St. Peter, Minn. – In some places, they mark spring by the arrival of the robins.

But in this southern Minnesota college town, a sure sign of the season is the flock of gawkers peering intently at the Minnesota River as it churns and surges under the historic Hwy. 99 truss bridge just south of downtown.

On a bright, sunny Thursday, they came by the dozens to check out the swollen current as it slowly rises in yet another season of flooding.

They compared the flow to historic flood years. They commented on the snowmelt and the rate of the river's rise. They speculated about the fate of low-lying homes on Front Street.

And they shouted with delight at the unexpected.

"I think that's a beaver!" said Jimmy Goede of Mankato, pointing at a dark object carried along on the swift current. "Or maybe a catfish?"

Even as weather forecasters issue comforting updates on the season's gradual, "perfect melt," people in the Minnesota River valley south of the Twin Cities are on edge.

Ronald and Sharon Hawker drove the 15-odd miles from their home in North Mankato to get a close look at the Minnesota and see what it was up to. They live about four blocks from the water's edge, and although their neighborhood is protected by a giant dike built after historic flooding some years ago, they weren't exactly comfortable.

"The water is still coming," Ronald Hawker said. "We're scared of what's coming our way."

Sharon Hawker called it "the worst winter I can remember since I was young." The couple took out flood insurance this year for the first time in many years, she said.

"All the rivers are bursting," said Julie Von Essen of St. Peter. "People already have water in their basements. It's gonna be bad."

For many area residents, the water level at the bridge — which was within a foot of the lowermost girder Thursday — was a reminder to begin plotting their detour routes.

They rattled off the names and numbers of country roads and byways that they take when the Hwy. 99 bridge is closed, which it already was.

Eric LaPlante drives a food delivery truck. When the bridge is closed, he has to detour through Le Sueur to make his deliveries in nearby Cleveland and Le Center adding about 25 miles to his twice-weekly trips.

Erik Attenberger remembered the thaw of 2010, when his wife bought him cigarettes in Kasota, usually a 2-mile trip. Except that time, it was 60 miles with all the detours.

A State Patrol officer stopped by at midafternoon to kick people off the bridge, which was cordoned off with yellow tape. But as soon as she left, they went right back out.

Ben Burg of St. Peter, who grew up across the river, recalled riding his bike across the span as a young man through a foot of rushing water.

"I had a girlfriend in town — I had to get there," he said.

As he spoke, giant chunks of ice sped under the bridge, sometimes whacking the ironwork with a loud thunk.

"That's gonna leave a mark," Goede said after one loud crash.

Ryan Whittaker, a state Department of Natural Resources hydrologist, had a team on the site measuring the volume of the current. On Thursday afternoon, it was running at 60,000 cubic feet per second.

"It's still going up, so I think it will be slightly more than a typical spring," he said.

His message to residents: "Just keep an eye on the river."

On the defense

That's exactly what folks in Stillwater were doing Thursday as 275 volunteers began pulling shifts filling sandbags to protect the downtown against potential flooding by the St. Croix River.

The work started at 11 a.m., with volunteers organized into shifts of 25 workers. Two shifts worked Thursday, and three shifts will work from 8 a.m. to 5 p.m. Friday, Monday and Tuesday. Washington County posted on its Facebook page that no sandbagging would take place Saturday because of the "overwhelming response."

Most volunteers came from the St. Croix River Valley, but some also came from Minneapolis, St. Paul, Bloomington, Richfield and Wisconsin, said Jennifer Kmecik, volunteer center director with Community Thread, which is coordinating the effort.

In St. Paul, crews began installing a floodwall Thursday at the St. Paul Downtown Airport to protect against a rising Mississippi River, which is expected to crest there at 18 feet next week.

"We're taking every precaution to stay ahead of the rising waters and keep the airport open and operating safely," said airport manager Joe Harris.

Airport officials will review the flood forecasts on Friday and decide whether to extend the wall to its full length across the southeast end of runway 14/32. That would shorten the runway by about 1,150 feet. Two shorter runways already have been closed to allow crews to stage materials for the wall.

In the west metro, crews spent the last week breaking up ice jams on Minnehaha Creek to prevent flooding along its banks. With the creek flowing, the Minnehaha Creek Watershed District has begun discharging water from the Gray's Bay Dam into the creek. Usually the dam isn't opened until Lake Minnetonka is ice-free.

But this year, with a foot of ice still covering the lake, the district opened the dam to lower the lake's water level and make room for more water.

"Lake Minnetonka water levels are rising as February's record snowfall melts and following recent rainfall," said Tiffany Schaufler, project and land manager for the watershed district. "By releasing water ahead of a large snowmelt or rain event, when it can be easily controlled, we can prevent flooding throughout the whole system."

The district released water before the lake's ice-out last year in anticipation of two record snowfalls in April. By doing that, the district avoided flooding on Minnehaha Creek and the lake, officials said.

Staff writers Dan Browning and Mary Lynn Smith contributed to this report.

John Reinan • 612-673-7402