Nine members of Washington state’s congressional delegation, all Democrats, called Wednesday for the U.S. Department of State to intervene in a simmering dispute with Canada over a company’s proposal for exploratory mining in the headwaters of the Skagit River.

“The potential for releases of copper and other heavy metals would pollute waters downstream,” the congressional leaders wrote to Secretary of State Mike Pompeo, declaring their opposition to the project.

The letter outlines concerns over potential harms to Washington’s tourism and recreation economy, public health and vulnerable fish populations, among others.

Sen. Maria Cantwell organized the letter. Sen. Patty Murray and Representatives Suzan DelBene, Rick Larsen, Derek Kilmer, Pramila Jayapal, Kim Schrier, Adam Smith and Denny Heck were signatories.

Their request to the State Department follows a cascade of sternly worded letters from tribal leaders, elected officials and environmental organizations to B.C. officials. The mining proposal has stressed a typically affable relationship between Washington state and British Columbia, and could test a longstanding treaty between the U.S. and Canada over the Skagit River.

“Deny this permit,” wrote Seattle Mayor Jenny Durkan to British Columbia Premier John Horgan.

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Mining in the headwaters of the Skagit “represents an unacceptable risk to a river that remains a bulwark to dwindling salmon,” wrote Washington Gov. Jay Inslee.

The mining applicant, Imperial Metals, “would irreparably damage the water quality of the Skagit River,” added Benjamin Joseph, chairman of the Sauk-Suiattle Tribe.

All told, more than 100 elected officials, tribal leaders and environmental organizations on both sides of the border have declared their opposition after the company filed for exploratory drilling permits, according to a tally by Tom Uniack, executive director of conservation nonprofit Washington Wild.

“We’re very hopeful, we have a lot of partners,” said Jeremy Wilbur, a senator for the Swinomish Indian Tribal Community, one of several tribes and First Nations with concerns. “We feel industrial mining and logging in this sensitive part of the Skagit headwaters ecosystem must be rejected.”

Imperial Metals, known for an environmental disaster at its Mount Polley mine in eastern British Columbia, applied for a permit to explore for gold in an area known as the “donut hole” last December. The Canadian government notified Seattle officials of the proposal in March. It’s currently under review, after a public comment period.

The company’s application shows that it hopes to access drill sites using roads carved through the area during controversial clear-cutting logging operations last summer.

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The donut hole is a Manhattan-sized patch of public land left unprotected because of historic mineral and logging rights. Provincial parks encircle the donut hole, and some conservationists argue it should be part of those parks. Alpine snowpack in the area feeds the Skagit’s headwaters.

The Skagit River is the top salmon-producing river in Washington state, and its waters churn through hydropower turbines at Ross Dam, among others, to bring Seattle much of its electricity. Endangered bull trout live in its upper reaches.

Conservationists cried foul after B.C. loggers last summer began to fell trees in the donut hole, after a B.C. government-approved timber sale. But the possibility of mining was always conservationists’ greatest fear because dissolved metals, particularly copper, are toxic to salmon.

Elected officials south of the 49th parallel contend that mining and logging in the donut hole are at odds with a 1984 treaty and agreement.

Before the treaty, British Columbia and Seattle were in dispute over the city’s plan to generate more electricity from the Skagit’s Ross Dam, by building the structure higher. A taller dam would send Ross Lake flooding farther into B.C., something environmentalists opposed. After protests, Seattle agreed to halt the dam project in exchange for inexpensive B.C. hydropower through 2065.

The U.S. and Canada cemented the local deal with a treaty. The agreement created the Skagit Environmental Endowment Commission (SEEC) to conserve the watershed on both sides of the border, enhance recreation there and seek its protection. The B.C. premier and Seattle mayor each appoint four commissioners to lead SEEC.

Last year, SEEC commissioners learned of ongoing logging operations in the Skagit watershed and raised concerns with B.C.’s forestry ministry. The commissioners also alerted Durkan.

After Durkan wrote to Premier Horgan, the ministry last summer put future timber sales in the donut hole on hold. Logging that had been already approved was finished by summer’s end.

This April, B.C. mining and forestry officials met with members of the SEEC commission, including Thomas Curley, a Canadian appointee.

“It was a productive meeting,” Curley said, adding that Forestry Minister Doug Donaldson committed to prevent future timber sales in the donut hole and to deactivate logging roads there.

“Restoration of vegetation and land contours on the blocks which have already been logged will occur in the next two years,” Curley said. Officials promised to notify SEEC of any changes planned for the area.

With the logging issue seemingly resolved, attention is now fixed on Imperial Metals’ application to drill for up to five years inside the donut hole. The company would set up trenches and build settling ponds for the exploratory work in an area believed to have gold and copper.

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Tribes, First Nations, governments and environmental groups, fearing the impacts to fish, have organizing opposition to the project for months.

Wilbur said the Swinomish community worries mining could have “ripple effects” into the Salish Sea. He said Swinomish leaders believe the B.C. government must consult with tribal governments about effects.

“We depend on salmon for ceremonial, subsistence and commercial fishing activities. The Imperial Metals gold-mining operation threatens those activities. That’s our way of life,” Wilbur said.

Seattle City Light, meanwhile, sent a list of more technical concerns to the B.C. chief inspector of mines, outlining possible environmental impacts of mining such as erosion and sedimentation from trail construction, water diversion for mining, heavy metal impacts from drilling and impacts on recreation, among others.

The SEEC commissioners have asked for mining to be prohibited in the watershed, Curley said. The commission wants the B.C. government to help facilitate the purchase of mineral rights, so the donut hole can be preserved for habitat and recreation.

Imperial Metals CEO Brian Kynoch did not immediately return a phone call or email Wednesday.

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Earlier this year, Kynoch said the company needed to explore to determine where its donut-hole mining rights fit into the company’s overall plans. Last year, he told The Seattle Times that he recognized the company would have to produce a sufficient plan to mitigate harms to fish in order to obtain mining permits.

“You need to come up with a plan that’s not going to harm the salmon,” he said.

It’s not clear when the B.C. Ministry of Energy, Mines & Petroleum Resources will decide on the Imperial Metals application. The ministry did not respond on-record to a request for comment.