A Blast to Sandy River’s Past
By Kathie Durbin, The Columbian, Vancouver, Wash.
Jul. 25–SANDY, Ore. — With a medium-sized boom and a billowing poof of yellow-white dust, the beginning of the end came Tuesday for Marmot Dam on the Sandy River.
Four thousand pounds of explosives, strategically embedded in the concrete face of the 47-foot-high dam, performed even more efficiently than expected, demolishing the top few feet of the structure in an instant.
Moments later, a convoy of backhoes climbed the dam and begun the weeks-long process of reducing it to rubble. Pneumatic hammers drilled and shovels chewed away at the cracked dam, cutting through tangled rebar. Dump trucks hauled away big chunks of concrete, which will be recycled into road surfacing material.
Tuesday’s event offered a preview of next year’s planned demolition of Condit Dam on Washington’s White Salmon River, which promises to be more spectacular in every way. At 125 feet, Condit will be the highest dam ever demolished in the United States.
For Portland General Electric, the owner of Marmot Dam, Tuesday was a day for celebration tinged with nostalgia.
“In one sense, it’s sad to see it go,” said Peggy Fowler, the utility’s chief executive officer, before she pushed a fake plunger just after noon to mark the occasion. “But the time has come to say goodbye.”
Natt McDougall, the contractor in charge of the demolition, felt nostalgic, too.
“We actually built this dam in 1989,” he said as he surveyed the heavy equipment tearing his handiwork apart from a vantage point behind the dam. “We didn’t know it was going to be torn down (less than) 20 years later. It’s a little sad, I guess, because we worked really hard to get this built. We worked around the clock.”
The present dam replaced a “timber crib” dam made of logs and boulders, built in 1913 to power a trolley that carried city dwellers out to the countryside. That primitive dam was damaged in a flood.
To prepare for Tuesday’s demolition, McDougall’s workers built an earthen coffer dam to divert the flow of the Sandy River — just as they had during its construction.
After Marmot Dam is gone and the fall rains come, the free-flowing river is expected to make short work of the coffer dam, said PGE spokesman Mark Fryburg.
“We think what will happen is, as the high water approaches, we will turn off the sump pumps and the coffer dam will quickly saturate and just wash downstream,” Fryburg said.
When heavy equipment and the rain-swollen river complete the demolition work in late fall, Marmot will be the highest dam ever removed in Oregon.
“People are placing bets on when the river will break through the coffer dam,” said Amy Cober of the environmental group American Rivers. She hopes to be there to witness that moment.
“There have been more than 250 dams removed nationwide,” Cober said. “This is definitely one of the bigger ones.”
Next summer, PGE will complete the job of returning the Sandy River to a free-flowing state when it removes the 16-foot-high Little Sandy Dam, a powerhouse and miles of tunnels and wood flumes and fills a 7,000-foot-long diversion canal.
Two dams, two scenarios
Like Condit Dam, which is scheduled for demolition in October 2008, Marmot Dam is a victim of changing economics and changing values.
Both were built in the early years of the 20th century to meet local demand for hydroelectricity. Both impeded fish passage, although the rebuilt Marmot Dam, unlike Condit, did have fish ladders. Because both rivers harbor threatened runs of native salmon and steelhead, both utilities faced requirements for expensive modifications when they came up for federal relicensing in the 1990s.
In the end, both utilities calculated that it would be less expensive to demolish the dams than to invest in improved fish passage. In the case of Marmot Dam, the cost of dam modifications would have exceeded the revenue generated by selling the power, Fowler said.
Both utilities brought conservationists and other groups on board to support their plans in formal settlement agreements.
But there the similarities between the projects end.
Over nearly a century of operation, sediment has built up behind both dams.
Condit Dam will be demolished in a single massive explosion, and its 2.2 million cubic yards of sediment will be released in a huge flush. According to state and federal fisheries agencies, shock waves from the blast will kill all fish in the vicinity. All aquatic life downstream in the White Salmon and in the Columbia River as far as Bonneville Dam, including endangered chum salmon, are expected to be killed or -displaced by the sediment plume.
But the damage will be short-lived, and federal fisheries officials have concluded that the benefits of opening 33 miles of habitat on the White Salmon to salmon and steelhead outweigh the temporary destruction of fish habitat.
Nothing like that sediment flush happened on the Sandy River Tuesday.
Over the years, the glacier-fed mountain stream has nearly filled the small reservoir behind Marmot Dam.
Once the dam is gone, nature will be allowed to take its course, moving the sediment behind it downstream in layers over a period of four or five years.
“It will be rapid erosion followed by gradual erosion,” PGE spokesman Mark Fryburg. “The Sandy River moves a lot of rock and sand naturally because of the glacier that feeds it. We expect the river will take care of the sediment naturally.”
But if the flush of sediment threatens the river’s salmon and steelhead runs, Fryburg said, PGE will act to limit the damage.
“There has never been a dam removal quite like this,” he said. “We have been modeling it for weeks, but there is still an element of uncertainty. We will have a group of engineers, sediment specialists, to make sure the sediment lands in a way that will be helpful for fish.”
The Sandy is home to threatened spring chinook, winter steelhead and coho salmon. In preparation for the demolition, fish biologists captured more than 400 adult spring chinook and 300 Pacific lamprey in a salvage operation last week, said Todd Alsbury of the Oregon Department of Fish and Wildlife. The wild chinook were transported above the temporary coffer dam to complete their journey upriver to spawn.
On Tuesday, with the fish ladder temporarily inoperable, a few spring chinook still lingered above a temporary fish trap below Marmot Dam. Biologists were netting them in an effort to limit mortality. “Any time you do a project like this, you have to do fish salvage,” said Stephanie Burchfield of the National Marine Fisheries Service.
Removal of the dam will open a 5-mile canyon of the Sandy to kayakers and rafters. And the entire area is slated to become a recreational corridor.
PGE has donated 1,500 acres of the Sandy River basin to the Western Rivers Conservancy, which hopes to sell most of it to the U.S. Bureau of Land Management, along with other land holdings, when federal appropriations become available. The land will become part of a 9,000-acre conservation and recreation area, with 14 miles of trails along the Sandy River, said the conservancy’s Sue Doroff.
“We have been working on the Sandy since 1999,” Doroff said. “We approached PGE when we realized there was an opportunity to leave a legacy on the river.”
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Copyright (c) 2007, The Columbian, Vancouver, Wash.
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