For 50 years, rivals fought over its size, location and whether it should be built at all.

When the new St. Croix bridge begins carrying thousands of cars and trucks over the shimmering river next month, the largest road and bridge project in Minnesota history will represent as much a triumph over politics as a feat of engineering.

The $646 million bridge, with a design so rare it's only the second of its type in the United States, required a congressional exemption to span the federally protected river, prized for its natural beauty.

Backers say it will provide faster access to jobs in the Twin Cities, unleash development in western Wisconsin and rid historic downtown Stillwater — a popular and scenic regional destination — of the noise and congestion that came with having the most convenient river crossing for miles around.

"I look at the bridge as a regional asset," said Bill Rubin of the St. Croix Economic Development Corp. "In the heartland, in the valley, we've got something everybody should be proud of. It accomplished a lot of what folks desired — safety, visual quality, easing the congestion."

Dignitaries will gather Aug. 2 to celebrate the opening of the four-lane bridge, 1½ miles south of downtown Stillwater. To many of them, the bridge with its enormous piers and cables represents an engineering feat ranking with famous bridges such as the Golden Gate in San Francisco.

To the people who opposed it, however, the towering bridge stands as a steel-and-concrete desecration of a natural wonder, and a costly enabler of urban sprawl that could well doom the river's special qualities.

One of the bridge's strongest critics is former Vice President Walter Mondale, who as a U.S. senator half a century ago joined with colleague Gaylord Nelson of Wisconsin to get Congress to permanently protect the St. Croix and other rivers under the national Wild and Scenic Rivers Act.

"It's a big, vast assault on the idea behind the federal river system," Mondale said recently of the new bridge. "We passed that bill all those many years ago to protect rivers so they didn't go the same way as those industrial eastern rivers, destroyed by sewage, blight and so on. It's a reckless, dangerous step."

Just what the new bridge will mean for the economic fortunes of the region remains a matter of speculation.

Stillwater hopes for a resurgence of spending in its historic downtown, Oak Park Heights will see increased traffic through its business district, and developers are showing interest in farmland on the Wisconsin side. Officials in New Richmond, Wis., have spent years positioning their city to accommodate growth anticipated from the bridge.

But a recent St. Croix County study, funded with money from the bridge project, cautioned that recent changes in demographics and spending might influence the county's future more than the new bridge.

"The trends suggest that it is less likely that St. Croix County will return to the growth rates experienced in the 1990s and 2000s, even with the improved crossing," the study concluded.

Bridge of compromises

The St. Croix bridge represents compromises by 28 diverse stakeholders on dozens of environmental, historical and cultural enhancements, as well as miles of new connecting highways on both sides of the river.

Talk of a new bridge began as early as 1951 after a flood closed the Lift Bridge in downtown Stillwater, which by then was already 20 years old.

By the 1960s, successive floods and growing use of the two-lane bridge by interstate commuters hastened debate over whether to build a new and bigger bridge. City leaders, tired of cars and motorcycles rumbling through their neighborhoods, saw a new bridge as their salvation.

The Lift Bridge wasn't designed to last as long as it did, former Stillwater Mayor Jay Kimble said. It also had environmental drawbacks, he said, leaking everything from motor oil to horse manure and stormwater into the St. Croix.

"The beauty of the new bridge is that it's much more environmentally sound," he said.

That argument didn't wash with many environmentalists. The Sierra Club twice sued to stop construction. The National Park Service, which oversees the St. Croix National Scenic Riverway, ruled that a new bridge would permanently alter the river's scenic and recreational values.

It took a vote in Congress in early 2012 to exempt the bridge from the laws governing use of the St. Croix. U.S. Sen. Amy Klobuchar, D-Minn., sponsored the Senate bill while then-U.S. Rep. Michele Bachmann, a Republican, introduced it in the House.

In a recent interview, Bachmann said the bridge will greatly improve transportation in the St. Croix Valley and "add to the economy beyond a shadow of a doubt."

"It is the legislative achievement I am most proud of in my time in Congress," she said.

Klobuchar said the bridge bill "met every environmental test" and won't lead to a rash of congressional exemptions on other protected rivers.

"The herculean effort that it took — the three decades that it took, a major-league debate in Congress — would lead one to believe this will not be an ordinary activity, that this was a rare exception," she said.

