The U.S. Environmental Protection Agency (EPA) issued a proposal under the Clean Water Act that would essentially block the development of Pebble Mine project in Alaska.

The state of Alaska and Pebble Partnership previously filed a lawsuit that argues the EPA should not be able to veto the project before a mine plan is finalized and evaluated through the permitting process. Pebble has asked that a judge block the EPA from taking any additional steps, but no ruling has been made.

The EPA is using its 404 (c) authority to initiate the process. The EPA has used the authority sparingly, but EPA regional administrator Dennis McLerran said the science is clear “that mining the Pebble deposit would cause irreversible damage to one of the world’s last intact salmon ecosystems. Bristol Bay’s exceptional fisheries deserve exceptional protection.”

The EPA said as part of its analysis it used plans filed by the mine’s owner, Northern Dynasty Minerals, with the U.S. Securities and Exchange Commission in 2011. That information indicated the Pebble deposit is likely to involve excavation of the largest openpit ever built in North America, reaching a depth that rivals that of the Grand Canyon at nearly a mile, the EPA said in its report.

The announcement is the next step in that process, The Associated Press reported.

EPA plans to take public comment through Sept. 19 and to hold public meetings in Alaska next month. After that, McLerran would have to decide whether to withdraw the proposed action or send it to EPA headquarters for consideration.

Tom Collier, CEO of the Pebble Partnership, said he was pleased the agency did not flat-out reject any development and instead proposed conditions on future development. However, he said Pebble does not believe EPA has the authority to do even that before a detailed mine plan is submitted and reviewed by state and federal agencies.

He said in a statement that Pebble believes the project must be developed in a way that protects clean water, fish and wildlife and Alaska Native communities that rely on them. But he called the restrictions proposed by EPA “misguided and not based on sound science or regulatory precedent.”

The EPA said as part of its analysis it used plans filed by the mine’s owner, Northern Dynasty Minerals, with the U.S. Securities and Exchange Commission in 2011. That information indicated the Pebble deposit is likely to involve excavation of the largest open pit ever built in North America, reaching a depth that rivals that of the Grand Canyon at nearly a mile, the EPA said in its report.

The agency looked at three mine scenarios, one based on the worldwide median size deposit that contains copper-, gold- and molybdenum-bearing minerals, which was the smallest scenario analyzed, and two that it said were based on statements made by Northern Dynasty, of mine sizes of 2 billion tons and 6.5 billion tons.

The restrictions proposed by EPA are in line with the estimated impacts of the smallest scenario, which McLerran said is one-eighth the smallest size contemplated in Northern Dynasty’s filing. They include loss of at least 5 miles of streams with documented salmon or loss of 1,100 or more acres of wetlands, lakes and ponds that connect to salmon-bearing streams or tributaries.

The EPA said if the proposed restrictions were finalized, mining of the Pebble deposit would still be possible if the environmental impacts were smaller than those laid out. It’s possible those behind the Pebble Mine could come up with something different from what EPA analyzed.