BONNER – There’s a new dam in the old Milltown Reservoir, and it’s evidence the 300-acre floodplain is coming back to life.
“Usually, I get angry when beavers are cutting willows on my riverbanks, but this is the perfect place for them,” restoration ecologist Amy Sacry said as she walked up a side channel of the Clark Fork River toward a line of branches in the water. “They won’t come if there isn’t food available. It’s the exact conditions we’d see in side channels upstream of the restoration.”
Sacry is vice president of Geum Environmental Consulting of Hamilton, and she’s evaluating regrowth at the former Milltown Reservoir Superfund Site.
The century-old dam that created the reservoir was breached in 2008, and excavators worked for four years hauling away sediment contaminated with old mine and smelter wastes.
They also built a new channel for the Clark Fork River, and planted thousands of trees and shrubs to keep it in place.
“We have 15-foot trees that were planted in this project,” said Doug Martin of the Montana Natural Resource Damage Program, which oversaw the Milltown site's restoration and rehabilitation. “But we also have cottonwoods and willows growing from the 2011 and 2012 high flows that are natural recruits. Those floods hardened the channel and planted thousands of cottonwood trees.”
Matt Daniels is the principal engineer at River Design Group in Whitefish. He was the design engineer for Milltown’s channel and floodplain drawings.
While his firm also did the channel reconstruction for the Jocko River near Arlee, Daniels said rebuilding the Clark Fork was a much more challenging assignment.
“There’s not a cookbook recipe to follow,” Daniels said. “We had a lot of lessons learned from our work on the Jocko five years before. But the floods are orders of magnitude more powerful on bigger rivers.”
River Design Group had to work with several conflicting needs. They couldn’t simply follow the method used on the Elwha River Dam removal in Washington, where the river was simply allowed to carve its own path through the reservoir.
The Clark Fork couldn’t be allowed to wash away some of the toxic sediments left in place along the floodplain. Nor could it undermine Interstate 90 on its northern bank, or inundate the private property along its former reservoir.
On the other hand, the project counted on the river to do much of its heavy lifting, so the trick was correctly predicting where it wanted to go. And the whole point of a floodplain is to give the river space to spread out and dissipate the energy of a spring runoff so it doesn’t erode its surroundings.
Daniels said he could have designed the new channel to resist a 100-year flood, but that would make the floodplain almost useless. Instead, the route was built with a 20-year flood in mind.
The year after the waters ran free in 2010, spring runoff delivered at least a 25-year flood that punched a new path through the curvy channel like the bar through a dollar sign.
The floods also revived plants that had been buried under the reservoir sediments for most of the 20th century. The excavators were able to find about 20 percent of the original floodplain soils with remarkable precision when removing the toxic material. And seeds and stems from that old vegetation sprung back to life when exposed to air and sunlight again.
“Those floods washed away at least 50 percent of what we planted,” Sacry said. “But because of those floods, we’re also ahead of where we would have been.”
That’s because while a willow or cottonwood seedling takes about 7 years of growth to produce seeds of its own, mature plant material can resume that activity in a single year. And those long-buried trees and shrubs picked up where they left off after Milltown Dam started blocking the river.
The result is both dense brush cover along the banks of the main Clark Fork and extensive thickets of native vegetation elsewhere in the floodplain.
Last Thursday, flocks of mountain bluebirds zoomed around the fence pole exclosures protecting young ponderosa pine saplings. Families of ducks paddled in some of the ponds left after the excavators ran out of fill dirt below the abandoned Milwaukee Road railroad grade.
After arsenic started showing up in drinking water supplies of residents near Milltown Dam in the 1980s, state and federal investigators traced the problem back to mine and smelter sludge deposited behind the dam in the flood of 1908.
The U.S. Environmental Protection Agency released a plan to remove the dam in 2004. Together with the state Natural Resource Damage Program, the federal agency developed a $120 million plan to demolish the dam, rebuild the confluence of the Clark Fork and Blackfoot rivers, remove 3 million tons of contaminated sediments and restore the floodplain above the confluence.
The excavation work wrapped up on Sept. 24, 2009, but state officials closed foot access to the Clark Fork riverbanks for the following four years to give new plants a chance to root undisturbed.
That closure quietly ended this summer, leaving the whole Clark Fork-Blackfoot confluence area open to floating, exploring and fishing.
“I’d cast a fly right along that foam line,” Sacry said of the water flowing along a pile of stumps and logs reinforcing one of the corners of the Clark Fork’s new channel. “There are some big pools right below there.”