Milltown Superfund remediation goals were relatively straightforward: remove the dam and sediment and then monitor to see if the aquifer recovers. The end goal for the state's restoration plan is a "naturally functioning channel and floodplain." But what does that really mean?
For the Milltown project, it means that the channel should be appropriate to its setting and connected to its floodplain. Native vegetation should grow along its banks, on its floodplain and in its wetlands. A naturally functioning river should do its job of moving water and sediment effectively, ideally by eroding and depositing sediment in a way that maintains a dynamic equilibrium between the channel and its floodplain. In other words, to make a river"naturally functioning," it's necessary to consider more than just the channel. A river is a three-dimensional ecosystem that includes the channel, the floodplain, and subsurface groundwater. In a naturally functioning river, all of these parts are usually connected.
It's especially important for a natural river to be able to spread onto its floodplain during high flows. Floodplains help moderate the size and speed of floods, thus lessening erosion and decreasing the hazard downstream. By slowing runoff, naturally functioning floodplains allow water to soak in and replenish groundwater below. The shallow water table means that streamside areas offer rich habitat for vegetation and wildlife, and thick vegetation near the stream helps filter out contaminants that would otherwise flow into the water. Together, the channel, floodplain, and groundwater trade water, sediment, and nutrients in a way that supports the web of life, whether it's westslope cutthroat trout in the channel, blue herons on the banks, or groves of cottonwood trees on the floodplain.
The features incorporated into the Milltown restoration plan - such as diverse native plant communities, side channels, and off-channel wetlands - mimic those found in a natural river system, but getting back to a truly natural condition will take some time because the Milltown project has unique constraints. Although the worst of the contam-inated sediments are being removed, there is no way to scrub all the metal from the Clark Fork; indeed, the river is affected by its mining legacy for more than 100 miles upstream.
To maintain water quality in the short-term, restoration designers are building a channel with temporary bank stabilization structures that will help control excessive erosion and vertical channel scour while native vegetation has a chance to become established on the floodplain. The newly constructed river is designed to transport sediment naturally through the site by eroding and depositing sediment within the channel boundaries, but initially, at least, the river won't erode sediment from the floodplain. And since they are starting with a blank slate in thereservoir area, the restoration designers want to ensure that a large or even moderate flood will not blow out their initial efforts to shape and align the river channel.
Given these constraints, restoration planners have different expectations for the short and long term. For the first decade or two, structures built of natural wood and rock will control the shape and location of the river channel, limiting excessive erosion and allowing vegetation to take hold on the river's banks and flood-plain. The wood structures - root wads and logs - are patterned after natural woody debris that's found in the Clark Fork upstream of the project area.
Over time, plants such as willows and cottonwoods will become well-established, providing deep roots to knit the river's banks together, branches and leaves to slow flood flows, and critical wildlife habitat. Eventually, the wood structures will decompose, erode away, or become buried, and the river will once again be allowed to meander naturally across its floodplain. It may take some time, but the restoration plan helps set the trajectory for a truly natural river in the future.
Check out more of the Dam News 2010.