– About 1 in the morning of Wednesday, June 20, 2012, the assistant manager at this popular camping and picnicking destination just south of Duluth decided to evacuate the 30 families and groups staying at the park in tents and RVs.

Already, Interstate 35 tunnels in downtown Duluth were filling with floodwater, and streets in Grand Rapids, Minn., were reported to be awash in 6 inches of rain.

Alerting the state’s Arrowhead region that the storm might grow even worse, the National Weather Service declared that a “life-threatening flash flood event appears to be developing across a large part of northern Minnesota.”

Still the rains came.

“I was on duty that night, and a deputy sheriff and I decided it was time to evacuate,’’ said Mark Luschen, assistant manager at Jay Cooke State Park.

Luschen wasn’t worried the campground itself would flood because it, like the park’s buildings, was far enough removed from the floodplain of the now-raging St. Louis River.

“We worried instead that the road getting out of the park might wash out,’’ Luschen said. “We waited until daylight, about 5:30, and awoke the campers and told them we had to leave.’’

Some RVs and other gear were left behind because their weight, Luschen worried, might stress the fast-deteriorating road out of the park.

Luschen was correct to be concerned.

Gates at the Thomson Dam upstream of the park were already wide open, allowing as much water as possible to rush through. But the dam was built more than a century ago without emergency spillways. So the St. Louis, whose torrents would peak at about 55,000 cubic feet per second, was rushing over the levee on the dam and around its sides.

Soon Hwy. 210 in Thomson would wash out, and the torrents would turn their sights on one of Minnesota’s most iconic landmarks, the Swinging Bridge at Jay Cooke.

The suspension bridge, originally built in 1924 by the U.S. Forest Service, has attracted hundreds of thousands of visitors to the park over the years, while also providing foot access to a vast, 25-mile web of hiking and cross-country ski trails on the south side of the St. Louis River.

As initially constructed, log cribs supported a “swinging’’ walkway of wood planks that hung 18 feet above the river. At its entrance, a sign read, “Not more than five persons should cross bridge at one time. Jumping, swinging, running or other unnecessary movement on the bridge is dangerous.’’

A decade later, a much stronger suspension bridge replaced the original structure. Built by the Civilian Conservation Corps (CCC) and completed in 1935, the new bridge featured stone pillars on either side of the river, replacing the log cribs.

In 1941, another CCC crew raised the bridge deck 4 feet.

Their work wouldn’t last.

In May 1950, during what is now the second-worst flood ever recorded in the region, spring snowmelt sent raging floodwater down the St. Louis. Measured at 42,000 cubic feet per second, the deluge wiped out the Swinging Bridge while leaving largely intact its stone support pillars.

A reconstructed Swinging Bridge would not open until 1953.

This time, engineers decided to raise the bridge deck still more, bringing the total height it had been lifted since 1924 to 7½ feet. The increase was accomplished by adding concrete “caps’’ to the otherwise stone-faced pillars, raising the suspension cables supporting the deck.

Total length of the reconstructed bridge: 200 feet, with a 126-foot span over the river.

2012 flood was worst ever

Last year by the time Luschen had decided to evacuate Jay Cooke, the raging St. Louis had risen to the level of the Swinging Bridge deck, and its loss seemed fated.

Unknown was whether the bridge’s stone support pillars would survive.

Some of the smaller pillars that supported the gangplank leading to the bridge had already given way to the flood. So survival of the taller support pillars seemed problematic.

But survive they did.

“The pillars lost some of their rock facing,’’ Luschen said. “But they came through the flood largely intact.’’

When the St. Louis had finally receded to normal levels, pieces of the bridge were strewn 7 miles downstream.

Some cleanup occurred last summer. But much of the salvage work didn’t start until this spring, after a reconstruction plan was approved.

The Swinging Bridge, one of only two suspension bridges in the state parks system, is listed in the National Register of Historic Places like other structures at Jay Cooke State Park,.

So “modernizing’’ it wasn’t in the cards.

Instead, in its rebuilding, the bridge will get a more retro look still.

In a nod to the original stonework completed by the CCC, the concrete caps on top of the support pillars will be removed with jackhammers and replaced with stone.

Construction will cost about $1 million, with completion expected by about Labor Day.

“The bridge should be open again by our fall color season,’’ Luschen said.