DULUTH – Ice has overtaken a great swath of Lake Superior's surface, edging it toward its first complete ice-over since 1996 and worrying the shipping industry.

By Friday, 93.5 percent of the lake's surface was covered by ice — about three times the average for this time in the season — contributing to early ice concentrations on the Great Lakes as a whole, according to the Great Lakes Environmental Research Laboratory. The average ice coverage across all five Great Lakes reached 79.6 percent on Friday.

"We've still got another solid month of cooling off, forming ice out there," said Jay Austin, an associate professor with the Large Lakes Observatory at the University of Minnesota Duluth. "It's much further ahead than its been for many, many years."

That kind of freeze could cut down on lake-effect snow, slow the evaporation that's led to lower water levels and provide better shelter to some fish species' spawning beds, experts say. It could also forecast a sluggish start to the shipping season, marked by the opening of the Soo Locks at Sault Ste. Marie on March 25.

"Lake Superior is likely going to ice over completely," said Glen Nekvasil, vice president of the Lake Carriers' Association. "In all probability, that means it's going to be a very tough breakout in March."

It's been since 1996 that Lake Superior was "essentially 100 percent ice-covered," said George Leshkevich, with the Great Lakes Environmental Research Laboratory. Coverage sways from year to year, but "back in the '70s and '80s, there was fairly good ice cover every year."

In 1979, ice cover on the Great Lakes reached 94.7 percent — the most on record. If frigid weather continues, there's a chance the lakes could hit that high again. Ice coverage tends to peak in late February or early March. But a big warm-up could change everything, Leshkevich said.

Shipping companies are hoping for above-freezing temps and maybe even some rain. "But right now we're not getting any relief," said Mark Barker, president of Ohio-based Interlake Steamship Co., which transports iron ore, coal and limestone.

Such companies rely on the U.S. Coast Guard's ice-breaking vessels to create "highways" through the ice, he said.

Usually, they're able to position the breakers in the "critical areas," Barker said. "In this case, we have heavy ice over multiple locations. That takes those assets and spreads them thin."

But it's important to remember that great ice coverage wasn't always uncommon, he said. "It's a good old-fashioned winter."

The near ice-over is a phenomenon better seen from the skies than the shoreline. While satellite images reveal whiteness across Lake Superior, photos snapped by residents along the north shore show sunlit waves, where strong winds pushed the ice from shore.

"The wind can do that," Austin said. "It's not like it's a solid block of ice."

Which brings him to another point: While ice covers most of the lake, some of it is very thin, and there's still open water, he said. "It doesn't mean you're going to be able to snowmobile across it or something like that."

Before sunset on Friday, Matt Silverness, 29, trudged in spiked boots onto the slippery ice off Park Point in Duluth. Carrying a Nikon camera, he had come to take photos of glassy mounds and shards of ice stacked by the wind.

Silverness, a Duluth resident, had heard that the ice cover on Lake Superior is extensive, but he was careful not to tread too far out. From his spot on the ice, he could see open water. "It's pretty cool what this lake will do," he said.

Streams of visitors to Lake Superior's south shore have been trekking to caves made accessible by the ice. For the first time in five years, the frozen shoreline has given winter entry to the Ice Caves of the Apostle Islands National Lakeshore. That's good news for nearby Bayfield.

"Thousands of people are coming on weekends where we might only have hundreds in a normal year," said Larry MacDonald, Bayfield's mayor. "This is filling up the lodging, filling up the restaurants, I'm sure the taverns and hopefully the gift shops."

The cold has also meant that the ice road between Bayfield and Madeline Island is intact this season. For two years in the past decade, he said, there was not enough ice to create the 2.6 mile road between the Wisconsin city and the island town of La Pointe. In others, the road's season was cut short by warm weather.

"I've heard that it's in the 2- to 2½-foot range this year," MacDonald said. "Which is good because it's over 200 feet of water," he added, chuckling.

Staff writer Pam Louwagie contributed to this report.

Jenna Ross • 612-673-7168