Unprecedented Seasonal Water Level Dynamics on One of the Earth's Largest Lakes
T he North American Great Lakes (Fig. 1) contain roughly 20% of the Earth’s unfrozen fresh surface water and cover a massive area (Lake Superior alone is the largest unfrozen freshwater surface on the planet). Water levels on the Great Lakes have been recorded continuously for more than 150 years, representing one of the longest sets of direct hydroclimate measurements. This dataset, synthesized by Quinn (1981) and Lenters (2001), among many others, indicates that water levels on each of the Great Lakes follow a strong seasonal pattern closely linked with the timing and magnitude of the major components of the regional water budget, with relatively low water levels in the winter months, rising water levels in the spring, and decreasing water levels in the late summer and early fall. Water-level measurements on Lake Erie during the 2011 and 2012 water years (October 2010 through September 2011, and October 2011 through September 2012, respectively), however, reflect dramatic and unexpected changes in the seasonal waterlevel cycle and in the Great Lakes regional water budget. In the 2011 water year, monthly average water levels on Lake Erie rose more than 0.8 m from February to June, an unprecedented amplification of the historical seasonal pattern (Fig. 1). Never before had water levels on Lake Erie risen as much during a fourmonth period. More specifically, the water level rose 0.28 m between February and March of that period. Only twice have water levels risen more between February and March (0.31 m in both 1976 and 1985). Furthermore, the water level between April and May 2011 rose 0.26 m. Only twice have water levels risen more between April and May (0.27 m in 1943 and 0.30 m in 1947). In the 2012 water year, however, the seasonal water level cycle on Lake Erie experienced a remarkable shift (as opposed to the 2011 water-year amplification) in the historical average seasonal pattern. From November to December 2011, for example, the monthly average water level on Lake Erie rose 0.19 m (Fig. 1), the second-highest increase for that time period in recorded history (the highest was 0.20 m in 1927). Water levels then decreased continu-