Normally, the waters of Lake Michigan sit around 580 feet.
Government agencies, industries, and navigational charts across the U.S. and Canada all use this figure.
But the height of the upper Great Lakes is actually off by a foot or so, says Laura Rear McLaughlin of the National Oceanic and Atmospheric Administration, or NOAA.
Current water levels are based on measurements done back in the 1980s. And since then the earth has moved.
McLauglin is leading efforts across the U.S. and Canada to recalculate the water levels of the Great Lakes and make sure everyone is on the same page.
These measurements need to be redone every few decades because of geologic processes that go back to the Ice Age.
Some 20,000 years ago, North America was covered by heavy, mile-high glaciers. After so long under pressure, “that land is going to slowly bounce back after that ice melts,” McLaughlin says.
That bounce back, called the post-glacial rebound or glacial isostatic adjustment, is still happening thousands of years later — as though the earth is a giant mattress where a glacier once lay, still reclaiming its original shape.
Over the next few years, researchers will combine satellite and field measurements from over 200 locations across the U.S. and Canada to come up with new elevations for the Great Lakes. They plan to release the new numbers in 2025.