With the Pacific Northwest poised for an epic political battle over fish and hydropower, federal agencies yesterday unveiled long-awaited reports that in the end offered an ambiguous answer to the question of whether tearing out four dams on the lower Snake River will save imperiled runs of salmon and steelhead.

"There are no silver bullets here," concluded William Stelle, regional administrator for the National Marine Fisheries Service as he and other officials released the reports, including a draft environmental study of different options for trying to restore severely depleted stocks of fish that for generations have virtually defined the Northwest.

Release of the environmental studies today will surely ratchet up what already has been a hugely contentious debate in the region over the fate of four dams built during the 1960s and 1970s along a 140-mile stretch of the Snake upstream of its confluence with the Columbia. Because of a court decision four years ago, federal agencies must by next spring make a final recommendation on the dams, though the ultimate decision will almost certainly rest with Congress.

In just a generation, the dams have become an important part of the regional economy, providing 5 percent of the Northwest's electrical power, transforming inland cities such as Lewiston, Idaho, into Pacific Rim exporting giants through the barging of wheat and other products, and allowing farmers to irrigate what were once desert landscapes.

But the dams, along with others on the Columbia, have also helped decimate once prodigious runs of salmon and steelhead, largely by transforming the Snake from a free-flowing river into a series of tepid reservoir pools. Since 1991, four fish species that travel up the Columbia and Snake have been placed on the federal endangered and threatened species lists: the Snake River populations of sockeye salmon, spring/summer chinook salmon, fall chinook salmon and steelhead.

Environmental groups have launched a national campaign to "breach" the four Lower Snake River Dams--Ice Harbor, Lower Monumental, Little Goose and Lower Granite. Under the breaching plan, which would cost $1 billion, the earthen portions of the four federal dams would be removed and the river would essentially be allowed to flow freely again around the concrete remainders.

The most important of the reports released yesterday examines the environmental, economic and social impacts of that proposal as well as three others under which dam operations would remain essentially unchanged; the U.S. Army Corps of Engineers would operate the dams as they do now but maximize artificial transportation of young salmon by barge and truck; and the Corps would accelerate technical fixes to the dams to improve fish passage.

The analysis was far more certain of the economic costs than it was of the biological benefits of removing the dams, an outcome that in part reflects the enduring disagreements among federal agencies and their scientists.

The essence of the report's message was that while dam breaching offers more hope than other actions, it alone is not sufficient to avert the salmon's and steelhead's extinction. "Breaching will not by itself recover dwindling salmon and steelhead numbers," said Gen. Carl A. Strock of the Army Corps of Engineers.

But the federal message was somewhat muddled. According to an appendix to the main environmental report, the U.S. Fish and Wildlife Service found that the dam drawdown option would provide many more benefits to fish than any other alternative. "The bottom-line biological conclusion is really a no-brainer," said Anne Badgley, the service's regional director, at a news conference in Portland, Ore. "For native fish and wildlife, a free-flowing river is better than a dammed river."

Stelle, the National Marine Fisheries Service official, said that the ultimate conclusion of the federal studies is that the Pacific Northwest faces a host of hard choices involving not just how to manage the river systems, but also how to improve spawning habitat, how much fishing should be allowed and how much of a role hatcheries should have.

"To be effective, we must be comprehensive," he said. "We need to make some tough choices here in the Pacific Northwest to be successful, and we must be successful."

In terms of the economic effects on the Northwest, the study said that tearing down the dams would have significant costs as well as benefits. Loss of barge transport would cost grain farmers and other shippers $24 million annually. Farmers would incur $15 million a year in new irrigation expenses. Replacing lost electricity could cost $250 million a year, or $1.20 to $6.50 a month for residential customers. And though the project would create nearly 21,000 temporary jobs, it would result in an overall long-term loss of 2,844 jobs in the region.

On the other hand, creating a free-flowing river, the report said, would generate $82 million in new spending on recreation.

Advocates on both sides of the dam breaching issue tended to see in the reports what they wanted to see.

"If you go through these documents, what you find is that the science is becoming more clear that dam breaching is a necessary ingredient of restoring these fish," said Jeff Curtis, western conservation director for Trout Unlimited, a national fish conservation group.

But Bruce Lovelin, executive director of the Columbia River Alliance, which represents commercial users of the river, said the main conclusion was that breaching remains an expensive option with an uncertain result. "It now appears the National Marine Fisheries Service needs more time to answer its own question," he said.