Fifty years ago this week, 16-year-old Jack Thueson helped save Stillwater.

Thueson and hundreds of other teenagers built a dike that saved the historic downtown in 1965 from the flooding St. Croix River. The “Teenagers’ Dike,” as it came to be called, was the 8-foot-tall, mile-long levee that held the river at bay.

“Without it, the water would have made it up to Main Street,” Thueson, now 66, said this week. “It would have been disastrous. The infrastructure would have collapsed. It literally saved that downtown area — from Main Street down.”

When the call went out for volunteers, Thueson, a sophomore at Stillwater High School, didn’t hesitate.

“Pretty much everybody at the high school was down there at some point or another,” he said. “There were hundreds of kids out there from all over the metro area. They had trustees from the prison working on it. It was a phenomenal time.”

Thueson and his friends were assigned sandbag duty, filling hundreds of bags to help build the dike. They were excused from school and worked six- to eight-hour shifts, he said. Volunteers who didn’t live in Stillwater slept on cots in the armory.

The American Red Cross provided hot sandwiches, and a civil defense truck delivered hot cocoa and candy bars, Thueson said. “So we were well-fed.”

Flooding stretched along the St. Croix and Mississippi rivers. The water level reached at the Stillwater Lift Bridge remains unmatched.

The St. Croix crested April 18 — Easter Sunday that year — about 7 feet above flood stage. That is almost 2 feet above the second-highest flood level ever reached in Stillwater, in April 2001.

The flood “brought the community together — the young and the old,” said Brent Peterson, executive director of the Washington County Historical Society. “They all worked together. They saved downtown Stillwater.”

The National Guard closed downtown “for three or four days” — the only time downtown Stillwater has ever been shut down to the public, Peterson said.

“It was a very intense week to 10 days, watching that water come up and up and up. There were people pacing back and forth on top of the dike making sure it wouldn’t leak. The other temporary dikes throughout the state all caved in, but this one held. It held back the waters, and everyone takes pride in that, and we still do today.”

“Cold, rain, more cold, rain and snow, rain and snow, more snow, warm temperatures, and finally rain.” That was the sequence of weather 50 years ago that led to “Minnesota’s greatest flood,” meteorologist Joseph Strub of the Weather Bureau, the forerunner of the National Weather Service, wrote in the June 1965 edition of the journal “Weatherwise.”

“It was kind of a perfect storm, as they say,” said Craig Schmidt, service hydrologist for the National Weather Service in Chanhassen.

“We started with an extremely cold and dry winter — just bitter cold; the frost depth was tremendous,” Schmidt said. “The soils froze, and they froze very, very, very deep. All the way through the month of February, we didn’t have much snow, so the ground was bare, and it tended to continue to just freeze and freeze and freeze.”

March stayed unseasonably cold — 12 to 15 degrees below normal — and then came the snow.

Four major storms fell that month, and “the snowpack went up from almost nothing to almost twice as much as normal in one month,” Schmidt said. “There was a ton of water in that snow, and then we got to April, and it warmed up, and we got a bunch of rain on top of it. It started warming up, and the snow started melting, and the rain fell on top of it. The ground was still completely frozen, so it all just ran.”

Mark Seeley, a University of Minnesota professor and climatologist, said rainfall during the fall of 1964 also was a factor. Already saturated by the fall rains, the soil couldn’t absorb the spring meltwater, he said.

Seeley, the author of “Minnesota Weather Almanac” and a frequent lecturer around the metro area, said the 1965 flood remains a vivid memory for many.

“It was unbelievable,” he said. “When you have a weather event of that magnitude, it stays with the residents for the rest of their lives.”

One of the most unusual preparations for the flood in Stillwater involved the intentional flooding of downtown businesses’ basements with clean water to equalize the pressure against the walls from the inside.

Roger Peterson, Brent Peterson’s father, was a newly elected Stillwater City Council member when the flood started.

“We got a call from the Weather Bureau saying that the chances of the river flooding were good,” said Roger Peterson, now 78. “They didn’t predict that it would be (as high as it got), but they thought it would be high enough that there would be water on Main Street.”

The city council and downtown business owners met and decided to build a dike on the railroad tracks that paralleled the river. A car wash and Muller Boat Works were the only businesses on the river side of the tracks, said Roger Peterson, a city council member from 1965 to 1981.

“We were lucky,” he said. “The kids worked together. The convicts worked together. The business owners worked together. The city worked together. Everyone worked together, and we really didn’t have any disagreements about what we should do. We wanted to build a dike, and that’s what we did.”

Dirt used to build the dike later was used to make the ball fields and tennis courts at Lily Lake, Peterson said.

“We’ve had other floods since then, but none as serious as ’65,” he said.

Former Oak Park Heights Mayor David Beaudet was an eighth-grader at Stillwater Junior High at the time of the flood. Now 62, he was put on sandbag duty and remembers feeling a great sense of urgency to get the dike built.

“There was a rumor that the dam at Taylors Falls might break,” Beaudet said. “It was a colder and wetter spring than last year. It was drizzling at least the last four or five days while we were building the dike. You had your raincoat on. There was no sunshine. I can remember just being perpetually wet from the waist down.”

Feeling a sense of accomplishment for saving downtown Stillwater didn’t come until later — “after it was successful,” Beaudet said. “That was rewarding.”

About 20 years ago, the Army Corps of Engineers began a project to protect Stillwater’s eroding shoreline, beach and slope, said Nanette Bischoff, a project manager for the corps.

The first phase, completed in 1997, consisted of repairing and rebuilding the six-decade-old retaining wall from Nelson Street to the gazebo in Lowell Park. The second phase, completed in 2002, extended it to the north around Mulberry Point.

The final phase called for a low floodwall along the western edge of Lowell Park to protect downtown. In 2002, Congress appropriated $2 million for the project, but the corps decided it was not worth the cost and diverted the money to other projects.

In 2003, however, lawmakers directed the corps to spend the $2 million and finish the Stillwater project. By then, though, Stillwater’s funds had already been allocated elsewhere, leaving the corps to find money when and where it could.

In 2010, it was determined there was not enough funding to build the floodwall, so the project was downsized to an $830,000 interior-drainage collection system. That project was completed in November 2013, Bischoff said.

Could another major flood like the one of 1965 occur?

“Of course,” said climatologist Seeley. “Records are made to be broken. It’s a convergence of all the right factors. … There’s a small chance, there’s a small probability for that, but we have to acknowledge that that probability nevertheless exists.

“It will probably happen again. We just don’t know when.”

Mary Divine can be reached at 651-228-5443. Follow her at twitter.com/MaryEDivine.

THE WORST OF THE WORST

The National Weather Service ranks the spring flooding of April 15-18, 1965, as the worst Minnesota has ever seen. The Mississippi, Minnesota and St. Croix rivers spilled over their banks after a cold, snowy spring, followed by a rapid warm-up and heavy rains. The Minnesota River crested 17 feet above flood stage at Savage, and the Mississippi crested at 12 feet above flood stage in St. Paul. Minnesota and four other states downstream along the Mississippi suffered flood damage totals estimated about $160 million at the time. Fifteen people died.