Hydrographs of lake levels for each of the Great Lakes from 1860 to 2010. The linear trend of the lake level time series are shown by the straight black lines. Some fluctuations are noted across the lakes while other variability is specific to a particular lake. Data from the NOAA Great Lakes Environmental Research Laboratory (NOAA GLERL 2012). The trend for Michigan-Huron is downward and statistically significant.