With the proposed four-lane St. Croix River bridge caught in a political crossfire, citizen interest in a tunnel to divert interstate traffic from Stillwater has resurfaced in recent months.

The vision is that eastbound traffic would disappear from sight on the Minnesota side of the protected St. Croix, leave the view on the river unblemished, and emerge in the bluffs on the Wisconsin side.

Building a tunnel, proponents say, would keep the river crossing in good graces with the U.S. Wild and Scenic Rivers Act while improving regional transportation needs.

The idea is far from new. Two tunnel routes proposed in 1990 were examined and discounted as being even more costly than a bridge, said Adam Josephson, east-metro manager for the Minnesota Department of Transportation (MnDOT).

"It would not be productive to revisit that," he said.

Several other river crossing proposals were discussed in the past 20 years in a furious debate over cost, location and intrusion on the St. Croix. With the current freeway-style proposal now being examined in Congress, alternatives to a large bridge are re-emerging in informal public debate.

The chief critic of the current proposal, the Sierra Club, wants to reopen consideration of other ideas. MnDOT, as the supervising agency on the bridge project, didn't allow "serious analysis of lower, slower, less expensive options," said Margaret Levin, state director of the Sierra Club's North Star Chapter.

Gov. Mark Dayton said earlier this year that he, too, wanted a fresh review of the various St. Croix proposals. However, he soon reversed his position, joining with U.S. Rep. Michele Bachmann, R-Minn., to push for congressional approval to build the bigger bridge.

Downsides of a tunnel

The prospect of building a traffic tunnel continues to captivate some people's imaginations but fell out of favor with planners for several reasons.

Engineering reports show a south tunnel was under study for a possible crossing at Oak Park Heights. A north tunnel would have entered the river from the junction of Hwys. 95 and 96, just north of Stillwater.

The many stakeholders in the longstanding bridge debate rejected the tunnels in 2003, Josephson said.

Documents show concern over disturbing the river's aquatic life, excessive fuel consumption when vehicles climbed out of a tunnel onto the river bluffs, "substantial negative short-term impact" to boating and fishing, and at that time as much as $100 million more costly than a bridge. The bridge estimate, however, has risen to $690 million.

In addition, Josephson said, soil testing showed that mucky soil on the river floor is about 100 feet deep, "not conducive to tunnel construction."

A tunnel also would bring long-term operational costs for lighting and ventilation, he said.

Of the two possible routes, the north tunnel would have caused more environmental harm, MnDOT records show. That tunnel "would have produced potentially severe impacts to the lower section of Browns Creek, a trout stream which flows through a scenic valley north of Stillwater," it was concluded.

Tunnel proponents tout perceived benefits in blogs, letters to the editor and other forums. A tunnel would eliminate traffic noise, prevent road contaminants from being washed into the river, and wouldn't have snow-clearing costs, some observers have contended. Tunnels have been built elsewhere and under more difficult circumstances, they point out.

Countering the views of the tunnel supporters are bridge advocates who question all the fuss over preserving a view on the St. Croix. They say the argument to preserve scenery doesn't mean much given that the Allen S. King Power Plant's 785-foot smokestack, built before the federal law in the 1960s, already dominates the river view at Oak Park Heights.

Kevin Giles • 651-735-3342 • Twitter: @stribgiles