Secular changes in Great Lakes water level seasonal cycles

The three primary scales of Great Lakes water level fluctuations are interannual, seasonal, and episodic. Of these three, the seasonal water level fluctuations have received relatively little attention. The Great Lakes water levels have a well defined seasonal cycle driven primarily by snowmelt in the spring and summer and lake evaporation in the fall and winter. The present average seasonal cycle ranges from 26 cm on Lake Superior to 38 cm on Lake Ontario. Great Lakes monthly water levels from 1860 to 2000 were used to assess changes in the seasonal cycle of each of the Great Lakes and Lake St. Clair over the past 140 years. Changes are found on all of the lakes during the period of record. They usually resulted in a decrease in seasonal range and a time shift in the months of seasonal maximum and minimum. The effects of lake regulation were found to be negligible in the case of Lake Superior and significant for Lake Ontario. The major changes on Lakes St. Clair and Erie are likely a result of changes in the connecting channels ice retardation rather than changes in seasonal hydrometeorology. Seasonal cycle regimes are delineated for each of the lakes and possible rationale for the changes discussed.