Water flows from Lake Superior's headwaters to the Atlantic Ocean, spanning 20 percent of our planet's freshwater. Beavers and wolves change the landscape. Loons, owls, moose, bears and people depend upon these lakes we take for granted.

The story of how winter has shaped life in the Great Lakes. A polar vortex paralyzes fish and ducks and attracts hundreds of bald eagles. Wolves hunt deer trapped by ice, but are manipulated by ravens. See how animals use snow to survive.