New York Times suggests that CO2 may cause low Great Lakes water levels

Low Waters and High Anxiety - NYTimes.com

When water levels approached record highs in 1997, the largest lake freighters were hauling 71,000 tons of cargo at a time. But all that tonnage weighs a ship down, and cargo loads declined after lake levels began a rapid decline in the late 1990’s.
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Since 1999, annual average water levels have been consistently below the historic averages recorded since 1860 in Lakes Superior and Michigan-Huron. A major research question for scientists is whether that trend will persist. Levels in Lakes Ontario and Erie have not fallen over the same period according to data from the Great Lakes Environmental Research Laboratory, and similar troughs during the 1930’s and 1960’s could suggest that this is a natural cycle.

But if the data are the beginning of a new normal, climate change could be the culprit, researchers say.
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Climate change could further complicate those operations by periodically increasing precipitation in the region. Lake Erie saw one of its largest-ever seasonal water level rises between February and June 2011 as result of record rainfall.