– Bill Chelmowski eased the airboat from Lake Pepin’s open waters onto a pockmarked sheet of ice, already certain of what he’d find beneath the surface.

In this crucial stretch of river, the roar of an airboat is the cadence of spring. Each year, the U.S. Army Corps of Engineers measures Pepin’s ice thickness to help divine when the first tows can break through and reach St. Paul, opening the navigation season.

Chelmowski’s auger jabbed through the slushy sheet like a rotten tooth Thursday, a sign that the sleeping titan of commerce called the Mississippi is ready for business — at least along the tract often slowest to melt.

Now if only the towboats could get there.

High water and flooding downstream have closed locks dotting the river from Iowa into Missouri, halting commercial traffic and setting up what could be the latest start to the shipping season on record.

The first tow normally punches through Pepin’s ice in the third of week March. But some in the towing industry worry the later melt and flooding could mean two or three more weeks before barges arrive in the Twin Cities.

Blame it on the wet fall, the deep freeze that followed and this year’s record-breaking late snow.

“It’s a recipe for a mess,” said Lee Nelson, president of Upper River Services in St. Paul, which moves and maintains barges in the Twin Cities.

And if it rains?

“Anything we get is simply going to exacerbate a bad situation,” Nelson said.

Barges carrying fertilizer, cement, steel and salt can’t make it north, marooned by high waters.

For goods like grain headed south, washed out rail beds in Iowa, Missouri and Nebraska have also slowed shipping, said Bob Zelenka, executive director of the Minnesota Grain & Feed Association.

“At least in the short term, we’re kind of stuck here,” Zelenka said.

The river, Zelenka said, is the cheapest way to move grain out of the region, much of it bound for New Orleans and beyond.

About 108 million tons of commodities traveled up and down the Mississippi River last year from Guttenberg, Iowa, to the port of St. Paul.

Shippers learned long ago to watch the corps’ work on Lake Pepin, which stretches from near Red Wing south to Wabasha. The slower current on the wide, 21-mile natural reservoir means ice here is often the last barrier between northbound tows and St. Paul.

The corps usually sends a survey crew onto the lake starting in mid-February. They tunnel into the ice in as many as 18 spots, jotting down measurements in a small notebook and posting them online until the first towboat breaks through.

Thick ice delayed last year’s opening to April 11, the latest in 50 years of record keeping, the corps said. The second latest opening was April 8, 2013, when navigation was pushed back by flooding.

Things were looking up this year, with only average ice on Lake Pepin. Then, February hit.

The late snow acted “like an insulating blanket” draped over the ice, slowing the melt, said Al VanGuilder, a supervisory engineering technician with the corps.

Along Hwy. 61, which hugs the curves of the lake, water lapped into park areas Thursday near Lake City.

Further south, the swollen river also covered the boat ramp at Camp Lacupolis that the corps often uses as a launch point for the airboat.

Dick Koch, the camp’s owner, has already lifted chairs and other furniture in his office building onto tables, bracing himself and his possessions for the rising water to come.

“We haven’t had this for many years,” he said, pointing his yellow glove to the submerged ramp.

Each spring, Koch listens for the boom of the first barges crushing through the ice. It’s an annual scene that leaves lookout points along the river packed with people eager to watch the leviathans slip north.

On Thursday morning, Koch helped the corps team get the airboat into the lake.

Even eyeing the distant ice sheet from shore, Chelmowski and his crewmate Kurt Schroeder could tell the outing would likely be their last of the season.

Last week, the lake’s ice still measured 25 inches at its thickest point. Towboats generally wait until the ice is about 12 inches or less to bust north.

The crew jots down measurements for two kinds of ice. They note the blue ice, which is clear and solid, as well as the softer white ice, filled with air bubbles.

In this science of inches, the corps still defers to nature, hazarding no predictions for the navigation season opening.

“Old Man Winter is kind of losing his grasp,” said Chelmowski, a boat operator for the corps. “But a lot of this comes down now to Mother Nature.”

Under sunny skies, the airboat weltered over an ice sheet already whittled thin by wind and current, easily cracking through the surface as it careened upstream.

The men took only one measurement Thursday, stopping near the spot where the ice had been the thickest. After Chelmowski drilled a hole, Schroeder dipped a stick labeled with inch marks into the lake.

“Solid 10,” he said, pulling it out. “More than likely all white.”

Bingo. Safe enough for the towboats, whenever they get here.