It's a battle against ice and time for ships in the northern Great Lakes

Sault Ste. Marie, Michigan

AN INTRICATE boat ballet plays out each January on the frigid waters of the northern Great Lakes.

Thousand-foot freighters scramble to make final deliveries of iron ore to steel mills. The Coast Guard carves paths through ice-clogged shipping routes. And the Soo Locks, the engineering marvel connecting Lake Superior with points to the south and east, shut down for 10 weeks of maintenance while the weather is at its most brutal.

On a recent morning, with the locks' annual closing looming and…