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The cofferdam was intended as a temporary structure to hold back water while building a permanent barrier as part of the mining company's tailings disposal plan for the Kensington Mine, about 45 miles north of Juneau. Tailings are the ground rock left over after the metal is extracted.

The permanent dam would have retained the snowmelt, but it was never finished pending a ruling on a lawsuit over the disposal plan.

Water may eventually flow out of the lake, Ross said.

The U.S. 9th Circuit Court of Appeals denied the mining company's plan for an "interceptor" ditch to divert snowmelt. The court said Coeur's tailings proposal would violate the Clean Water Act.

The Coeur plan also included a 30-foot-wide, 1,600-foot-long road to be built adjacent to the ditch and the filling of roughly 4.5 acres of wetlands.

Other options to deal with runoff initially included building a spillway over the cofferdam, adding additional pumps and reinforcing the cofferdam.

The structure was not designed to retain spring runoff, and Corps of Engineers officials feared that the snowmelt would swamp it, resulting in "catastrophic downstream environmental damage and water quality violations," according to its permit evaluation and decision document.

"We are pleased," said Mark Rorick, head of the Juneau chapter of the Sierra Club. "They are putting the lake back into a natural drainage situation."

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"Our information is that they have dodged a bullet in that we have had a very slow snowmelt process and that they have had no indications of any threatening flood situations occurring there," Rorick said. "They were very lucky."

Coeur spokesman Scott Lamb said Thursday the company had "elected not to comment on the runoff plan."

Ross said that since that ruling has not been handed down as a final decision, it has been difficult to know how to deal with the snowmelt.

"I don't know what we are going to be asked to do tomorrow," he said.

"They are doing everything they can to reduce turbidity and sediments. They are working with the agencies and within the parameters that the court has given them," he said.

Environmentalists hope that the lake can be restored even further.

"We would urge them to take the next steps, which would be reshaping the lake, restoring it to its natural levels as soon as possible, reseeding the banks with grass and willow to start the process of forest regeneration," Rorick said.

• Brittany Retherford can be reached at brittany.retherford@juneauempire.com.