The Environmental Protection Agency issued a proposal Friday under the Clean Water Act that would limit mining activity in Alaska’s Bristol Bay watershed, striking a major blow to a project that would rank as one of the world's largest open-pit mines.

The proposed determination, which will now be subject to a public comment period until Sept. 19, represents the latest step by the Obama administration to impose restrictions on a massive gold and copper mining project, called Pebble Mine. Native Alaskan tribes, commercial fishing operations and environmentalists who have been seeking to block the venture on the grounds that discharge from its operations could harm the area that supports nearly half of the world’s sockeye salmon.

Dennis McLerran, the regional administrator for EPA Region 10, told reporters Friday the agency had concluded that even a mine much smaller than the one currently envisioned by Pebble's sponsors would produce "almost unfathomable amounts of rock" which "posed significant risks to the fragile ecosystem" in Bristol Bay.

The EPA was taking this step "to protect the world’s largest salmon greatest fishery what would certainly be one of the world’s largest open pit mine developments ever conceived of," he said.

A Canadian-based firm, Northern Dynasty Minerals, is trying to start construction on the project, which it predicts will create 1,000 direct jobs and generate up to $180 million in state revenue.

The mine's developers have not filed a permit application yet, so EPA based its analysis on the company's filings with the Securities and Exchange Commission. It based its conclusion on a .25 billion ton mine, which is significantly smaller than the project the Pebble Partnership said it is likely to pursue.

"It if further disappointing when you consider that many of the peer reviewers of the Bristol Bay Watershed Assessment noted that the Assessment – the only “science” EPA has to justify its action – was not a sufficient basis to support any regulatory decision," Collier said. "We will continue to fight this unprecedented action by the agency, and are confident we will prevail.”

In February, EPA invoked its authority under the Clean Water Act to determine whether it should permanently bar the U.S. Army Corps of Engineers from issuing a discharge permit for the mine. Bristol Bay is home to a critical fishery. Its draft proposal bars certain filling and dredging activities associated with the operation.

Sen. Lisa Murkowski (R-Alaska) has repeatedly warned EPA not to issue a “preemptive veto” against Pebble Mine, though Sen. Mark Begich (D-Alaska) and other Democratic senators from the Pacific Northwest such as Maria Cantwell (Wash.) have argued it poses too great a risk to the wild fishery.

“The EPA is being disingenuous in saying that this decision is only going to impact mining in a particular area of Alaska,” said Murkowski in a statement, noting that she is visiting the U.S.-Mexico border right now and is still reviewing the documents. “The EPA is setting a precedent that strips Alaska and all Alaskans of the ability to make decisions on how to develop a healthy economy on their lands. This is a blueprint that will be used across the county to stop economic development.”

But opponents of the mine emphasized the prescribed nature of the proposal: Begich issued a statement Friday saying it "applies only to the Pebble deposit."

"The limited scope is critical and means the determination would not affect mining or any other resource development project in other parts of the state. As I’ve often stated, I believe Pebble is the wrong mine in the wrong place," he said. "However, I remain a strong supporter of the mining industry and mines in other regions of Alaska and remain committed to ensuring that this process does not allow any precedent to be set that could restrict other responsible mining projects in Alaska or the U.S.."

Tim Bristol, who directs Trout Unlimited’s Alaska program, praised the agency’s move.

"Thousands of Alaskans requested that the EPA review threats to the Bristol Bay fishery from the proposed Pebble Mine, and hoped they would recommend strong protections for Bristol Bay salmon and jobs as they have,” Bristol wrote in an e-mail. “Far from a preemptive veto, the EPA’s actions simply place an understandably high standard for any mining company wishing to apply for permits in Bristol Bay. These restrictions will ensure that no unacceptable adverse impacts will occur from mining development in Bristol Bay; the very standard that has been the law of the land for over 40 years.”

Chris Wood, Trout Unlimited's president and CEO, said he had just visited Bristol Bay last week, where commercial fishing operators are finishing a season that will net 30 million fish. "If there was ever a time to stop this ill advised and myopic proposal to mine Bristol bay's headwaters, it is now."Throughout the EPA's review, backers of the mine have questioned whether the agency has used adequate science to support its findings.

Joel Reynolds, Western director for the advocacy group Natural Resources Defense Council, said he did not think the mine's sponsors would be able to overturn the EPA's determination because it was based on "a comprehensive, science-based process."

"Their chances of being able to overturn it are minimal at best," he said. "Today's one more nail in the coffin in a project that deserves to die."