The St. Louis River in northeastern Minnesota was named one of the most endangered rivers in the country Monday by a national conservation group that every year names 10 rivers it considers to be at a tipping point from imminent environmental threats.

The St. Louis, the largest U.S. tributary to Lake Superior and an important wildlife breeding ground, is already contaminated with sulfate and mercury and now faces additional risks from a proposed copper-nickel mine nearby, said the national group American Rivers.

It was one of five rivers nominated because of risks from some kind of mining, and the second Minnesota river to make the list in the last three years because of copper-nickel mining proposals in northeastern Minnesota.

“We can’t afford to destroy the largest river feeding Lake Superior,” said Jessie Thomas-Blate of American Rivers.

Officials from PolyMet Mining Corp., which is pursuing the copper-nickel project, say the mine, as planned, would meet all state and federal water quality rules.

Toronto-based PolyMet has proposed a $650 million mine near Babbitt and Hoyt Lakes that, if approved, could be the first to tap into one of the largest untouched reserves of copper nickel in the world.

The proposed mine would take over the tailings basin and crushing facility of a long-defunct taconite operation and dig a new open pit mine where a massive wetland exists now.

The operation would drain into streams that lead to the St. Louis River, which meanders for 194 miles through a 3,634-square-mile watershed before spreading out into the St. Louis River estuary near Duluth.

The estuary, which has also been heavily contaminated by a century of industry around its edges, is the primary incubator for fish, birds and wildlife for the western half of Lake Superior.

The estuary is now undergoing a multimillion dollar cleanup effort funded by federal, state and local governments.

PolyMet and the state last year completed a large-scale preliminary environmental impact review for the project, and the final document is expected to be released by the Minnesota Department of Natural Resources sometime this spring.

Environment vs. jobs

The proposal has pitted environmental concerns against jobs and economic development in the wildest and most treasured corner of the state.

Copper mining carries different and greater environmental risks to water than Minnesota’s long-standing taconite industry.

PolyMet’s open pit mine and processing plant would operate for 20 years, but it could require decades, or even centuries, of expensive water treatment to protect the lakes and rivers that drain into Lake Superior.

“This is a critical year for the future of the St. Louis,” said Steve Morse, executive director of the Minnesota Environmental Partnership, one of the groups that nominated the river for the list. “There is intense political pressure to move this mine forward. It’s important that citizens take their role seriously and engage and be heard.”

The Fond du Lac Tribe of Lake Superior Chippewa, which joined in the petition to nominate the St. Louis, relies on the river for fishing and wild rice.

“This endangered river designation should cause all Minnesotans to join the band in its effort to restore the river,” said Karen Diver, tribal chair.

PolyMet spokesman Bruce Richardson said the company has demonstrated through the environmental impact statement that it will not affect the St. Louis or other streams in any way.

He also said that the project would reduce water pollution that is now coming from the existing tailings basin.

“I don’t think that the St. Louis River is threatened by our project in any way,” he said.

American Rivers in 2013 listed the Kawishiwi River near Ely, Minn., as threatened due to pending copper mining projects along its bank.

Some of the other rivers on the list are the Colorado River in the Grand Canyon, which is threatened by development and uranium mining; Oregon’s Illinois and Rogue Rivers watershed, which is threatened by nickel mining; the Smith River in Montana, which is threatened by a 12,000-acre copper mine on one of its tributaries.