3 quadrillion gallons.
Or, as the headline reads in the print edition: As the Earth Warms, Duluth is Looking Pretty Cool. Duluth, which sits at the western end of Lake Superior, the greatest of the Great Lakes by volume, also has fresh water. A lot of it. Superior is so voluminous that, if poured out, it would submerge North and South America under a foot of water.
Just how big is Lake Superior? (Lake Superior Magazine)