A third and more tentative theory points to extremely deep lows arriving every 150 years. Searching for that longer view, U.S. and Canadian scientists study lake records to see if this year's retreat is part of a more profound pattern, and, if so, what course future levels are likely to take. The lakes have been here for more than 10,000 years and we have only this tiny, 140-year window of recorded data to study. It's hard to come to useful conclusions. Looking for patterns As a rule, lakes swell to their highest in summer, as spring melting rolls down the system.
Levels sink to their lowest in winter, when snow piles up and freezes flows from tributaries.
This tends to be a reliable annual pattern. Reviewing their records, Gauthier and others have noticed broader patterns of lake declines that seem to take place about every 30 years. When historic lows were set on Lakes Michigan and Huron in 1964, the word drought was used freely by journalists and scientists. That year, residents near Lake Charlevoix were treated to a scene from Michigan's logging past. Axes, sawmill tools and millions of feet of sunken boards from the mid-1800s came to light from the lake's drying bed.