On the Old Mission Peninsula in Lake Michigan's Grand Traverse Bay, Ted Cline gave up guessing years ago. Since 1957 he and his wife, Jean, have lived here in a home overlooking the bay. From the edge of the lawn, steps go down to a fine sand beach, almost to the water when the level's up. But not long after the Clines moved in, they had to take a walk to wet their feet—the level of Lake Michigan had fallen lower than at any time since surface measurements were first recorded in the 19th century. A shade over 20 years later the lake was up again, higher than it had been in a century, lapping at the foot of the Clines' steps.

Last summer I stood with Cline above the lake and saw how it might have looked back in that earlier record-low time of 1964. For now, after years of drought and simmering annual temperatures, the levels of the Great Lakes had fallen once again, all the way from Duluth, Minnesota, to Kingston, Ontario, at the head of the St. Lawrence River. This is not just a matter of inconvenience to a hundred thousand riparian landowners along U.S. and Canadian shores, though more than a few of them are being put to the expense of extending their docks. It is a matter of concern to the multitudinous cities and farms dependent on lake water, to the boating and fishing segments of the region's multibillion-dollar tourism industry, and to the operators of deep-draft ships that ply these inland ports and waterways to hitch North America's heartland to the markets of the world.

And right here the wide, weedy beaches and rocky shoals of the Old Mission Peninsula said it all: Another couple of years of climatic deprivation and the greatest of these lakes might well bottom out at levels lower than any recorded in historic times.

"Oh, the lake will come back up someday," Cline said. A retired surgeon and World War II leatherneck dive-bomber pilot, he has seen much of the upper Great Lakes from the cockpit of his private plane, flying aerial photography assignments for books and magazines. Now, at the top of the steps above his dehydrated beach, I could only say to Cline, "I hope you're right." 

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While the tidal currents in the Great Lakes are weak as compared with ocean currents, other forces can act upon the lakes to cause significant movement of water. The Great Lakes can form seiches—oscillations of surface water that can change water levels by three feet in just moments. A seiche, a French word meaning "to sway back and forth," is formed when wind and air pressure change and cause the surface of a lake to rock back and forth. While small-scale seiches go unnoticed, large seiches are powerful enough to bang anchored ships together. Seiches can also impact the biology of lakes by pulling nutrients and other sediments into the upper layers of the lakes.

—Nora Gallagher