It’s official:  Lake Michigan has broken the 1964 record low.  That includes Grand Traverse Bay, of course.

The hot, dry summer of 2012 is a factor~but not the only one.   Executive Director Andy Knott says Great Lakes water levels have been declining for at least a decade.

“Water levels fluctuate because of weather and weather systems,” he says.  “Low levels affect near shore habitat, but fluctuating water levels are natural…That’s nature, and the eco-system adapts. It’s a long-term thing.”

Here are other factors that affect Great Lakes water levels:

  • Winter~levels can be 12 to 18 inches lower in winter.  In fall and early winter, the air above the water is cold and dry, and the water is relatively warm.  This results in the greatest evaporation and water level decline of the year.  If temperatures stay low enough for the water to freeze, that helps slow down evaporation.
  • Climate change~climate change could be a factor.  Records of winter ice cover in Grand Traverse Bay show a steady decline in the bay freezing over since the 1970s.  Less ice cover in the Great Lakes means more winter evaporation.  See chart below.
  • Summer~levels can be 12 to 18 inches higher.  When snow melts in spring, runoff increases.  Evaporation is low since the water is relatively cold, and the air is warm and moist.  Lake Michigan is generally highest in July.
  • Decade to decade~the Great Lakes experienced extremely low water levels in the 1920s, mid-1930s and mid-1960s.  They experienced extremely high levels in the 1870s, early 1950s, early 1970s, mid-1980s and mid-1990s.
  • Daily~water levels can change a few inches depending on precipitation, runoff and evaporation rate.
  • Storm surge~Levels can even change locally in a matter of minutes depending on which way the wind blows.  Sustained high winds from one direction can push the water level up at one end of Grand Traverse Bay and drop at the other end.  This is called a wind set-up or storm surge.
  • Seiche~pronounced “saysh.”  When the wind abruptly subsides or barometic pressure changes quickly, the water level will vary until it stabilizes.  Storm surges and seiches can seem like tides and last for days after the winds and pressure have stabilized.
  • Centuries~levels have risen and fallen due to basin-wide, continental and global climactic varions that affect evaporation and precipitation.  The Great Lakes have shown a relatively small range in water levels, approximately 6.5 feet from the recorded monthly maximum to the monthly minimum.

Knott says snow and rain will help bring Lake Michigan levels back up~not lake-effect rain or snow, but what is called system snow or rain.  System snow or rain brings moisture from other parts of the country or Canada; but lake-effect snow and rain draws moisture from the Great Lakes, exacerbating the problem.

There is some debate about whether dredging in the St. Claire channel is lowering Lake Michigan.  There is also some concern about the release of 2 billion gallons a day in Chicago.  Neither of these activities is significantly lowering Lake Michigan.   It is important to stress that drought and increased evaporation, and the balance between precipitation and evaporation basin-wide is the major driver of lake levels.

A December 27, 2012 Ticker News article by Al Parker contributed to this report

Photo:  Great Lakes Discovery Center dock on West Bay in Greilickville taken on January 10 by Denise Baker

Chart by Craig W. Williams 

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