Environmental activists challenging plans for a long-delayed new bridge over the St. Croix River on Thursday said the recent placement of a stretch of the river on a new state impaired-waters list supports their contention that the bridge is a bad idea.

At the same time, legislators from Minnesota and Wisconsin pushed the Sierra Club to drop its legal challenge to the bridge near Stillwater.

Lake St. Croix -- a stretch from Stillwater to Prescott, Wis. -- was included on the list of impaired waters because of conditions that caused excessive phosphorous in the water, said Nancy Miller, a spokeswoman for the Minnesota Pollution Control Agency.

Although the report didn't specify the source, phosphorous usually comes from urban runoff, stormwater discharge, farm fields and septic systems, she said.

The St. Croix River is federally protected as a wild and scenic river.

Mat Hollinshead, the Sierra Club's transportation chairman, said the listing shows why a new four-lane bridge shouldn't be built over the St. Croix River. Such a "superbridge" would make water quality worse, he said.

"What we need to be doing now is increasing protections of the St Croix, not decreasing," he said.

In a suit filed in June, the Sierra Club sought an injunction against federal funding, the primary source of money for the controversial proposed bridge. The suit argued that the bridge would intrude on views of bluffs along the river and promote development that would hurt the river's ecosystem.

Three legislators who held a news conference in Stillwater on Thursday said that any delays in building a new bridge place dangerous stress on Stillwater's 1931 lift bridge and invite pollution from mile-long traffic backups.

About 16,000 drivers, many of them commuters, cross that bridge daily.

"It's an environmental issue of smog and noise for Stillwater and the area," said Rep. Matt Dean, R-Dellwood. "It's also an issue of commerce for our economic vitality, to be able to bring people and goods across the river in a safe manner."

Joining him were Sen. Ray Vandeveer, R-Forest Lake, and Wisconsin state Sen. Sheila Harsdorf, R-River Falls. All said they understood the Sierra Club's concern that a new bridge might hurt the sensitive river basin but said that's why dozens of "stakeholders" worked out compromises before the Sierra Club went to court.

Dean said Thursday evening that he hadn't seen the impaired-waters list and wanted to know more about the source of the problem before he commented. He did say, however, that he trusted the work of environmentalists and government agencies who prepared an environmental impact statement for the proposed bridge.

In a letter to Sierra Club leaders in Minneapolis and San Francisco mailed last week, a bipartisan group of eight legislators chided the suit as "a great disservice" to residents and businesses, as well as dozens of stakeholders who worked out compromises on the bridge proposal over several years.

Hollinshead dismissed the legislators' criticism as political posturing and said that some of them should instead invest their energy in developing transit to reduce the number of cars crossing the river. He also said that the Sierra Club suit hasn't delayed the bridge because the federal government hasn't committed funding anyway.

"By far the biggest obstacle to this bridge is the money," he said. "There is no money."

The proposed St. Croix bridge would cost an estimated $484 million in federal appropriations if built within 10 years. The Minnesota Department of Transportation, however, said federal funding might not be available until much later.

"We're defending our environmental values of the river," Hollinshead said. "That's our mission, and we'll be true to that mission."