Phosphorous contamination that landed the federally protected St. Croix River on Minnesota's impaired-waters list for the first time has been building for decades, several experts familiar with the river said Monday.

One of them, Craig Affeldt of the Minnesota Pollution Control Agency, said that large algae blooms in Lake St. Croix -- the portion of the St. Croix River between Stillwater and Prescott, Wis. -- have resulted from excessive phosphorus, which can alter a river's ecosystem.

"Whether that's runoff from streets, parking lots, livestock or fertilizers, this is what people introduce to the landscape because that's how we live," said Affeldt, a specialist on St. Croix basin matters.

Lake St. Croix appeared last week on the state's 2008 draft list on the same day that eight Minnesota and Wisconsin legislators called on the Sierra Club to end a suit that seeks to block construction of a new bridge across the St. Croix. The club responded that the river's arrival on the impaired list is further evidence that the bridge would pollute the river.

Scott Elkin, the club's Minnesota president, said Monday that the proposed bridge that would lead to Wisconsin from Oak Park Heights, just south of Stillwater, is a "sledgehammer" that will hurt the river basin. He said the Minnesota Department of Transportation dismissed the club's suggestion for a smaller, less invasive bridge.

"They have been incredibly stubborn and narrow-minded," he said of MnDOT officials.

But Todd Clarkowski, a MnDOT engineer involved with the bridge's environmental impact statement, said the proposed bridge would improve the river's water quality because of new stormwater treatment ponds that would contain runoff. Some would be built in areas where water now runs into the river unrestricted, he said.

Sen. Kathy Saltzman, DFL-Woodbury, one of the legislators asking the Sierra Club to drop the suit, said the MPCA findings concern her.

However, she said the collapse of the Interstate 35W bridge reinforces concerns about safety on the 1931 lift bridge in Stillwater, and said that both a new bridge and improved water quality need attention.

"We have to address the river crossing, somehow, some way," she said.

Across the river, Wisconsin legislator Sheila Harsdorf said she can care about the river and still support the new bridge. She said safety is paramount and that congestion continues to grow even without a new bridge.

The Sierra Club argued in its suit, filed in June, that a new bridge would bring additional development that would damage the St. Croix watershed. But Harsdorf, R-River Falls, disagrees.

"Obviously they're latching onto this as another excuse not to build another bridge," she said.

The MPCA's Dennis Wasley, one of the scientists who evaluated Lake St. Croix data, said that phosphorus has gathered in that portion of the river because it's far deeper -- from about 40 feet to 70 feet in some places -- than the rest of the river. The algae worsens in drought years, he said.

"We're kind of at a nuisance level on the St. Croix," he said, because people are less willing to swim but algae blooms aren't so bad that they would kill fish and interfere with boating.

Ken Harycki, Stillwater's mayor, said he's not happy to hear of the scenic St. Croix's problem with phosphorus but said traffic backups at the lift bridge are fouling Stillwater's air.

Kevin Giles • 651-298-1554

Kevin Giles • kgiles@startribune.com