Chris Goepfert, senior program manager of the National Parks Conservation Association's field office in St. Paul, disagreed. She said the exemption hurt the St. Croix and showed the vulnerability of laws protecting the river.

"Decisionmakers can still pave the way for massive development despite those protections. That's not how we should be doing business," she said.

Construction finally began in early 2014, with a scheduled completion date of late 2016. However, engineering complexities, weather delays and other unforeseen problems pushed the work into 2017.

"It was more difficult to build because of everything around it," said Terry Zoller, bridge construction manager for the Minnesota Department of Transportation.

To reduce both the number and height of supporting piers in the water, officials decided on a so-called extradosed design that combines cable-stay engineering with box-girder construction. Connecticut has the only other large bridge of similar design in the country.

A "reed-like" pier design makes the supports look slimmer to boaters on the river. Bridge towers were kept at or below the bluffs on the Wisconsin side. An earth-tone color was chosen to make the bridge blend into its surroundings. Corners were rounded to give it a streamlined look, and night lighting was subdued.

Still, the St. Croix bridge stands more than two times higher than the Interstate 94 bridge just 6 miles downriver at Hudson, Wis.

Stillwater native Charlotte Przybylski, 40, said she had been "quite frightened" by the prospect of a big concrete structure on the skyline. Now that the bridge is finished, she thinks it will make Stillwater a more walkable town.

"The bridge is beautiful. I didn't expect it to turn out so stunning," she said. "The river is stronger than we think it is and hopefully this won't be too harmful to it."

Impact on Stillwater

It's predicted that Stillwater, a city of about 19,000 residents, will see an immediate reduction of traffic when the new bridge opens.

Washington County Commissioner Gary Kriesel, a former Stillwater City Council member, said exhaust and congestion from traffic backups at the Lift Bridge endangered the river and its surroundings.

"The bridge was absolutely necessary, unless you want to concede that historic downtown Stillwater wasn't worthy of being saved," he said.

The picturesque Lift Bridge will remain as part of a 5-mile loop trail that will replace the steep highway on the Wisconsin side and cross the new bridge as well. It will close to motor vehicle traffic as soon as the new one opens, but continue to lift for watercraft.

The St. Croix County study concluded that the new bridge will give Wisconsin commuters access to tens of thousands more jobs in the metro area. No longer constrained by an average 20-minute delay at the Lift Bridge, commuters will be inclined to drive farther into the metro area to work, the study said.

The new bridge is expected to handle up to 71,500 crossings a day. By comparison, the Lift Bridge typically counted about 16,000 crossings a day — many of them by commuters going both ways. Planners believe an uptick in river crossings could happen right away, as drivers who have long avoided the Lift Bridge decide to use the new one.

On the Wisconsin side of the river, a new four-lane highway will link the bridge with New Richmond and Somerset. Drivers crossing the bridge into Wisconsin will travel a mile or so before they see an exit, another nod to what the stakeholders envisioned.

"They wanted to limit the development near the bluff because of the scenic riverway," said Tim Mason, project manager for the Wisconsin Department of Transportation.

Near the bridge, family farms cover most of the land and the underground utilities needed for big-box retailers don't exist. St. Croix County has hired consultants to explore rezoning the land, but growth comes at a cost — requiring more taxes to pay for schools, roads, utilities, policing and other government services.

Kriesel rejects the argument that the bridge will encourage an explosion of commerce. "You don't control growth with bridges, you control growth with land-use policies," he said.

Mondale predicted the new bridge will bolster arguments for more development on the St. Croix and the nation's other 207 waterways under federal protection.

"When it's all done, the idea behind this pristine public river could be gone," Mondale said. "I don't say that's going to happen, because there's a lot of will yet to resist, but it's a sad lesson for us."

But in Oak Park Heights, where hundreds are expected to congregate in a couple weeks to cut a big red ribbon on the bridge, the many decades of turbulent debate might well evaporate when traffic begins flowing high and fast above the St. Croix.

"The residents are ready for all of this to be done," said Marcia Nelson, who lives in Oak Park Heights and looks forward to the bridge. "It's a sculptured masterpiece — a wonderful, contemporary miracle."

Kevin Giles • 651-925-5037