A U.S. Environmental Protection Agency watershed assessment of the proposed Pebble Mine in the Bristol Bay region of Alaska suggests the mine and its related infrastructure could destroy as much as 94 miles of streams and up to 5,300 acres of wetlands, ponds and lakes.

Backers of the controversial mine say the project—which could include a large containment pond, waste treatment facilities, roads, a pipeline and a highly engineered earthen dam to store waste by-products, or tailings—could cost up to $8 billion to construct.

The site contains billions of metric tons of copper, gold and molybdenum that, potentially, could be extracted for more than 60 years. But public opposition in Alaska is strong, and critics of the project say it would devastate not only the landscape but also the resident salmon population and local fishermen's way of life.

Nelli Williams, deputy director of Trout Unlimited's Alaska program, says the region is a "salmon powerhouse," home to more than half the world's fishable sockeye salmon.

The mine, which encompasses an area about 40,000 sq miles in size, sits directly atop the headwaters of two of the largest rivers that feed into the bay, Williams says. Trout Unlimited contends the proposed dam and containment pond alone could hold between 2.5 billion and 10 billion tons of mine waste, which would need to be stored on-site in perpetuity.

The EPA assessment was prompted by several Bristol Bay Alaskan tribes requesting the agency take action to protect the bay. The assessment, which was conducted over three years, was peer-reviewed by a panel of scientists and mining-industry experts and should serve as a technical resource for governments and tribes that are wrestling with the challenge of developing large-scale mines without harming local watersheds, the EPA says.

The assessment was based on a preliminary plan submitted to the U.S. Securities and Exchange Commission by the mine's chief backer, Vancouver-based Northern Dynasty Minerals Ltd. The report does not make any recommendations one way or the other.

While environmental groups lauded EPA's assessment, the Pebble Mine Limited Partnership, the mine's developer, says the agency should have waited until the partnership submitted a final plan.

Mike Heatwole, a spokesman for the partnership, says, "We've maintained throughout that, if the EPA were going to undertake this assessment, they should do it right. It remains our view that they did not."

He says the agency did not dedicate sufficient financial resources or time to the assessment and did not take into account the environmental safeguards the team planned to put in place.