– By the time the men loaded the airboat onto Lake Pepin’s icy shell, the clouds had cleared. Nicholas Lorenz looked up. “Sun’s out,” he said, “no wind.” His crewmate shook his head, as if not believing their luck.

For the first time this year, a U.S. Army Corps of Engineers team traversed the lake Wednesday morning, stopping at 18 points to measure the thickness of the ice on this crucial tract of the Mississippi River. Because the numbers hint at when the first towboat might break through to St. Paul, opening the navigation season, shippers watch them carefully.

But residents along the river, too, keep a close eye. The team’s measurements, jotted down in a little journal and later posted online, portend the start of spring. So if this is Minnesota’s version of a groundhog’s shadow, good news: The biggest number the team recorded Wednesday was 19 inches — below average.

“This is about the thinnest I’ve ever seen it,” said Bill Chelmowski, a small-craft operator for the corps, after bringing the airboat ashore.

Two years ago, the team fought windchills of 30 below in search of signs of spring and found not one. Ice reached 32 inches that year, the thickest reading since the survey began. The shipping season started late and then came to a halt on Nov. 20, the earliest closing in 45 years. “We’ve been at this where it’s been zero” degrees, Chelmowski said, “where our helmets are frozen right to our face.”

But this year, signs of spring surrounded them on Lake Pepin, which extends from near Red Wing to Reads Landing, where open water waved Wednesday morning. Because of its width and slower current, the 21-mile natural reservoir, about an hour’s drive southeast of St. Paul, is often home to the thickest ice and is the last linchpin in moving commodities up and down the river. Nearly 5.5 million tons of commodities — much of it grain — passed through the Twin Cities river terminals in 2014. (Final numbers for 2015 have not yet been compiled.)

The river is “the cheapest way to move grain,” said Bob Zelenka, executive director of the Minnesota Grain and Feed Association. His group tracks the ice measurements because “when the river opens up, it’s a whole new market for us.”

Last year’s large crop and low grain prices mean millions of bushels are sitting in storage, he said. “There is a lot of grain waiting to move.”

A towing company typically waits until about 12 inches of ice remain before breaking through. So thinner-than-average ice — coupled with warm weekend forecasts — could mean an earlier date for the first towboat’s arrival in St. Paul, which over the past 10 years has averaged March 24. Last year, the New Dawn was the first tow to pass through Lake Pepin, making it to St. Paul on March 25.

“We’re shut down when Mother Nature says it’s time to shut down,” said Lee Nelson, president of Upper River Services Inc. in St. Paul, which moves and maintains barges on the river. “Typically, the stick she holds over us is how much ice on Lake Pepin.”

But this year, work on a man-made structure — Lock and Dam 9, near Lynxville, Wis. — could, as much as the weather, dictate the key date. That work is scheduled to run until March 17, according to the corps, which is in charge of the river’s lock-and-dam system.

‘A big deal here’

The corps team’s weekly — or biweekly, depending on the weather — ice measurements are a rite specific to Minnesota.

“I don’t think this is done anywhere else in the country,” said Dan Cottrell, channel maintenance coordinator with the corps.

On Wednesday morning, Chelmowski glided the 385-horsepower airboat across the lake, spinning it in a few tight circles before letting it settle on the ice.

The other equipment is basic: A handheld GPS device pinpoints the spot, an auger tunnels through the ice, a metal stick gauges the thickness. That old stick was meant for sport, with images of fish alongside the inch marks.

“Itsy bitsy,” it reads at 6 inches. “Nice dinner plate” at 11.

“We’re glorified ice fishermen with no fishing pole,” Chelmowski said, chuckling.

A few ice anglers approached the crew members at their first spot along the sailing line, where they had stuck a green and orange stake for future reference. Many locals know about the work the corps does each year, the crew said.

“It’s kind of a big deal here,” said Dan Bender, mayor of Red Wing. One reason: “A large part of our commerce here in Red Wing is by river barge.”

Even some residents with little connection to the navigational system regularly check the corps website (tinyurl.com/Pepin-ice), passing along the news, he said.

“When it gets to be this time of year,” Bender said, “we’re looking for any sign of spring.”