3. Results and Discussion
We identify robust, statistically significant increasing trends in the year-over-year variability of lake levels in three of the four Great Lakes from 1970 to the present (Figure 1; Superior, p < 0.001; Michigan-Huron, p < 0.001; Erie, p < 0.05) with conflicting statistical significance results for Lake Ontario (p > 0.05 with Kendall rank correlation, p < 0.05 with Mann-Kendall trend test). While the long-term increases in lake level variability for Lakes Superior, Michigan-Huron, and Erie may be attributable to the non-stationarity of environmental conditions, it should be noted that the long-term decrease in Lake Ontario variability is attributable to human implemented controls, as Lake Ontario has been regulated since ~1960 (Wilcox et al., 2007). Lake Superior has also been subject to a level of human control since the early 20th century, though to a lesser extent, with evolving regulation that “attempts to maintain natural variability” (Clites and Quinn, 2003). Beginning in 1973, regulations mandated that the levels of Lake Superior be maintained in a manner that considers the impacts of regulation on the rest of the Great Lakes system. This is a factor to be considered when examining recent trends in lake level variability for both Lake Superior, as well as the rest of the Great Lakes. In spite of this change, all four lake basins have attained relative maxima in interannual variability over the past decade. Lake Superior set its all-time maximum in variability, while the remaining lakes endured high water marks in variability not seen since around 1970.