The U.S. Environmental Protection Agency (EPA) proposed a legal a legal determination that would ban the disposal of mining waste in Alaska’s Bristol Bay watershed, a move that could put an end to the Pebble Partnership’s plans for the proposed Pebble Mine.

The EPA cited the 1972 Clean Water Act in proposing the legislation that would create permanent protections for the waters and wildlife of Bristol Bay, about 321 km (200 miles) southwest of Anchorage. The New York Times reported that the proposal will be finalized later this year.

The determination would prohibit any entity from disposing mine-related waste within 797 km² (308 sq miles) around the site of the proposed Pebble Mine project. That’s an area about four times as large as Washington, D.C., but just a small fraction of the entire 103,600 km² (40,000- sq mile) area of the Bristol Bay watershed.

“The Bristol Bay watershed is a shining example of how our nation’s waters are essential to healthy communities, vibrant ecosystems, and a thriving economy,” said Michael S. Regan, the administrator of the Environmental Protection Agency. “E.P.A. is committed to following the science, the law, and a transparent public process to determine what is needed to ensure that this irreplaceable and invaluable resource is protected for current and future generations.”

Blocking the Pebble Mine would be a promise kept for President Biden, who pledged on the campaign trail to “listen to the scientists and protect Bristol Bay.”

However, the move would also keep in place an enormous amount of copper, a metal critical to the ongoing energy transition.

Last fall, a spokesman for Pebble Mine Partnership said that doing so could have the unintended consequence of hampering the Biden administration’s goals to combat climate change, by restricting domestic extraction of a crucial mineral used in making batteries for electric vehicles and other low-carbon technologies.

“As the Biden Administration seeks lower carbon emissions for energy production, they should recognize that such change will require significantly more mineral production — notably copper,” said Mike Heatwole, the spokesman, in a statement. “The Pebble Project remains an important domestic source for the minerals necessary for the administration to reach its green energy goals.”

The fight over the fate of Pebble Mine and Bristol Bay has raged for more than a decade. The plan to mine Bristol Bay was scuttled years ago under the Obama administration, then found new life under President Trump. But opposition, from Alaska Native communities, environmentalists and the fishing industry never diminished, and even Mr. Trump’s son, Donald Trump Jr., a sportsman who had fished in the region, came out against the project. The waters are thick with chum, coho, sockeye and pink salmon.

In 2020, the Army Corps of Engineers denied a permit for the project that was seen as critical for it to proceed.

The company seeking to build the mine, the Pebble Limited Partnership, appealed that decision and is also expected to challenge the legality of the Biden administration’s new plan to protect Bristol Bay.

The company wants to dig an openpit mine more than a mile square and one-third of a mile deep where it would process tens of millions of tons of rock a year to extract metals estimated to be worth at least $300 billion. The project would include the construction of a 270-megawatt power plant and 265-km (165-mile) natural-gas pipeline, as well as a 132-km (82-mile) road and large dammed ponds for the tailings, some of them toxic. It would also require dredging a port at Iliamna Bay.

Both federal and state agencies found that the proposed Pebble Mine, which would be located in two watersheds that feed fish-spawning rivers, would cause permanent damage, harming the breeding grounds for salmon that are the basis for a sport fishing industry and a large commercial fishery in Bristol Bay. Salmon are also an important part of the diet of Alaska Natives who live in small villages across the region. Scientists say the mine would destroy more than 209 km (130 miles) of streams, 2,800 acres of wetlands and 130 acres of open water.

Agency officials said they would accept public comments on the proposal until July 5 before publishing a final legal determination.