Apostle Islands celebrates century of National Park Service
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3 | History repeating itself
Long-term water level fluctuations are shown on a hydrograph, a plot of lake levels versus time. The hydrographs on this page show average monthly water levels for each of the Great Lakes and Lake St. Clair since the middle of the 19th century. Levels have been measured on most lakes since 1865, with the current network of water level gages operating since 1918.
As the hydrographs illustrate, Great Lakes water levels tend to follow a cyclical pattern. The Great Lakes system experienced extremely high water levels in the 1870s, early 1950s, early 1970s, mid-1980s and mid-1990s. Extremely low water levels were experienced in the late 1920s, mid-1930s, mid-1960s, and in the late 1990s leading up to today.
Graphics courtesy U.S. Army Corps of Engineers, Detroit District.