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.