Springtime in Duluth isn't signaled by birds or butterflies or flowers.

Duluthians know it's spring when the Coast Guard icebreakers arrive, announcing the start of another Great Lakes shipping season.

Last week, two cutters carved a track from the Soo Locks on the eastern edge of Lake Superior all the way to Duluth. The Mackinaw then cut a path up the North Shore, breaking open paths in the ice and clearing the way for thousand-foot freighters to carry their first cargo of iron ore, coal and grain.

But the U.S. Coast Guard cutter Alder stayed behind to break up the ice in the Duluth-Superior harbor.