The National Park Service designated the St. Croix a wild and scenic river in l976 to protect it from degradation caused by human activity, including commercial development.
The Sierra Club lawsuit argues the National Park Service is violating its own rules by allowing the plan for a new bridge to move forward.
Sierra Club spokesman Mat Hollinshead of St. Paul says the river is a unique public resource.
"The St. Croix is within an hour's travel time from almost three million people. It's hard to identify another such river resource available to so many people in such an unspoiled condition," says Hollinshead.
The new crossing would replace the current two-lane lift bridge in downtown Stillwater, which is more than 70 years old and nearing the end of its useful life.
The bridge would relieve congestion that fills the downtown every day, as commuters line up to cross the lift bridge. The lift bridge would remain open to pedestrians and bicyclists.
A new bridge has been sought for years by developers and residents in the Stillwater area and in fast-growing western Wisconsin counties.
A Sierra Club lawsuit in l996 stopped an earlier St. Croix bridge plan, and prompted a National Park Service review which claimed the bridge would harm the waterway.
In November 2006, the federal government signed off on the environmental impact study for the span, which cleared the way for Minnesota and Wisconsin officials to begin planning in earnest.
The new Sierra club lawsuit, filed Tuesday in federal court in Minnesota, names the National Park Service as a defendant and seeks an injunction to stop bridge construction.
Mat Hollinshead says the Sierra Club offered an alternative to the plan.
"One of the alternatives was called a twin bridge alternative, by which a second bridge would have been built close to the existing lift bridge and used for traffic in the other direction," says Hollinshead. "We felt that would have been in the same corridor, so a new corridor for a structure would not have been created in the scenic riverway. That was dropped."
The proposed bridge would be built a mile south of Stillwater, starting in the Twin Cities suburb of Oak Park Heights.
Twelve years ago, the government bought out owners of more than 60 homes on that city's property tax roles to make way for the bridge. The homes were destroyed, but the land sits unused as discussions about bridge construction continue.
At one time, federal officials charged with protecting the St. Croix's wild and scenic river status wanted the old bridge out of the river. Some Stillwater residents want to keep the old structure.
Five years ago the Bush administration said it wanted seven controversial transportation projects, including the St. Croix bridge issue, settled. Since then, the federal government has relented on some of its environmental concerns.
The l995 pricetag for the new bridge over the St. Croix was $120 million. Two years ago the estimate was more than doubled to $350 million, to include money for some additional farmland acquisition in Wisconsin, bigger roads leading to the bridge and environmental mitigation costs.
The proposal for a new St. Croix river bridge is one of Minnesota's longest-running environmental controversies.
The National Wild and Scenic Rivers System includes more than 11,000 miles of U.S. waterways on 160 rivers or just over one-fourth of 1 percent of the country's rivers.