On the side of earth that faces the sun and moon, it’s mainly the gravitational pull of these two bodies that draws the ocean water into a bulge. On the opposite of the globe it’s the centrifugal force generated by the rotation of the earth that causes a bulge of the ocean water. These simultaneous bulges of water are what we know as high tide.
Low tides occur on other parts of the globe, where water drains away to fill those bulges. During new and full moons, tides range the widest because the earth, moon and sun are all lined up, so the pulls of moon and sun work together.
In lakes and ponds, the same forces are at work, but in miniature. How much tides range depends partly on the size, depth and slope of the water’s basin. On seacoasts, where the basin stretches halfway across the globe, tides range about 6 to 10 feet. Compare that to Lake Michigan, where tides range just a few inches! Smaller lakes and ponds also register the effect to lesser degrees, in most cases imperceptibly. Even when we can’t see it, the whole earth responds to the gravitational pulls of the sun and moon.