Temporal behavior of levels of the Great Lakes and climate variability

Levels of Lakes Superior, Michigan-Huron, and Erie for the 1861-2001 period were assessed to define key temporal fluctuations in their averages and extremes. Behavior of levels of Lakes Michigan-Huron and Superior has been extremely different since 1861, including vastly different long-term distributions, differences in the amount of variability over time, and differences in when their record high and low levels occurred. Record high or low 15-Year events were present on one or more lakes in 64 years, and record events based on 25-year periods were present in 96 of the 141 years, both representing the Presence of records during much more time than if the record events had occurred simultaneously on all lakes. These lake level differences reflect significant between-basin differences in climate conditions, and principally precipitation over time. There were two eras, 1923-1938 and 1973-2001, with exceptional variability and extremes of levels on all lakes. These findings are relevant to planning for future water level conditions, for understanding recent extremes, and for considering how the sizable spatial and temporal shifts of the past could relate to future changes in the basin's climate.