Five Senate Democrats are calling on US President Barack Obama to consider blocking construction of a copper mine in Alaska, arguing it could jeopardize the pristine environment of America's biggest salmon fishery.

The concerns by lawmakers from the western states of California, Oregon and Washington could add to pressure on the Environmental Protection Agency to scupper the huge Pebble Mine project.

EPA is currently conducting a study of the impact of the project, which would disrupt several square miles of watershed around Bristol Bay, where an estimated 40 million salmon return every year.

"Water contamination and habitat loss from the construction and operation of a hard rock mine in Bristol Bay would put thousands of fishery-related family wage jobs at risk," wrote the lawmakers, led by Senator Maria Cantwell of Washington state.

The senators pointed to a new study by the University of Alaska's Institute of Social and Economic Research (ISER), which valued the commercial fishing activities in the region at $1.5 billion in output, including $500 million in direct income.

"Additionally, Washington, Oregon and California benefit from $674 million in economic activity from Bristol Bay salmon fishing and processing," the lawmakers said.

The three western states hold over 1,100 fishing permits for Bristol Bay in southwest Alaska, supplying thousands of jobs in the industry.

Pebble Mine owners Northern Dynasty and Anglo American say the project's operational investments would top $1.2 billion per year, which would provide substantial tax revenue for the remote state, and expand US copper production by 20 percent.

EPA released a peer review report last November in which several experts noted the "significant impacts" that such a project would have, including the loss of dozens of miles of streams that are spawning areas for sockeye, chinook and other salmon species.

The EPA is given authority under a rarely invoked section of the Clean Water Act to block the issuance of federal permits for large developments if they are deemed to have adverse impacts on the environment.

A final EPA assessment is expected by the end of the year.

Film icon and activist Robert Redford has weighed in against the project, writing in the Los Angeles Times last month that Bristol Bay is "just too important, environmentally and economically, to be sacrificed for the sake of foreign mining profits."