Last year, the Obama administration permanently banned oil drilling in Alaska’s Bristol Bay, America’s richest salmon fishery and the heart of a $2.2 billion regional fishing industry. One huge threat to this extraordinary ecosystem remains: a proposed gold and copper operation known as the Pebble Mine. If built, it would affect a huge area of clear-running headwater streams and wetlands that feed the bay.

Responding to urgent requests from nine native tribes that depend on the headwaters for subsistence, the federal Environmental Protection Agency has now announced that it will assess the risks to the bay from mining and commercial projects in general.

This is very good news. The agency obviously cannot prejudge the study’s outcome, but its announcement pointedly called attention to Bristol Bay’s “extraordinary importance” as a salmon fishery and source of food and income for local residents. It also called attention to its obligation under the federal Clean Water Act to block any project that would have an “unacceptable adverse effect” on water quality and wildlife.

Anglo American, the London-based multinational powerhouse behind the project, says it can extract the minerals safely. But historically the mining industry has done a sloppy job of protecting the environment. Mining residues, like sulfide-laced rock, are toxic. No matter how hard the company tries to sequester them — it proposes to build a 740-foot-high dam to contain the waste — an earthquake or other disturbance can jar them loose.

The people of Alaska came close to blocking the project themselves in a 2008 referendum. Three former governors, including two Republicans, and Senator Ted Stevens spoke out against the mine. Industry, however, spent $12 million on advertising about the mine’s economic benefits; that, plus a last-minute pro-mining push by Gov. Sarah Palin and her administration, turned the tide in industry’s favor.

The E.P.A. is right to do this study. We are certain it will find that the mine presents unacceptable risks and should not be allowed to proceed.