By Nicole Mordant

VANCOUVER (Reuters) - Large-scale mining in the Bristol Bay watershed poses serious risks to salmon and native cultures in this pristine corner of southwest Alaska, the U.S. Environmental Protection Agency said in a report released on Wednesday.

The EPA said a mine could destroy up to 94 miles of salmon-supporting streams and thousands of acres of wetlands, ponds and lakes. The report focused on the impact of mining in an area where a Canadian-based company wants to build a large copper and gold mine.

Polluted water from the mine site could enter streams, causing widespread damage in a region that produces nearly 50 percent of the world's wild sockeye salmon, the EPA said.

The Bristol Bay region supports all five species of Pacific salmon found in North America, which include sockeye, Chinook, chum, coho and pink salmon. It is also home to bears, moose and caribou.

There is also the risk of accidents and pipeline failures that could release toxic copper concentrate or diesel fuel into salmon streams or wetlands, the EPA said.

"Our report concludes that large-scale mining poses risks to salmon and the tribal communities that have depended on them for thousands of years," Dennis McLerran, the EPA's regional administrator in the Pacific Northwest, said in a statement.

The report, which concludes a three-year study and follows two drafts that also warned of widespread ecological damage from mining, does not recommend policy or regulatory decisions.

Northern Dynasty Minerals Ltd's Pebble project would develop an open-pit mine in the region, which has one of the world's largest copper-gold deposits.

The Vancouver-based company was swift to condemn the report, and said the EPA had repeatedly failed to meet its own guidelines and policies for watershed assessments, risk assessment and peer review.

"We believe EPA set out to do a flawed analysis of the Pebble Project ...," Northern Dynasty Chief Executive Ron Thiessen said in a statement.

Alaska's U.S. Senator Lisa Murkowski, the ranking Republican on the Senate's Committee on Energy and Natural Resources, said the EPA has weighed in too soon.

"If the EPA has concerns about the impact of a project there is an appropriate time to raise them, after a permit application has been made, not before," Murkowski said in a statement.

"It is clear that a preemptive veto is still being considered by EPA. Such a veto is quite simply outside the legal authority that Congress intended to provide EPA."

The company vowed to press on with the project, located some 200 miles southwest of Anchorage. Northern Dynasty lost its project development partner last September when mining group Anglo American pulled out of the venture.

(Reporting by Nicole Mordant in Vancouver; Additional reporting by Steve Quinn in Juneau, Alaska; Editing by Toni Reinhold)