Bracing for a Superfund flood

Since the removal of the Milltown Dam in 2008, the state has worked to restore the floodplain at the confluence of the Clark Fork and Blackfoot rivers. But that floodplain isn't quite ready to be flooded—which is a real possibility as the region's unusually deep snowpack begins to melt.

"The biggest issue we have with the newly constructed floodplain is that we have not had a chance to have vegetation grow on it," says Doug Martin of the Montana Natural Resource Damage Program.

If there's no vegetation, floodwaters can significantly erode the ground they spill on. And that ground, about eight miles upstream from Missoula, is part of the largest Superfund complex in the West, contaminated by mining waste from Butte.

The floodplain is now being covered with trees to create "roughness" that mimics the fallen cottonwoods and pines in other sections of the Clark Fork. The trees are intended to reduce the floodwater's energy and minimize erosion, and to encourage deposits of sediment, which might lead to vegetative growth in the future.

"One of the main goals of the whole design was to have the system in equilibrium: stuff that comes in equals what's going out," Martin says. "So if we have material that's being scoured off the floodplain, we are also hoping we get the same amount of material being deposited on the floodplain."

According to the Natural Resource Conservation Service, the Upper Clark Fork River Basin's snowpack stands at about 130 percent of average. Up-to-date stream flow data wasn't available, but NRCS hydrologist Brian Domonkos says the longer the cool and wet weather persists, the greater the risk of flooding when the weather finally turns warm.

Martin's hoping to avoid what he says would be the worst-case scenario at the former Milltown Dam site: the river cutting side channels, or avulsions, where engineers don't want it to. That could be expensive to undo. Robert Williams, the construction manager for Envirocon, the state's contractor, adds his worst-case scenario, in jest: "We watch the water come up and we all go home."

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