July 18 (Reuters) - U.S. environmental regulators laid out proposed protections for Alaska's Bristol Bay watershed on Friday, outlining restrictions that would effectively block the development of the Pebble deposit, which could be one of the world's largest copper mines.
In a 214-page document posted to its website, the Environmental Protection Agency (EPA) proposed to restrict the use of certain waters in the region for disposal of dredged or fill material associated with mining the Pebble deposit.
It is the latest step by the agency to use a rare process under the Clean Water Act to try to stop work on the proposed copper-gold mine, which they say would threaten the region's valuable salmon fisheries.
"The science is clear that mining the Pebble deposit would cause irreversible damage to one of the world's last intact salmon ecosystems," said Dennis McLerran, regional administrator for the EPA, in a statement. "Bristol Bay's exceptional fisheries deserve exceptional protection."
Project owner Northern Dynasty Minerals Ltd, meanwhile, has sued the EPA for trying the derail development before the company has even applied for its permit or environmental assessment.
A company representative was not immediately available to comment on the proposed restrictions, but the Vancouver-based miner has repeatedly said the mine can be developed safely.
Pebble is one of the richest undeveloped copper deposits in the world. Northern Dynasty has said if the project goes ahead it will employ thousands and filter hundreds of millions in tax dollars to federal, state and local governments each year.
But opponents have long said the environmental risks outweigh the benefits, citing the potential for widespread damage if polluted water were to enter streams in the region.
The EPA's proposal would make it difficult to build and operate the project by restricting the discharge of dredged or fill material related to mining that would result in the loss of streams, wetlands, lakes and ponds, or alter the flow of streams where salmon have been documented.
The agency made clear that the proposed restrictions impact only the mining claims surrounding the Pebble deposits and will not affect other projects in Alaska or the United States.
Northern Dynasty's shares were down 6.6 percent at 85 Canadian cents on Friday on the Toronto Stock Exchange. The stock has fallen more than 60 percent in the 12 months, driven down by a series of setbacks including the loss of its joint venture partner Anglo American and the EPA action.
Reporting by Julie Gordon; Editing by Nick Zieminski