Q: Does leaving the Powerhouse at Milltown Dam negatively effect restoration of the confluence of the Blackfoot and Clark Fork Rivers?

A: Yes. It is in the 5 year flood plain and seasonal high water would cause a “backwater effect,” depositing sediments damaging to aquatic life upstream of the Powerhouse.

Effects on the River

(The following information is excerpted from The Draft Conceptual Restoration Plan Section 5.3.1)

Keeping the powerhouse and associated structures in place and constructing a natural channel are not compatible because:

• With the powerhouse in place, the limited width of the flood prone area would create a severe constriction causing a backwater effect (excess sediment deposition during even moderate flood events).
  
  
• Excessive maintenance would be required after even small floods. This would add a large cost to the project after each flood, an expense that would continue indefinitely into the future.
  
  
• Aquatic habitat would be damaged following flood events as a result of the sediment deposition and the subsequent maintenance.
  
  
• The constriction would increase velocity during all flood events, which would likely preclude fish migration during that period. Species of fish that could be affected include bull, cutthroat, rainbow and brown trout, as well as mountain whitefish and two species of suckers that are important to the ecology of the river.
  
  
• The constriction and backwater would result in increased flood stage upstream from the powerhouse as well as increased shear stress and scour at the constriction and immediately downstream. The sudden expansion downstream from the constriction would create back-eddy erosion on the stream banks downstream from the constriction.
  
  
• A major (100 year) flood would create backwater conditions that would bury the entire channel and some of the floodplain with large cobble and small boulder sized bedload sediment.
  
  
• Following each major flood, within the area affected by backwater deposition, all structures would be filled with sediment. This would increase the risk that the floodwaters would attempt to flank structures and create new channels.

Problems with the Powerhouse

Under EPA's remediation plan, not only the Powerhouse, but the Divider Block, the Right Abutment and the Forebay would be left in place, resulting in a highly engineered channel at the site.

The Forebay and Divider Block are both concrete structures extending about 60 feet upstream. Presently a portion of the Divider Block and all of the Forebay are submerged, but they would be visible after the Clark Fork is in its new channel.

This overhead view of a to-scale plan of the dam, shows the areas that would be left in place.

After the reservoir is gone, the Powerhouse would present a much different view than we are used to. This photo, taken Aug. 16, 2002, during the drawdown, foreshadows the view of the upstream face of the Powerhouse. The actual water level after remediation would be lower.

The State's conceptual restoration plan proposes an active and natural flood plain for the river. Because of its location in the 5-year flood plain, the Powerhouse would need extensive protection from flood events. Periodic flooding would require ongoing and expensive maintenance forever.

Before the effects on the river of preserving the Powerhouse, the Right Abutment, the Divider Block and the Forebay were understood, many thought the Powerhouse would make a good site for a Visitor's Center.

The consequences to the river and the expensive and ongoing maintenance required to secure the structure offer convincing evidence that the Powerhouse and its associated structures should be removed and their historical importance commemorated appropriately in another way, possibly a nearby interpretive center located outside of the 100-year flood plain.