With many of the nation's dams no longer making economic or environmental sense, old dams are being dismantled, in a slow-moving but remarkable reversal of fortune for rivers and fish.

The latest announcement came from the Pacific Northwest when Portland General Electric inked a $16 million deal last week to remove Marmot Dam on the Sandy River and another smaller structure on the Little Sandy in Oregon about 50 miles east of Portland.

The deconstructions in Oregon are unusual because they are fully functioning hydroelectric dams still producing power -- a move that once upon a time would have been almost unimaginable.

"This is a major development in the environmental history of the West," said Eric Eckl, a spokesman for American Rivers, an advocacy group. "It is one of first removals of hydroelectric dams, and they are not only taking down the dams, but restoring habitat for threatened salmon and steelhead."

Conservationists hope it is the beginning of a long-term trend. Hundreds of dams are slated for possible demolition; more than 60 are scheduled to be removed this year alone, the highest number since American Rivers began keeping count five years ago.

"I recognize there are risks involved. We've never been down this road before," Oregon Gov. John Kitzhaber (D) told reporters at the signing ceremony for the deal to dismantle Marmot Dam. Kitzhaber was referring to a central problem with removing old dams: the huge buildup of silt behind them that must be slowly siphoned off so as not to damage downstream habitat.

But the governor added, "We do know this one small piece of Oregon will be wilder. . . . We're restoring a part of why we love this state."

Since the deconstruction of the Edwards Dam on the Kennebec River in Maine three years ago, more than 250 impoundments have been dismantled, many of them in eastern and midwestern states such as Pennsylvania and Wisconsin.

Most of the dams are small, and almost all of them have done their jobs for irrigation, flood control or water supply -- and not hydroelectric power.

But environmentalists, working closely with utility companies and other agencies (when they aren't suing them), are pressing ahead to take on ever bigger dams.

In the next few years, three more large dams may come down in the Pacific Northwest: the Elwha Dam on the Washington peninsula, the Savage Rapids Dam on the Rogue River in Oregon and the Condit Dam on the White Salmon River in Washington.

The issue of fish and dams has been most heated in the Pacific Northwest because of the dwindling runs of salmon, which are the totemic species there, the symbol of how well wildness can live alongside Microsoft and Boeing, wheat farmers and wine growers.

Part of this new momentum is generated as public and private institutions are struggling to restore the runs of salmon, steelhead trout, alewife and other migratory fish, which spend part of their lives in the ocean, but return to their natal streams and rivers to spawn.

The dams impede these natural migrations because they block fish from swimming upstream. To help the fish pass over the dams, elaborate measures have been employed. There are "fish ladders" at many dams, and some salmon are even loaded into barges and trucks and transported upstream. In other settings, such as the Marmot Dam, workers pluck salmon from a pool, select the wild subspecies and allow them to pass upriver.

Although much of the controversy over dam removal has traditionally pitted environmentalists against big utilities, the U.S. Army Corps of Engineers or the Bureau of Reclamation, there appears to be a growing consensus that many water impoundments have served their purpose and should be decommissioned.

"There is this shift in thinking, that there are these outmoded dams that have run their cause, and the environmental benefits of removing them vastly outweigh the cost of preserving them, and those are the dams that people are taking the hardest look at," said Alan Moore, a spokesman for Trout Unlimited's western conservation programs.

Environmentalists argue that removing dams is not just good for fish, but bolsters a new service economy based upon free-moving rivers -- the fishing and rafting guides, and the tourists and new residents who seek out less spoiled areas.

The new look at old hydroelectric dams is occurring now because the facilities must renew their licenses every 50 years with the Federal Energy Regulatory Commission. Many dams are coming up for review, and more owners and operators may conclude that there are economic as well as environmental reasons to demolish dams.

To renew their licenses, older dams must often be renovated to provide state-of-the-art access for migrating fish. And that cost can be high.

Those are the calculations that Peggy Fowler, president of Portland General Electric, said she made when the utility decided to remove its two dams in the Sandy River system. That, and some obvious public relations. PGE is owned by Enron Corp.

The largest of the two dams, Marmot, was built in 1912 and would have needed extensive maintenance and alterations to improve fish habitat.

Together, the two dams produce about 10 megawatts of electricity, less than 1 percent of the 2,000 megawatts of electricity that PGE generates. They are expensive to operate because power is produced by an elaborate system that involves first sending water from Marmot Dam to Little Sandy Dam, then to Roslyn Lake, and then to a generating station on nearby Bull Run River.

Fowler says the turn-of-the-century engineering was "amazing." And, she said, "it was the wonderful thing to do in the beginning," building the dams in 1912. But, she added, there is "the right thing to do now, which is take them down."

The Marmot Dam will go in 2007, and the Little Sandy Dam will go in 2008. The dams lie just outside the western edge of the Mount Hood National Forest. The utility also will transfer 1,500 acres of land near the dams to a nonprofit organization toward the creation of a 5,000-acre nature reserve.

But as Julie Keil, director of hydroelectric licensing at PGE, says, "It's harder to take out a dam than you might think." She describes the removal of the Sandy River dams as the most complex dismantling ever attempted in the United States. There are two challenges. First, there is the silt. Then, there are the fish.

The headwaters of the Sandy River begin in the glacier fields of Mount Hood east of Portland. Before the dams were constructed, a salmon or steelhead could swim from the ocean to the mountainsides.

After the dams were constructed, the state began to release salmon from a hatchery below the structures. These salmon are different -- genetically -- from the wild salmon that want to swim past the dam to spawn in upstream waters. So to limit possible genetic damage to wild stocks, the state will let the old hatchery fish slowly die off, and replace them with new fish more genetically akin to the wild stocks.

It is complicated, and no one is sure exactly how well this act of manipulating Mother Nature will work.

Oregon's Marmot Dam, a hydroelectric facility that still produces power, is slated to be destroyed in 2007. To help fish pass over dams, elaborate measures have been employed. At Marmot Dam, left, a worker sorts salmon, selecting the wild subspecies and allowing them to pass upriver. The issue of fish and dams has been most heated in the Pacific Northwest because of the dwindling runs of salmon.